Matv Smatv Project

Contract Title: Digital Television Project:

Provision of Technical Assistance

Contracting Authority: Department of Trade and Industry

Action Plan Task 5.9 Title: Survey of MATV and SMATV systems

DECEMBER 2003

DIGITAL TELEVISION PROJECT PROVISION OF TECHNICAL ASSISTANCE Survey of MATV and SMATV Systems 1st December 2003 Final Report Release 1.0 Project Team: John Ross and Peter Barnett DTG Management Services Ltd 2003

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Prepared for: Facilitated by: Ian Dixon DTG Management Services Ltd Department of Trade & Industry 7 Old Lodge Place Twickenham TW1 1RQ Tel: +44 20 8891 1830 Fax: +44 20 8891 1999

Document Status: The preparation of this report has been facilitated by DTG Management

Services Ltd a wholly owned subsidiary company of the Digital TV Group. The report has been compiled by acknowledged industry experts with the

aim of providing answers to specific questions posed by the UK Department of Trade and Industry. While every effort has been made to

ensure accuracy and to provide a consensus view when required, it should not be assumed that all member organisations of the Digital TV Group support all aspects of the report. The views expressed in the report are

those of the authors and not the collective views of DTG Council.

DIGITAL TELEVISION Page 3 of 44 Release 1.0, December 2003 PROJECT

0. Index

0.1. Index of Headings 0. Index .....................................................................................................................3

0.1. Index of Headings..........................................................................................3 0.2. Index of Figures .............................................................................................4 0.3. Index of Tables ..............................................................................................4

1. Executive summary...............................................................................................5 2. General introduction..............................................................................................7 3. Technical characteristics of existing systems .......................................................8

3.1. Introduction to MATV systems.......................................................................8 3.1.1. System design requirements.................................................................. 8

3.1.1.1. Signal levels ....................................................................................8 3.1.1.2. Noise levels .....................................................................................9

3.1.2. Impulsive interference ............................................................................ 9 3.1.3. System architectures and components .................................................. 9 3.1.4. Aerials .................................................................................................. 10 3.1.5. Masthead amplifiers ............................................................................. 10 3.1.6. Additional services ............................................................................... 11 3.1.7. Processed systems .............................................................................. 13

3.1.7.1. Equalisers......................................................................................13 3.1.7.2. Channel changers .........................................................................14

3.1.8. Launch amplifiers ................................................................................. 16 3.1.8.1. Noise .............................................................................................16 3.1.8.2. Intermodulation distortion ..............................................................17

3.1.9. Distribution networks ............................................................................ 18 3.1.9.1. Loop networks ...............................................................................18 3.1.9.2. Tree and branch networks.............................................................18 3.1.9.3. General considerations .................................................................19

3.2. SMATV-TM systems....................................................................................20 3.3. SMATV-IRS: Integrated Reception Systems ...............................................21

3.3.1. Satellite reception................................................................................. 21 3.3.2. IRS architecture.................................................................................... 22

3.3.2.1. Satellite dish aerials ......................................................................23 3.3.2.2. LNBs..............................................................................................24 3.3.2.3. Adding orbit locations ....................................................................24

3.3.3. Typical IRS installation ......................................................................... 24 4. Upgrading Systems for DTT ...............................................................................26

4.1. Introduction ..................................................................................................26 4.2. Upgrading MATV systems to carry DTT ......................................................26

4.2.1. Wideband systems............................................................................... 26 4.2.2. Frequency selective systems ............................................................... 26 4.2.3. Distribution networks ............................................................................ 26

4.3. Upgrading SMATV-TM systems to carry DTT .............................................28 4.4. Upgrading to IRS .........................................................................................28 4.5. Upgrade resources ......................................................................................28 4.6. Conclusions and Recommendations ...........................................................29

5. Issues towards switchover ..................................................................................31 5.1. Equipment advances ...................................................................................31 5.2. Internet access ............................................................................................31 5.3. Recommendations.......................................................................................32

6. Implications of switch-over..................................................................................33 6.1. Assumptions about switch-over ...................................................................33


6.2. Effects of frequency changes ......................................................................34 6.2.1. MATV systems ..................................................................................... 34 6.2.2. IRS systems ......................................................................................... 35

6.3. Effects of power level changes ....................................................................35 6.3.1. MATV systems ..................................................................................... 35

6.3.1.1. Delivered signal levels...................................................................35 6.3.1.2. Intermodulation product levels ......................................................35

6.3.2. IRS systems ......................................................................................... 36 6.4. Conclusions .................................................................................................36 6.5. Recommendations.......................................................................................37

7. Introduction of new services after switchover .....................................................38 7.1. Recommendations.......................................................................................38

8. Commercial issues..............................................................................................40 8.1. General issues.............................................................................................40 8.2. Tenant demand............................................................................................40 8.3. Costs............................................................................................................41

8.3.1. MATV Systems..................................................................................... 41 8.3.2. Integrated Reception Systems ............................................................. 41

8.3.2.1. IRS with Internet access................................................................42 8.3.3. Maintenance costs ............................................................................... 42 8.3.4. Total investment ................................................................................... 42

9. Summary of conclusions.....................................................................................43 10. Acknowledgements .........................................................................................44

0.2. Index of Figures Figure 1: Distribution of communal aerial system sizes ..............................................7 Figure 2: Wideband MATV system structure ............................................................10 Figure 3: MATV system with mast head amplifier.....................................................11 Figure 4: Dual-band MATV system ...........................................................................11 Figure 5: Dual-band system with dual launch amplifier.............................................12 Figure 6: Typical use of 4-input launch amplifier.......................................................13 Figure 7: Typical cluster equaliser response.............................................................14 Figure 8: Two reception paths causing pre-echoes on analogue signals .................15 Figure 9: Use of a 5-channel analogue channel changer .........................................15 Figure 10: Conventional amplifier preceded by attenuator .......................................17 Figure 11: Inter-stage amplifier .................................................................................17 Figure 12: Loop network topology.............................................................................18 Figure 13: Tree and branch topology ........................................................................19 Figure 14: Typical SMATV-TM architecture ..............................................................20 Figure 15: Typical 5-wire IRS architecture ................................................................23 Figure 16: An example of a well designed IRS headend ..........................................25 Figure 17: Typical modification to an MATV distribution network .............................27

0.3. Index of Tables Table 1: Recommended signal level ranges at outlets ...............................................9 Table 2: Example of noise levels with conventional and inter-stage amplifiers ........17 Table 3: Band segment selection criteria ..................................................................22


1. Executive summary About 20% of households in the UK obtain their television signals via communal aerial systems. Of this population, only about 20% have so far been converted to receive digital signals. Current rates of conversion will have to rise sharply if the goal of converting all communal aerial systems is to be achieved before switch-over to exclusively digital transmission. This report examines the technical characteristics of the various types of existing systems, the implications of upgrading to carry digital signals, the likely effects of switch-over, and some of the consequences of introducing new services after switch-over. It also examines some commercial issues, such as ownership and motivation, tenant demand and system costs. There are three principal categories of communal aerial systems:

MATV (Master Antenna TV) systems, which primarily provide access to UHF terrestrial analogue and digital signals;

SMATV-TM (Satellite Master Antenna TV: TransModulation) systems, which provide the same services as MATV, but add one or more satellite signals which have been converted to UHF;

SMATV-IRS (Satellite Master Antenna TV: Integrated Reception System, more commonly known as IRS) systems, which provide access to a wide range of satellite services, as well as UHF terrestrial TV.

