January 2000 Wavetek Wandel Goltermann, Inc. 1

Digital TV Re-Multiplexingat the CaTV Head-End

Digital TV benefits and constraints

Multiple TV programs over a single transmission channel:The main benefit of Digital TV is the combination of video compressionand digital transmission which offers the opportunity to transmit 6 to12 TV programs over a frequency previously used for a single analoguechannel.Not only does Digital TV allow for more TV programs also known asvirtual channels - but it can offer advanced services such as data, multi-language movie broadcast, multi-video sport programs and an unlimitedoffer of CD-quality radio programs.

Combining heterogeneous inputs into a consistent offer:

As described in the above figure, the offer to the subscriber is no longera set of basic modulated TV channels. It includes digital services fromvarious sources:

l Redistribution of satellite and terrestrial feeds, already scrambledand multiplexed,

l Local programming to be encoded and multiplexed,l Near Video on Demand services hosted on video servers,l Interactive applications and Electronic Program Guide.

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The redistribution of multiple feeds from different sources generates aneed for repackaging the corresponding programs - includingConditional Access and a common Electronic Program Guide andfinally re-multiplexing the various components.

Two-step DVB or ATSC Digital TV process

How does it work in DVB (Digital Video Broadcast) or ATSC systems?The figure below summarizes the operation for video (and audio)transport mechanism as defined in MPEG-2/DVB standards, usingMPEG Packetized Elementary Streams structures for the transmissionprocess.

Fragmentation into MPEG transport packets:The 1st step is a data fragmentation mechanism. Multi kilobyte pieces ofdata - such as a video frame - are fragmented in fixed-size (188 bytes)MPEG transport packets with minimal overhead. A series of transportpackets for a given data stream such as the video for one movie - isidentified by a user-defined PID (Packet ID.) value, such as PID 0x0A00as seen in the example.Notice that each packet will automatically benefit from a standard ReedSolomon forward error correction mechanism later during the digitalmodulation to recover from most transmission errors.

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Multiplexing the various data-streams:The 2nd step in the process is the multiplexing of the transport packetsfrom multiple video and audio streams - possibly along with datastreams for interactive applications - in a single MPEG TS (TransportStream). A TS is a flagged bit-stream with a fixed global bit-ratecorresponding to the performance of the digital modulation. As the rateof individual components (audio, video or data) is dynamically allocatedby the multiplexer, stuffing with Null packets is provided to reach thefixed total bit-rate.

Critical Service-Information

Self-contained protocol:The receiving equipment needs to filter the expected video and audio fora TV program from the entire multiplexed TS without getting externalinformation or pre-negotiation Therefore, appropriate descriptiveService-Information is embedded in the multiplex along with the TVpayload.Such Service-Information has a specific structure similar to filemanagement tree structure on a PC disk and uses specific PID valuesfor transmission and identification by any receiving equipment. A roottable lists the existing programs and services, and each service and itscontent is further described through additional sub-tables. These typesof tables are named MPEG-PSI, DVB-SI and ATSC-PSIP depending onthe standards, they are described in the table hereafter.

Award-winning test solution:Only an MPEG-2 Transport Stream Analyzer such as the WG DTS-Aoffers access to these critical Service-Information status and decodes aswell as PID-based bit-rate statistics and confirmation of encryptionmechanisms.DTS-A by Digital Transport Systems, a Wavetek Wandel Goltermann,Inc. company, received a "Best in Test" award from Test &Measurement Worldwide magazine for recognition as a unique MPEG-2/DVB and MPEG-2/ATSC analyzer.

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PSI-SI-PSIP tables and descriptions:

Purpose Table Link toMPEG-PSI tables

Lists the programs carried in thecurrent TS

PAT programassociation table

PMT and CAT tables

Lists the audio and videocomponents for a program in thecurrent TS

PMT program map table Elementary streams

Lists the data-streams with theaccess rights and keys for aprogram in the current TS

CAT conditional accesstable

EMM and ECMencryption data

DVB-SI tablesDescribes all the TSs composingthe entire service

BAT bouquetassociation tableNIT networkinformation tableIncluding a descriptor forthe modulation schemeand parameters.

SDT

Describes the programs containedin each TS

SDT servicedescription table

EIT

Describes the scheduled eventsfor each program

EIT event informationtable

Present and followingevents

Describes the changes in thescheduled events

RST running statustable

Date and time TDT time and datetableTOT time offset table

ATSC-PSIP tablesInformation for set-top-boxoperation

MGT master guidetable

EIT and ETT (PID)

Information for navigation andtuning

VCT - virtual channeltableIncluding a field for themodulation mode andparameters.

EIT (program sourceidentifier)

Information for the electronicprogram guide

EIT - event informationtable

Events per range of 3hours

Extended message information todescribe programs and events

ETT - extended text table

Parental guidance reference RRT - rating region tableDate and time STT - system time table

Main Service-Information tables

For more information, please send your query to mpeg@wwgsolutions.com.