Note that in principle, all categories of communal aerial systems are capable of carrying other services such as FM radio and DAB (Digital Audio Broadcasting). The bulk of existing MATV systems were installed between the late 1960s and early 1980s, and in most cases need some attention to be able to carry digital terrestrial television (DTT) signals. This is due to the use of frequency selective components, such as equalisers. Of the million or so outlets on communal aerial systems that have been converted, about 90% have been converted to IRS. The remaining 10% only carry analogue UHF TV and DTT. Conversion to IRS, while more expensive than upgrading, holds a number of clear advantages. Tenant choice is maximised, and self-installation, notoriously dangerous and damaging to buildings, is discouraged. The challenge for the installation industry is to increase the rate of installations substantially; if sustained at current levels, conversion of all communal aerial systems will not be achieved for about nineteen years. If switch-over occurs in, say, six years time, about three million homes will still not have access to any digital signals. We expect demand to increase dramatically after the switch-over plan is made public. The installation industry is capable of some degree of increase in capacity, but it is not clear that this will be adequate. Of particular concern is the ability to recruit and train suitable installation staff, most critically commissioning engineers. We therefore recommend that the industrys ability to cope with the expected increase in demand should be studied in detail. If no clear solution is found, an alternative plan should be developed for increasing the proportion of MATV upgrades, in order to increase the total rate of conversions. These upgrades may then be converted to IRS at a later date.


A relatively new development is the integration of Internet access into IRS. Glasgow Housing Association is conducting a particularly interesting pilot scheme in 600 homes. This addresses such issues as social exclusion, improved services including landlord and tenant communications, and the objectives of the e-Envoy for access to government services via the television. This pilot scheme is not widely known about yet, so we recommend bringing it to the attention of relevant departments, and to the DigiTV Project. After examining the probable effects of switch-over we conclude that most communal aerial systems that have been converted or upgraded should cope well with the anticipated frequency and power changes. However, when new channels are brought into use, due for example to existing analogue services being in the released spectrum, remedial work will be required. We therefore recommend that details of the spectrum plan are made available to installers at the earliest opportunity, so that during upgrades or conversion equipment can be fitted that will accept the new channels. We also recommend that a range of launch amplifiers are tested under anticipated load conditions, to verify that their performance will be acceptable. We also recommend testing a range of channel changers, as the performance of some models is believed to be inadequate for digital signals. Introduction of new services into the released spectrum after switchover may cause some problems to communal aerial systems. We recommend that wideband systems are avoided, and that filtering is used to protect systems against new signals that might compromise carriage of the existing services. In general, social landlords (local authorities and housing associations) are ahead of private landlords in converting systems. However, there is a wide range of attitudes, with some moving ahead enthusiastically, and others waiting until there is an announcement from government. Some social landlords are particularly under pressure from ethnic groups to provide delivery of TV programmes related to their cultural origins. This can be expensive to provide, and providing it to one group can imply having to provide it to all. Not offering the service tends to encourage self-provision. Costs are difficult to compare between MATV upgrades and IRS conversions. In MATV upgrades, the cost tends to be independent of the number of outlets, and particularly for IRS, cable containment requirements can strongly influence costs


2. General introduction Television services are available from a wide range of sources, including satellite, terrestrial, cable, and DSL. This report is concerned exclusively with systems for the reception and distribution of signals off-air, from both satellite and terrestrial transmitters. According to the 2001 census there were 5.1 million flats, maisonettes and apartments in England, Wales, Scotland and Northern Ireland, representing about 20% of households. Extrapolating current rates of growth, by 2009 this number will have risen to 5.6 million. Virtually all of these will have some means of receiving off-air television services. In the great majority of cases, this will be some form of communal aerial system. Communal aerial systems provide a number of advantages to both landlords and tenants. They provide access to a range of television services, often at lower cost than an individual tenant would be able to achieve acting alone. Particularly in tower blocks, self-installation of UHF receiving aerials or satellite dishes can be unsightly, dangerous, and can cause damage to the fabric of the building. For example, there have been many instances where damp-proof membranes have been punctured by the installation of a satellite dish, and the consequent repair costs have been very high. Communal aerial systems are found distributing signals to anything from clusters of four individual detached houses, to whole estates and tower-blocks. The chart below for example shows the distribution of system sizes to be found in the Borough of Enfield, but the distribution does vary significantly from one area to another.

020406080

100120140160180

4 6 8 12 14 16 18 20 24 26 28 30 32 36 40 48 50 52 60 72 78 84 88 92 102

132

Number of outlets per block

Num

ber o

f blo

cks

Figure 1: Distribution of communal aerial system sizes

in the Borough of Enfield Communal aerial systems are also found in such places as hospitals, nursing homes, shops, offices and universities. This is important as far as installation and maintenance resources are concerned, because the same businesses address these markets, as well as domestic installations. However, this report focuses only on the largest market: domestic installations.


3. Technical characteristics of existing systems Communal aerial systems can be subdivided into three categories:

Master Antenna TV systems (MATV), which primarily distribute UHF TV signals, but also frequently carry radio services;

Satellite Master Antenna TV systems (SMATV), which in turn can be subdivided into two categories:

o SMATV-TM, which TransModulate satellite services into the UHF TV band;

o SMATV-IRS (Integrated Reception Systems) which provide the full range of both satellite and terrestrial signals to users. This category represents about 90% of current installation work.

Within these categories there is a wide variety of possible combinations of equipment. No single manufacturer makes all the parts required for systems in all circumstances, and individual installers tend to stay with items of equipment with which they have become familiar. Installation sites vary considerably: some are exposed to the weather, and some are indoors; some have plenty of space, and some are very cramped. Distribution networks clearly are closely dependent on the number and relative locations of dwellings, and means within buildings for routing cables. These factors have prevented the production of a limited range of models of headend that will address all applications. Custom build is therefore virtually universal.

3.1. Introduction to MATV systems MATV systems are intended primarily to deliver the full range of UHF TV signals to users. The principles and practice of MATV systems have been established for a long time. Indeed, a significant proportion of systems still in use today were installed over 30 years ago.

3.1.1. System design requirements The purpose of any TV distribution system is to deliver signals to all potential users with appropriate levels of quality. This means that levels of signal, noise, interference and distortion must all be controlled.

3.1.1.1. Signal levels For MATV systems, requirements for signal levels delivered to outlets are well established, and are shown in Table 1 below:


Minimum signal level

per channel, dBV Maximum signal level

per channel, dBV Analogue television 60 80

DTT 451 70 FM radio 54 74

DAB 30 70

Table 1: Recommended signal level ranges at outlets The purpose of a launch amplifier is to drive the full set of signals, both analogue and digital, into the distribution system at a sufficiently high level that, taking into account the losses of splitters, cable and taps, signals are delivered to consumer outlets at the desired levels. In practice, the launch amplifier has to drive individual analogue signals into the distribution network typically at levels of up to 114 dBV.

3.1.1.2. Noise levels Not only must the signals be delivered with sufficient level, they must also have adequate carrier to noise ratio. This is particularly important for DTT signals which commonly are transmitted at power levels in the range 15-20 dB below analogue signals. It is therefore important that the system designer pays attention to the level of noise in the system. The level of noise in a system can be greatly increased by poor design technique. The subject is explained further in Section 3.1.8, Launch Amplifiers. MATV systems are readily amenable to a straightforward analysis of noise levels, the techniques for which were established a very long time ago. Although such analysis is not difficult to carry out for those trained in the art, the level of understanding of this subject in the MATV industry appears generally poor. However, some simple rules of thumb, such as ensuring that gain occurs before losses, will usually suffice.

3.1.2. Impulsive interference It has been conclusively demonstrated that impulsive interference can cause severe disruption to DTT signals. Techniques for minimising the levels of impulsive interference are generally well understood by those installing MATV and IRS systems, and by the equipment designers, and have been described in a recent report2.

3.1.3. System architectures and components At its simplest, an MATV system comprises a UHF aerial to receive the TV signals, an amplifier to raise the signal levels, and a distribution network of coaxial cables to deliver the signals to users. This is known as a wideband, unprocessed system, and is illustrated in Figure 2 below. 1 In a laboratory environment, DTT signals using 64QAM rate 2/3 can be decoded at levels down to about 30dBV. The level of 45dBV is generally accepted as the minimum practical in real systems, affording a reasonable level of protection against impulsive interference and other distortions. However, it is common practice to design for 50dBV, giving enhanced ruggedness. Note that for 16QAM rate 3/4, these figures could theoretically be reduced by about 4dB. 2 Action Plan Task 5.14: Improving UK Aerial Installations, D Fisher, J Ross and P Barnett.


Figure 2: Wideband MATV system structure The amplifier, commonly called a launch amplifier, is intended to overcome the signal losses in the distribution network, so that signals delivered to users are of appropriate levels and quality. There is another category of MATV system, which contains frequency selective components, such as equalisers or channels changers. These are commonly known as processed systems. There are many variants of MATV system design, and the following sections will discuss the main types and their components.

3.1.4. Aerials The requirements for UHF TV aerials for digital reception have been described in some detail in a recent report3. The great majority of MATV systems use aerials that pre-date the introduction of digital transmissions, and many of them may need replacing for satisfactory digital reception. Certain characteristics of aerials, such as the ability to reject signals from unwanted directions, can be particularly important for aerials installed on tall buildings as they may have visibility of distant transmitters that cannot be seen by aerials mounted at normal house rooftop levels. Aerials for reception of FM and DAB services are generally considerably less critical in their performance requirements than UHF TV aerials, because FM and DAB services are intended to be received by portable equipment with low gain aerials. In practice, care must be taken not to exceed the recommended signal levels in Table 1.

3.1.5. Masthead amplifiers Masthead amplifiers are sometimes introduced when the cable loss between the aerial and a launch amplifier is excessive. They can also be used to good effect to overcome the loss of a passive equaliser (see equalisers below). The gain of the amplifier should ideally be chosen to be just greater than the loss that it is intended to overcome and the amplifier should be mounted close to the aerial, ahead of the lossy component for which it compensates as shown in Figure 3.

3CAI/DTG: Guidelines for the use of Benchmarked Aerials, D Fisher

Distribution Network

UHF Aerial

Launch Amplifier


Figure 3: MATV system with mast head amplifier

3.1.6. Additional services It is quite common to find that MATV systems are used for delivering FM radio signals, and less commonly also DAB. One technique for achieving this is illustrated in Figure 4 below. Signals from the UHF aerial and the FM aerial are combined using a diplexer and the result is fed to the launch amplifier. The diplexer contains some filtering elements so that, for example, the small amount of UHF TV signals that will inevitably be received by the FM aerial will not be passed. This can be important for analogue TV because the path length for signals received via the UHF aerial may be different from those received via the FM aerial, which can result in so-called ghosting on analogue pictures. For digital signals, this is generally of less importance due to the high degree of immunity to reflections that has been designed into the digital modulation format.

Figure 4: Dual-band MATV system

The single amplifier technique described above does tend to suffer from one notable problem. Non-linearity in the launch amplifier (see Section 3.1.8, Launch Amplifiers) can cause the FM signals to generate distortion products that interfere with reception


UHF Aerial

Launch Amplifier

Mast Head Amplifier

UHF Aerial FM Aerial

+


Launch Amplifier

Diplexer


of TV signals in the UHF band. This effect is overcome using a dual launch amplifier (see Figure 5 below). One amplifier handles the FM signals, and the other amplifier handles the UHF TV signals. The outputs of the two amplifiers are combined using a diplexer, which prevents distortion products from the FM signals appearing at significant levels in the UHF band.

Figure 5: Dual-band system with dual launch amplifier

More complex still, there are systems that carry both FM and DAB, and on top of that may also have other services such as door entry cameras. Quad amplifiers with built-in filtering and independent gain controls can be used in these situations (see Figure 6).

UHF Aerial FM Aerial

+


Dual Launch

Amplifier


Figure 6: Typical use of 4-input launch amplifier

3.1.7. Processed systems In many cases it is advantageous or even necessary to make some changes to the signals before they are applied to the launch amplifier and distribution network.

3.1.7.1. Equalisers The best signal quality (specifically signal to intermodulation product ratio) is obtained from a launch amplifier when all the analogue signals feeding it are applied at the same level. It is rarely the case that received signal levels are all equal, and so it is common practice to use an equaliser to achieve this condition. An equaliser is a frequency selective device typically having four or five pass-bands, where those pass-bands can be tuned to the frequencies of the incoming analogue services, and the gain or attenuation of each pass-band can independently be adjusted until all analogue signals are at the same level. Some equalisers are active devices providing amplification of the signals, but the majority are passive devices providing only loss. Some equalisers are intended to operate only with single channels in each section, and others, called cluster equalisers, can operate with a group of channels in each section. A typical cluster equaliser frequency response is shown in Figure 7.

UHF Aerial

FM Aerial

DAB Aerial

CCTV UHF MOD

+ Distribution

Network

4-Input Launch Amplifier


Figure 7: Typical cluster equaliser response Passive equalisers introduce a loss, and this can adversely affect the system noise figure (this effect is explained in more detail in Section 3.1.8.1). In areas where the signal level provided by the aerial is marginal, this can be important. However, the use of an appropriate masthead amplifier can significantly improve performance. Filtering provided by equalisers is generally highly beneficial because it protects the system from unwanted signals. However, since equalisers are tuned to pass only certain selected channels, adding new services in new channels usually requires hardware changes. For example, thousands of headends had to be upgraded by replacing 4-channel equalisers with 5-channel equalisers when Channel 5 was added. For similar reasons, hardware upgrades are generally also required to allow satisfactory reception of digital signals. This is discussed at greater length in Section 4, Upgrading Systems for DTT.

3.1.7.2. Channel changers Many MATV systems have been constructed using relatively inexpensive coaxial cable. The shielding factor of this cable is often poor, and if the MATV system is located reasonably close to the transmitter, the leakage of the cable will allow a significant amount of signal to be received directly by the distribution network. Because the path is more direct, this signal will arrive slightly ahead of the intended signal, which has to pass through the headend equipment. The result is that


analogue pictures appear to have a so-called pre-echo, a fainter picture to the left of the wanted picture.

Figure 8: Two reception paths causing pre-echoes on analogue signals

This effect could clearly be overcome by using a higher quality coaxial cable. However, it is not usually possible to ensure that fly leads used to connect the television to the outlet plate are of adequate quality, so instead additional equipment is installed at the headend to move each analogue signal to a vacant UHF channel. As there is no direct path on this new channel, no echoes will be visible. Figure 9 illustrates the use of a typical 5-channel analogue channel changer. The equipment is modular, with input signals connected in a daisy-chain manner to all five individual channel converters. The outputs are similarly connected together to give a single output line for connection to the launch amplifier. Although block conversion of the entire group of channels in a single step would in principle be cheaper to implement, this technique is not generally used due to the difficulty of finding corresponding vacant destination channels.

Figure 9: Use of a 5-channel analogue channel changer

Direct path

Path via head-end

Transmitter

UHF Aerial


Launch Amplifier

5-channel analogue channel changer


Since channel changing requires the use of vacant destination channels, it is no surprise that many systems installed before digital transmissions began chose destination channels that subsequently were used for digital signals. In many such cases the digital signal caused analogue reception to appear very noisy, and urgent attention was required to select new, clear destination channels for the headend channel changers, and also to re-tune all televisions connected to the system.

3.1.8. Launch amplifiers The purpose of a launch amplifier is to raise the level of all the signals to be carried, to overcome the losses that will be encountered in the distribution network. It is not uncommon to find that the distribution network loss to the worst outlet is in the region of 40 - 50 dB. Noting that analogue signals must be delivered to all outlets at a level of at least 60 dBV, we can see that the launch amplifier must be capable of delivering at least 110 dBV per analogue signal to the distribution network. The most frequently used launch amplifier until recently is known as a 1 volt amplifier. Such an amplifier is capable of delivering two analogue TV signals at 120 dBV (1 volt) each, with the level of intermodulation products sufficiently low that they cause no noticeable degradation to analogue pictures. However, when the amplifier is used to carry more than two analogue TV signals, as is usually the case, its output level per signal must be reduced if the level of intermodulation products is to remain adequately low. This process is known as de-rating. For five equal analogue signals, the de-rating factor per signal is about 6 dB. The amplifier is therefore capable of delivering 114 dBV per analogue signal. The 1 volt amplifier has for some years been the workhorse of the MATV industry, and large numbers of them have been installed. Advances in semiconductor technology have however brought down prices and improved performance, and great use is now being made of 123/124 dBV amplifiers, which can be bought for less than 90. In large distribution networks, the network loss can exceed 50 dB. In principle it is possible to use an amplifier with a higher power capability to overcome this extra loss, but we have not identified any such product on the market. It is usual therefore to find repeater amplifiers placed within the distribution network, raising the signal levels, but positioned where they can operate within the constraints of their output capability. Generally these amplifiers are fed with DC down the coax for their power supply, to avoid having to run mains cabling in addition to the coaxial cables. As described in Section 3.1.6 it is also quite common to find split band amplifiers (see Figure 6), which amplify signals at different frequencies (e.g. FM, DAB, and UHF TV in Bands II, III and IV & V respectively) separately, and consequently to control mutual interference due to intermodulation.

3.1.8.1. Noise An important advance in the last two or three years is the wider recognition that under certain circumstances so-called inter-stage amplifiers can give a greatly improved system noise level compared to fixed gain amplifiers with an attenuator placed before the amplifier. These two cases are illustrated in the diagrams below.


Figure 10: Conventional amplifier preceded by attenuator

Figure 11: Inter-stage amplifier

An inter-stage amplifier has a fixed gain stage followed by an attenuator, which in turn is followed by a further amplifier normally capable of handling high signal levels. Prior to the availability of inter-stage amplifiers, the required overall gain was achieved by placing attenuation at the amplifier input, and in cases where the attenuation required was high, this resulted in rather poor system noise performance. The use of an inter-stage amplifier reduces this problem considerably. Table 1 below shows an example of typical calculated noise levels in systems using fixed gain and inter-stage amplifiers, illustrating clearly the advantage of using an inter-stage amplifier when the attenuation is high.

Attenuation, dB

Noise level referred to receiver input, with fixed gain amplifier,

dBV

Noise level referred to receiver input, with inter-stage amplifier,

dBV 40 48.5 28.6 30 38.5 19.7 20 28.6 14.5 10 19.3 13.4 0 13.3 13.3

Table 2: Example of noise levels with conventional and inter-stage amplifiers

In many cases where an existing MATV system is being upgraded to handle DTT signals it will therefore be necessary to exchange the existing launch amplifier for an inter-stage launch amplifier in order to deliver DTT signals with sufficient carrier to noise ratio to all outlets.

3.1.8.2. Intermodulation distortion A further constraint on amplifiers is intermodulation distortion. Intermodulation is generally particularly significant when the amplifier becomes non-linear due to being driven at high output levels. The result of intermodulation is to generate unwanted signals (known as intermodulation products) that can interfere with the wanted

Amplifier Attenuator Launch Amplifier Network

loss Receiver

Inter-stage amplifier

Attenuator Launch Amplifier Network

loss Receiver

Conventional amplifier


signals. Typically analogue TV shows patterning on the picture, and for DTT signals the decoding margin may be reduced, or decoding even prevented completely. The levels of unwanted intermodulation products can be reduced relative to the wanted signals by reducing the output level of the launch amplifier. The design process therefore becomes a matter of simultaneously achieving sufficient drive level to overcome network losses, whilst also achieving sufficiently low levels of intermodulation products.

3.1.9. Distribution networks The function of the distribution network is to deliver all signals to all users within the maximum and minimum amplitude limits defined in Table 1. Distribution networks generally are comprised of coaxial cables, taps, splitters and outlet plates, but their exact topologies vary considerably to suit local conditions.

3.1.9.1. Loop networks Probably the simplest form of distribution network is the loop network. The coaxial cable simply loops from one outlet plate to the next, as shown in Figure 12. The outlet plate and has an in-built tap that samples and attenuates the signal from the coaxial cable. Different tap values may be selected so that as signal levels decrease along the cable, the correct levels of signals may be delivered to each outlet plates socket.

Figure 12: Loop network topology

This form of distribution network is seldom installed nowadays as a single fault near the front of the network can cause all subsequent outlets to fail also. In addition, this type of network is vulnerable to tenants changing outlet plates to a type without an internal tap, and failing to re-connect the onward loop.

3.1.9.2. Tree and branch networks A more commonly used topology is shown in Figure 13, and is called a tree and branch network. In this example, a splitter is used to drive four separate cables. Tap values varying typically from 10 dB to 35 dB are selected to pull off suitable levels of

Launch Amplifier

Outlet/ tap


signals to feed down cables to individual outlet plates. Taps are usually located in service ducts that can be reached without having to gain entry to individual premises. This considerably simplifies maintenance and repair of the system.

Figure 13: Tree and branch topology

Outlet plates often contain diplexers that separate out the various services, presenting them on separate sockets.

3.1.9.3. General considerations The construction of the distribution network can be a major influence on the overall system costs. Use of an existing cable tray in a riser will clearly keep costs down, but many local authorities require a higher level of containment, such as the use of galvanised steel trunking, and LSZH (low smoke zero halogen) cable which costs about 10% more than standard cable. Cables should be compliant not only with EN501174, but also with the CAIs5 cable benchmarking scheme. This was developed originally as an endorsement of cables of appropriate quality for satellite reception, but has been adopted also for DTT. The designer of the distribution network must be concerned not only with end-to-end loss, but also with variations of loss due to the slope of the cables attenuation characteristic with frequency, and to standing waves. Care must also be taken with

4 EN50117: CENELEC standard Coaxial cables used in cabled distribution networks 5 Confederation of Aerial Industries, the UK trade association.

Splitter

Taps Outlet Plates

Launch Amplifier


safety aspects such as the system earthing and supply fusing, so that for example excessive currents do not flow in the coaxial cable. The CAI has published a Code of Practice covering these issues.

3.2. SMATV-TM systems In the late 1980s satellite transmission of TV services intended to be received directly in homes began. The dominant players were Sky, at the time using analogue FM on the Astra satellite network, and BSB, who used D-MAC on their own satellite. A number of other channels also became available, mostly on the Astra and Eutelsat satellite networks. The presence of all these channels as alternatives to terrestrial television gave rise to demand for carriage of these services on communal aerial systems, in addition to the available terrestrial TV services. This demand was met by SMATV-TM systems, which generally are similar to MATV systems, but have at least one channel provided by a satellite receiver. The modulation formats of analogue satellite signals of that time were not compatible with analogue UHF TV, so it was not possible to translate the satellite signals directly to unused channels in the UHF TV bands. Instead, a satellite receiver was used to demodulate the satellite signals and recover a composite analogue PAL signal, which in turn was applied to a UHF modulator. Most receivers actually had a modulator built in, so in principle it would have been possible to use this, but in practice the quality of the output was not regarded as satisfactory.

Figure 14: Typical SMATV-TM architecture

UHF TV uses a form of modulation known as vestigial sideband, or VSB, in which most of the lower sideband is removed (i.e. only a vestige remains). As a result, 5.5MHz of video, together with the analogue and NICAM sound carriers, can fit in a single 8MHz UHF channel. VSB modulators are however rather expensive in comparison to a double sideband, or DSB, modulator, in which upper and lower

UHF Aerial

Satellite Receiver

UHF Modulator

Satellite Receiver

UHF Modulator

+ Distribution

Network

Launch Amplifier

Satellite Aerial


sidebands are a mirror image of each other in the frequency domain. DSB modulation is compatible with conventional VSB TV receivers, but the signal occupies both the wanted channel, and the channel immediately below. Consequently, fewer of them can be accommodated in the UHF TV bands. SMATV-TM systems suffer from a number of constraints that have led to them becoming largely obsolete. These include

Limited channel capacity, perhaps ten to fifteen satellite channels at best. The numbers of satellite channels available nowadays vastly exceeds this.

Difficulty of handling pay-TV channels. If an authorised receiver is used to decode an encrypted pay-TV service, its output is in the clear on the whole distribution network. It is unlikely that a pay-TV operator would agree to this.

Decoding digital signals for re-modulation loses access to all the related digital services, such as electronic programme guides, interactivity, etc.

It is believed that about 5,000 SMATV-TM systems are still in use, many of them re-distributing Channel 5 (now branded five) in areas where terrestrial coverage is unavailable. In the shadow of the tower building at Canary Wharf in east London for example, SMATV-TM systems are used to provide all five analogue signals

3.3. SMATV-IRS: Integrated Reception Systems As the number of satellite channels increased, and the limitations of SMATV-TM systems described above were encountered, it was recognised that a new approach was needed. The result was the Integrated Reception System, which has the capability of delivering FM radio, DAB, analogue and digital terrestrial television, analogue and digital satellite. They therefore offer the maximum flexibility to tenants, albeit at some cost (see Section 8.3.2), giving them the greatest choice of off-air sources for viewing. Integrated Reception Systems build on the ideas and practices of MATV systems to add delivery of satellite signals. Roughly 90% of all recent communal aerial installations have been Integrated Reception Systems.

3.3.1. Satellite reception Satellite transmissions occupy bandwidth from 10.7 GHz to 12.75 GHz. This spectrum is actually used twice, on orthogonal polarisations (vertical and horizontal), giving a total of over 4 GHz of spectrum of satellite signals. Add to this roughly 1GHz of terrestrial signals, and the total bandwidth becomes about 5GHz. Cables capable of operating at frequencies this high tend to be both extremely expensive and very lossy, so the usual practice is to convert these frequencies to a much lower intermediate frequency (IF). However, the total bandwidth is so wide that it is necessary to break it into four segments in order to carry it effectively within an IF range of 950 MHz to 2150 MHz. The four segments are low-band (10.7 GHz to 11.7 GHz) and high-band (11.7 GHz to 12.75 GHz) on vertical and horizontal polarisations. In domestic receiving systems that do not use communal aerial systems, the satellite receiver is connected to the dish aerial via a single coaxial cable. At the feed point of the dish is an LNB (low noise block down converter) which performs the down conversion and selection of one of the four segments described in the previous paragraph. The selection is controlled by the receiver sending a 22 kHz tone or a change of DC supply voltage up the coax to the LNB. The presence or absence of


the 22 kHz selects between low-band and high-band, and the change of DC supply voltage selects between vertical and horizontal polarisation, as described in Table 3.

13v DC 18v DC No tone Low-band,

vertical polarisation Low-band,

horizontal polarisation 22kHz tone High band,

vertical polarisation High-band,

horizontal polarisation

Table 3: Band segment selection criteria

3.3.2. IRS architecture A typical 5-wire IRS architecture is shown in Figure 15. The four satellite band segments are fed in parallel down four cables forming the backbone of the system. A fifth cable carries the output of what is effectively a conventional MATV system which can take any of the forms described in the previous sections of this report, according to local needs. The switches, often referred to as multiswitches, react to tones and DC voltages coming back through outlet plates from the consumer equipment, and select the appropriate satellite band segment. The terrestrial signals carried on the fifth backbone cable are not switched, but are fed at all times to all outlets. An important benefit of Integrated Reception Systems is that they operate with standard consumer terrestrial and satellite receivers. The consumer will generally be unaware that they do not have their own private receiving system.


Figure 15: Typical 5-wire IRS architecture

3.3.2.1. Satellite dish aerials Integrated Reception Systems are much more complex than typical individual consumer installations not using shared aerial systems. As a result, the satellite signals can undergo a small amount of degradation, even in a well-designed system. It is therefore common practice to use dishes for Integrated Reception Systems that are a little larger than would be used for an equivalent individual installation: a 60 centimetre diameter dish instead of a 45 centimetre, or an 80 centimetre instead of a 60 centimetre, for example. When located on top of a particularly high building it will be important to pay close attention to the ability of both the dish and the method of mounting to withstand the high wind speeds that are likely to be experienced.

Satellite Aerial

MATV System

Taps

Switch

To Outlets

Taps

Switch

To Outlets


3.3.2.2. LNBs In single outlet domestic installations not using communal aerial systems, the LNB performs the switching functions, responding to the tone and the DC inputs. In Integrated Reception Systems, all four band segments are required to be present at the same time on separate outputs for feeding into the backbone. Four-output LNBs exist that will fulfil this function.

3.3.2.3. Adding orbit locations Figure 15 shows the typical architecture of a 5-wire system which will receive satellite signals from only one orbit location. Quite commonly there is a requirement for systems to provide signals from more than one orbit location. A second orbit location can be added simply by installing another satellite dish aerial and expanding the backbone to 9 wires. In principle this approach can be extended; the authors have heard reports of one 17-wire system in London. About 95% of IRS systems are 5-wire, 4% are 9-wire, and 1% are 13-wire. 13-way systems and above present two or more wires to the customer, as switches of these widths do not exist. In these cases, customers must make their own selection between the wires presented.

3.3.3. Typical IRS installation Figure 16 is a photograph of a typical well designed IRS headend, located on the roof of a tall residential building. The equipment is mounted inside a weatherproof steel cabinet, with components laid out accessibly and all cables with connectors to facilitate maintenance.


Figure 16: An example of a well designed IRS headend6

6 Photograph by courtesy of SCC International


4. Upgrading Systems for DTT

4.1. Introduction There are three paths to upgrading systems which we will consider here:

upgrading an existing analogue-only MATV system to a digital-capable MATV

system; upgrading an existing SMATV-TM system to a digital-capable system; upgrading an existing analogue-only system to full IRS.

4.2. Upgrading MATV systems to carry DTT In Section 3.1.3 we identified that MATV systems can be categorised into two types: wideband and frequency selective.

4.2.1. Wideband systems In principle wideband systems should be able to carry DTT signals without modification, because they handle the entire spectrum of Bands IV and V. Although this has been the case in many instances, a number of systems have failed to deliver sufficient digital signal level particularly in locations where the ratio of analogue to digital ERP is particularly high. For example, if the ERP ratio between analogue and digital transmissions is, say, 20 dB, and the signal level for analogue TV signals delivered by the MATV system is 60 dBV, then the digital signals will be delivered at 40 dBV. This is generally insufficient for reliable reception. The remedy will come from understanding exactly why a system is failing to deliver adequate signals, and this requires measurements of signal levels to be taken at key points in the system, such as the output of the launch amplifier, and at the worst outlets. If the ratio of analogue to digital signal is too high, then it may be necessary to convert the system to a channelised or frequency selective system, to amplify the digital signals relative to the analogue.

4.2.2. Frequency selective systems About 80-90% of MATV systems contain some frequency sensitive component such as a cluster equaliser or channel changer. As stated in Section 3.1.7.1 this can ensure that maximum performance is obtained from the launch amplifier, and also protects the system from unwanted interfering signals. This is generally regarded as good practice. However systems containing such components are less flexible as far as the introduction of new digital services is concerned. Equalisers fitted before the introduction of digital signals will almost certainly require at the very least re-tuning, and most likely replacement.

4.2.3. Distribution networks In a high proportion of MATV systems, the cable was installed between the late 1960s and early 1980s, but nonetheless is still usable in most cases. ONdigital/ITVdigital converted large numbers of systems, amounting to over 100,000


outlets, and never had to replace a distribution network. In a few cases, the distribution network loss at high frequencies was excessive, but this was circumvented by using channel changers to move signals on high frequency channels to lower frequencies. As a rule, if the distribution network is delivering reasonable quality analogue signals, it should be capable of delivering DTT signals. One Housing Association we interviewed had been told by an installer that cabling over 10 years old should be replaced as a matter of course, but this appears to have been motivated by the desire to extend the scope of the work. Re-use of old cable might also be considered counter to current recommendations for the use of benchmarked cable, but provided that adequate signal level is delivered, this practice does not seem to be causing undue problems.

Figure 17: Typical modification to an MATV distribution network

Another technique employed if adequate digital signal levels cannot be achieved is to reduce the effect of distribution network loss by reconfiguring the network as illustrated by the changes between Figure 13 and Figure 17. In Figure 13, the output of the amplifier is split four ways, reducing the maximum signal that can be driven into the four cable drops by 7-8 dB. In Figure 17, each amplifier can drive its full output into each cable drop. Therefore, if the amplifiers are all rated similarly, the network in Figure 17 will deliver 7-8 dB more signal level to the outlets. However, the advances in launch amplifier power output (see section 3.1.8) mean that this technique is now being used less frequently. It is easier to replace the original launch amplifier by a new higher powered model, although this will only gain 3-4 dB. Since in virtually all cases the distribution network does not require replacement when upgrading an analogue MATV system to carry DTT (see Section 4.2.3 above), this kind of upgrade can be carried out quickly. The work is centred on replacing the aerials and headend electronics. There is no need to visit all dwellings, as outlet plates are unchanged (visits are usually made to a few, however, to perform signal level checks). As a result, an experienced 2-man crew should be able to upgrade

Splitter

Taps Outlet Plates

4 Launch Amplifiers


between 1 and 3 analogue MATV systems per day, and the time taken is more or less independent of the number of outlets.

4.3. Upgrading SMATV-TM systems to carry DTT As SMATV-TM systems differ from MATV systems only in that some of the programme feeds come from satellite receivers equipped with UHF modulators, they are therefore subject to broadly the same constraints as MATV systems, when considering the introduction of DTT signals. In the same way as in MATV systems, where channel changers are likely to use empty channels that are later used for DTT transmissions, SMATV-TM systems may use channels for the satellite signals that are subsequently used for DTT signals. This tends to require fairly urgent re-tuning of the satellite signals, and of televisions using them, as the DTT signals that come upon the same channel as a translated satellite signal will cause the satellite signal to appear very noisy.

4.4. Upgrading to IRS Upgrading to IRS is the preferred option, in that it provides the full range of signals to users, and reduces the likelihood of tenants erecting their own external satellite aerials. Unlike upgrading an MATV system to carry DTT, installing an IRS generally requires replacement of all equipment: headend, distribution network and outlet plates alike. The only exception is when an MATV upgrade has previously taken place. Referring to Figure 15, it can be seen that Integrated Reception Systems handle UHF terrestrial TV signals in the headend in the same manner as for MATV systems, so it may be possible to re-use the MATV headend, integrating it into the IRS. Installing an IRS in a block of 30 flats would take a 2-man crew at least a week. Access to all flats is needed in order to fit new outlet plates and pull in new cable. Gaining access can be very difficult when the occupant is out during normal working hours, and this can significantly extend the duration of an installation.

4.5. Upgrade resources Practices vary somewhat in the industry, depending on the size of an installation company and the size of the job concerned. Small jobs could be dealing with a single system, typically from four homes upwards. A large job might involve 10,000 homes, perhaps synchronised to a building refurbishment programme, or to new construction. A range of skills are required, and for larger jobs may include:

Wiremen, for pulling in cables, fitting new outlet plates, fitting pre-constructed equipment boards and cabinets;

Aerial installers, who fit and align all terrestrial and satellite aerial hardware,

Fitters, who construct equipment boards and cabinets either in the company

workshop or on site;

Field supervisors, who monitor quality and health and safety issues;


Commissioning engineers, who survey sites, design systems, and troubleshoot and commission systems;

Facilitators, who arrange access to buildings and flats;

Contract managers, responsible to customers for the progress of the contract.

In order to keep costs down, the lowest appropriate skill levels are used, and as a result, IRS installations tend to be treated differently from MATV upgrades. The IRS installation crew may actually be less skilled than the MATV upgrade crew, because the IRS is commissioned by a commissioning engineer, whereas the MATV upgrade typically is commissioned by the more experienced member of the MATV upgrade crew. The MATV upgrade crew is therefore generally a fairly experienced installer together with a less experienced assistant.

4.6. Conclusions and Recommendations Roughly 1 million dwellings using communal aerial systems have so far been converted for digital reception, and about 90% of these are on Integrated Reception Systems. The current rate of conversions is said to be in the region of 250,000 dwellings per year, although there is some evidence that it may be rather less than this. However, even if we assume that a rate of 250,000 dwellings per year is sustained, it will be about nineteen years before conversion of all communal aerial systems is complete. If, in addition, switch-over goes ahead in say six years time, about three million homes will not be able to receive any digital services. Clearly, a substantial increase in the rate of installations is required if we are to convert all communal aerial systems before any of the dates currently under consideration for switch-over. At present demand is modest, but can be expected to increase dramatically once the government announces a firm plan for switchover. The industry seems to have capacity to cope with some degree of increase in demand, but it is by no means clear that it will necessarily be able to find the resources (suitably qualified and experienced manpower, investment in equipment and premises, etc.) to be able to cope with peak demand. The first limit likely to be encountered is the availability of qualified commissioning engineers. It is said that there are presently less than 100 active commissioning engineers in the industry. One estimate is that this number will be required to double. The level of experience and training required of a commissioning engineer is quite high; to train these people quickly would therefore require them to be drawn from among the most experienced installers in the industry, and it is not clear how this might be done, given the limited quantity and size of companies involved. If the industry is unable to increase the rate of installations sufficiently to avoid leaving some people without access to any digital services at switch-over, the total rate of conversions can be raised by increasing the proportion of MATV upgrades. We have shown above that conversion of existing analogue MATV systems to receive DTT takes roughly one tenth of the time of replacing them with Integrated Reception Systems. A limited number of additional installation staff would therefore be able to cover a much greater number of installations. This approach is not without training requirements too, but they are less demanding than for commissioning engineers.


Conversion of these upgraded systems to IRS should take place once the initial upgrade programme is complete. In conclusion, we have identified that there is a massive gap between the current rate of upgrades and installations, and the rate required to complete conversions before switch-over. We expect a dramatic rise in demand after the switch-over plan is made public. The installation industry is capable of some degree of increase in capacity, but it is not clear that this will be adequate. We therefore recommend that the industrys ability to cope with the expected increase in demand should be studied in detail. If no clear solution is found, an alternative plan should be developed for increasing the proportion of MATV upgrades, in order to increase the total rate of conversions. These upgrades may then be converted to IRS at a later date.


5. Issues towards switchover

5.1. Equipment advances One of the most notable advances in equipment for MATV systems is electronically tuneable cluster equalisers. These are able to offer electrical performance characteristics similar to conventional fixed equalisers, but are tuned by connecting a PC or dedicated controller to a data port (conventional equalisers require considerable skill and test equipment for alignment). Consequently, when any changes are made to either transmission frequency or power, only a brief visit is needed to realign the headend. Contacting the users to warn them of the need to re-scan their digital receivers may take rather more time, however. As an example of the capabilities of new equipment, one model of equaliser can provide equalisation in 10 clusters, with each cluster being adjustable in bandwidth from 1 7 UHF channels. In addition, 20 dB of AGC (automatic gain control) range ensures that levels at its output are held constant even if propagation changes cause variation in incoming signal levels.

5.2. Internet access Glasgow Housing Association has installed a pilot scheme in six multi-storey buildings containing a total of 600 flats that integrates Internet access into their Integrated Reception Systems. 10 Mbit/s Ethernet is injected onto the cables from the multiswitches to the outlets for each dwelling, and occupies otherwise unused bandwidth below Band 2 (i.e. up to about 50 MHz). Each tenant is provided with a small converter box that connects to a data port on the IRS outlet plate, and to the television as a display device. A cordless infra-red keyboard is also provided. Bandwidth is limited in the range 64 - 128 kilobits per second, and a local server provides access to certain Council services as well as the Internet. Council web pages have been designed for presentation on televisions, but in the brief demonstration we saw, translation of general Internet web pages was generally quite satisfactory. The Council sees a number of advantages flowing from this innovation, including:

The law7 requires landlords to hold regular consultation with leaseholders, particularly before entering into long term agreements or contracts over a certain value. Currently the Council spends over 0.5 million per year on postage for this purpose alone. E-mail is seen as a method of dramatically reducing these costs.

By organising home help more effectively the Council believes it can make savings in the region of 2 million per year.

The Council expects to be able to improve building maintenance by integrating various monitoring functions, thereby reducing reactive maintenance.

At the time of writing (November 2003) the pilot scheme has been running for approximately three months. Feedback from tenants has been very positive, and the

7 See Sections 20 and 20ZA of the Landlord and Tenant Act: 1985, as amended by the Commonhold and Leasehold Reform Act: 2002


Council sees this service as a major step towards social inclusion, and a reduction of the digital divide. The Office of the e-Envoy has published a policy document8 describing its aims and vision for giving the widest possible access to government services via digital television. It argues that while PCs are ideal for Internet access, the penetration of PCs across all households is unlikely to reach very high levels. In contrast, over 97% of households have at least one television, so, particularly after switchover, DTV can form an important channel for delivering access to government services with a very high level of penetration. The policy document describes how access to a portal carried within DTV signals gives an entry point and top level navigation for users. It would be impossible to carry the volume of data for all services on DTV, so very quickly the user is connected to the Internet via for example a PSTN line, establishing a point to point fully interactive service, rather than continuing to interact with one-way broadcast data. Integrating Internet access into IRS clearly gives an even more convenient always on connection, potentially without call charges.

5.3. Recommendations The Glasgow Housing Association pilot installation of Internet access over an Integrated Reception System appears to offer solutions to a number of problems, including social exclusion, enhanced Council services, and the objectives of the e-Envoy for access to government services via the television. It is not yet widely known about, so should be brought to the attention of relevant departments, and to the DigiTV Project9.

8 Digital Television: A policy framework for delivering e-government services to the home. 9 A national project aiming to demonstrate how digital interactive TV can be used as a channel to deliver government services.


6. Implications of switch-over

6.1. Assumptions about switch-over At the time of writing, the Action Plans Spectrum Planning Group is nearing completion of the development of a plan for digital switch-over. Their report is imminent but has not yet been published. Therefore, the following assumptions have been made, but may require revision in the light of the Spectrum Planning Group's final report. The conceptual model on which the Spectrum Planning Groups work is based divides the six UK multiplexes into two groups: three Public Service Broadcast (PSB) multiplexes and three commercial multiplexes. These two groups will be handled slightly differently. When the analogue transmissions are switched off, the PSB multiplexes will be moved to channels formerly occupied by analogue transmissions. The power levels of the digital transmissions will be in the range -7 dB to -10 dB relative to the power of the previous analogue transmissions. This is generally a higher power level than at present, but is in line with the constraints of the Chester 97 agreement, and therefore no international co-ordination processes will be required10. Modulation will be either 16QAM or 64QAM as at present. At present digital transmissions are available from 80 transmitter sites. After switch-over, the PSB multiplexes will be available from up to 1100 former analogue sites. As a result, coverage should be greatly increased because power levels from the existing sites will be raised, and transmissions of digital signals will be available from a greatly increased number of transmitter sites. The transition strategy is still being developed. Consultation is due in the Spring of 2004, and it is expected that a timetable for switchover may be published by the end of 2004. As a result, we have had to anticipate some elements of the strategy, and draw conclusions accordingly. We could expect that the switchover process will progress on a regional basis, as the resources required to convert the entire transmitter network at once will be very large. At present, the analogue network comprises main transmitters, fed by line, and relays, which re-broadcast signals received from the main stations. Neither analogue main stations nor analogue relays can handle digital signals without considerable re-engineering. It seems likely, then, that a main station will be converted by bringing a temporary digital transmitter into service, while modifications are carried out to the existing analogue transmitter. This gives some options for the re-broadcast relays linked to a main station that has just been converted: Ahead of the switch-over of the main station, temporary digital transmitters could

be established at each of the re-broadcast relays. The relays could then be converted at the same time as the main stations. In this way, digital

10 It should be noted than an ITU conference in 2004/5 is due to revise the Stockholm 61 and Chester 97 agreements, and this may impact frequency planning decisions made in the UK.


transmissions over an entire region could be established quickly, but considerable hardware and manpower resources would be required.

Ahead of the switch-over of the main station, alternative analogue feeds (such as satellite or terrestrial) could be established to continue feeding the re-broadcast relays. As a result, they could be converted to digital in a similar manner to the main stations, even one at a time if required. This would considerably reduce the number of temporary transmitters and people required.

A major goal of the switch-over process is to release a total of 14 of the 46 UHF channels for other purposes. These released channels will be in two groups: channels 31 - 40 (omitting channels 36 and 38) and channels 63 - 68. In virtually all cases three former analogue channels can be found in the retained spectrum, and so it should be possible to keep PSB transmissions within their existing aerial group. A similar process will be applied to the three commercial multiplexes, but the PSB multiplexes will have priority. The aim is to keep the commercial multiplexes in group, and to transmit them at -10 dB relative to the former analogue transmissions, but only initially at the best 200 sites. Commercial broadcasters will be able to request that the plan be extended beyond these 200 sites, according to their commercial needs. As yet, the uses to which the released channels will be put have not yet been agreed. It is understood that any new service must be of broadcast type, such as high-definition television, or services to hand-held devices using a new standard, DVB-H, currently in development.

6.2. Effects of frequency changes

6.2.1. MATV systems Since wideband MATV systems effectively reproduce the whole of Bands IV and V, changes of frequency should not have a significant effect on the operation of this type of system. However, the great majority of systems have some degree of frequency selectivity, arising from components such as channel changers or equalisers. As described in Section 3.1.7.2, channel changers are used to avoid problems of pre-echoes seen on analogue systems having leaky coax. They are designed to move an entire analogue signal to another channel, and should in theory be able to do this to a digital signal also. A potential difficulty could be that the phase noise of the conversion oscillator may be inadequate for OFDM signals, and could introduce excessive amounts of signal degradation. Depending on severity, the outcome could be degraded reception, or complete failure on the part of receivers to be able to decode the digital signal at all. Unlike analogue signals, digital signals are particularly insensitive to short delay echoes, and so the problem that channel changers were originally intended to solve is generally insignificant for digital reception. Therefore, if the channel changer is found to be seriously degrading signals then it should simply be removed. Section 3.1.7.1 of this report describes the use of equalisers to achieve equal launch levels for analogue signals. Their effect is to change the gain of a selected channel or clusters of channels, typically by no more than a few dB. The frequency response across a single equalised UHF channel is generally fairly flat, and should cause little degradation to a digital signal. Some manufacturers tune filters for digital signals a


little wider than for analogue. It is our impression, however, that analogue filter bandwidths are generally sufficient to avoid serious degradation to digital signals, but we have not been able to confirm this. Problems are most likely to arise when existing analogue or digital channels are in the spectrum that is to be released. New channels in the retained sections of the band will be found for these digital services, and it is likely that the new channels will not be in the passbands of existing equalisers. The equalisers will therefore require re-tuning or replacement.

6.2.2. IRS systems As has been stated previously, Integrated Reception Systems treat UHF TV signals in a manner very similar to MATV systems. We do not therefore anticipate any special class of problems arising in Integrated Reception Systems.

6.3. Effects of power level changes

6.3.1. MATV systems Achieving appropriate signal levels in MATV systems is generally the most demanding aspect of their design. It is therefore important to assess the impact of received signal level changes due to switch-over. In a MATV system there are two criteria that must be complied with simultaneously. The first is that the delivered signal level at all outlets should be adequate for satisfactory operation of a typical receiver, and the second is that signal levels at the output of the launch amplifier must not be excessive, in order to constrain the levels of intermodulation distortion.

6.3.1.1. Delivered signal levels Existing MATV systems have generally been designed to deliver a minimum of 60 dBV of signal for each analogue signal. Particularly in larger systems, this signal level is not generally exceeded by more than a few dB at the worst outlets due to the high launch power that would be required. If we presume that any existing MATV system delivers the minimum 60 dBV signal level to the worst outlet in the system, and we know that when the analogue signal is replaced by a digital signal, the power of the digital signal is, say, -10 dB relative to the analogue, then we can say that the delivered level of the digital signal at the worst outlet will be 50 dBV. Since the recommended minimum digital signal level is 45 dBV (see Table 1) we can be sure that any such system receiving digital signals on a channel formerly occupied by an analogue signal, and at a power level in the region of -10 dB relative to that analogue signal, will deliver appropriate levels of signal to users. Systems with AGC (automatic gain control) could encounter difficulties at switchover. AGC is used to ensure that if input signals vary, within certain constraints, then the output signal levels remain constant. This process requires detection of signal levels, and typical detectors react to peak power levels, not average power levels. The result may be that the AGC system holds the digital signal level constant, but at an inappropriate level. Fortunately, AGC systems are uncommon.

6.3.1.2. Intermodulation product levels In a properly constructed MATV system, levels of intermodulation products generated by the launch amplifier should be higher than those generated by any other


component. We should therefore need only to consider the impact of signal level changes at switch-over on launch amplifiers. In installing typical MATV systems, it is common practice to reduce the drive level of the launch amplifier until patterning on analogue pictures can no longer be seen. Providing at this point the system is delivering sufficient signal level to all outlets, then satisfactory operation is regarded as having been achieved. This technique works because the level of intermodulation products generated in an amplifier changes at a much greater rate than changes in the wanted signal level. In other words, a 1 dB reduction in the level of wanted signals will cause the levels of intermodulation products to fall by significantly more than 1 dB, thereby improving the ratio of wanted signal to intermodulation product. Fortunately, analogue signals are particularly sensitive to the presence of intermodulation products arising from combinations of other analogue signals. This means that for intermodulation products not to cause visible effects on analogue pictures, they must be at relatively low levels. At switch-over, all of the analogue signals will disappear and at least some will be replaced by digital signals at typically 7-10 dB lower than the previous analogue signals. The total signal power that must be generated by the launch amplifier will therefore be considerably less after switch-over than before, and as a result intermodulation product levels should be even lower. We can therefore conclude that the proposed changes in transmitted power level for digital signals at switch-over will not cause problems for launch amplifiers in MATV systems. This, however, a theoretical analysis, and tests should be made to confirm the outcome.

6.3.2. IRS systems Again, due to the similarity between IRS and MATV systems in the way that UHF TV signals are handled, we do not anticipate switch-over causing any significant problems that are unique to IRS systems.

6.4. Conclusions To summarise the conclusions from the above: Removal of analogue signals and their replacement by digital signals typically

7 dB to 10 dB lower in power should not cause problems to launch amplifiers. Moving digital signals into former analogue channels should generally not cause

problems for MATV systems. Possible exceptions are where frequency changers are used.

In cases where the destination channel for a moved digital multiplex is a new channel, MATV systems with frequency selective components such as equalisers are likely to need attention. Transmitting stations where this happens are expected to be identified when the Spectrum Planning Groups report is made public in 2004.

So far, about 1m dwellings have been upgraded, leaving about 4.85m yet to convert. Once the Spectrum Planning Groups plan is finalised, it should be possible to identify areas where new channels will come into use after switch-over, and thereby ensure that any conversions carried out before switch-over in these areas fit equalisers that are pre-tuned to accept the new channels. This should greatly reduce the number of systems that need to be visited after switch-over.


Channel changers have been identified as another source of potential problems for switch-over. Channel changers are generally fitted only in areas of particularly high field strength (i.e. close to transmitters), so this should help with identifying systems with channel changers fitted. By testing the ability of popular models to handle digital signals, it should be possible to determine with only a brief visit whether a system is likely to need attention after switch-over, owing to the poor performance of its channel changer.

6.5. Recommendations A range of launch amplifiers should be tested to verify their performance under

typical load conditions that will arise immediately after switch-over. The implications of the Spectrum Planning Groups final plan should be made

known to installers at the earliest possible time, so that equipment can be fitted in MATV and IRS upgrades that accommodate any new frequencies that may come into use.

Popular models of channel changer should be evaluated for their ability to handle digital signals, particularly in terms of their phase noise performance, to assist with identifying systems in which channel changers will cause problems after switch-over.


7. Introduction of new services after switchover One of the goals of the switch-over plan is to release fourteen channels for other broadcast type services. At present there is no certainty how these channels may be used, but it is important that recommendations for the adaptation of MATV systems for switch-over should take account of new services in these fourteen channels. The most likely possible uses for the 14 released channels could include:

more standard definition television multiplexes high definition television services to hand-held devices.

Use for both standard definition television and high-definition television would probably require networks rather similar to the existing ones. That is, transmissions would most likely originate from the same transmitter sites, and at similar sorts of power levels. Clearly and frequency selective components in the head-end would require attention to admit these new channels, but we should also consider the effect on the launch amplifier. It is at least theoretically possible that all fourteen channels could be filled with such multiplexes, increasing the number of multiplexes from six to twenty. This represents an increase in total power of 5.2 dB. Rough calculations indicate that under these circumstances, a launch amplifier designed originally to handle four or more analogue services, but now handling twenty digital multiplexes should have adequate intermodulation performance. However, it would be wise to carry out some tests for reassurance that this is the case in practice. However, the networks required for services to hand-held devices are likely to be quite different, more closely resembling cellular networks for mobile telephone services. The reason for this is that hand-held receiving devices will have relatively low aerial gain, and so high field strengths will be required. In addition, the nature of the services carried may contain elements of unicasting, and this would be handled most effectively by a network having a relatively small cell structure. If a network comprising a large number of small transmitters operates on frequencies close to the network of existing broadcast transmitter sites, then it is likely that a number of MATV systems will be located close to a transmitter for the new services whilst being at some considerable distance from the existing broadcast transmitter site. In a broadband MATV system this could result in very large levels of signal being received from the new transmitter and amplified by the launch amplifier. The resulting overload could cause high levels of intermodulation products to be generated, compromising or even preventing reception of the broadcast signals. Even in systems with some degree of frequency selectivity, unwanted signal levels could be sufficiently high that further filtering would be required in order to restore satisfactory service for the broadcast signals.

7.1. Recommendations In order to minimise difficulties introduced by new services using the fourteen

released channels, MATV and IRS systems should ideally avoid wideband UHF sections. Use should be made of filtering designed only to admit the required services while providing a high level of rejection of potentially interfering services on nearby frequencies. This approach is clearly good


practice in any case, but compared to wideband systems may incr

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