Part 7a - AM & FM Broadcasting - Handout

8/6/2019 Part 7a - AM & FM Broadcasting - Handout

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Part 7a

Broadcast Engineering:

AM and FM Broadcasting

ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 2

Broadcasting

Broadcasting is the distribution of audio and/or videosignals (programs) to a number of recipients("listeners" or "viewers") that belong to a large group.

Broadcasting to a very narrow range of audience iscalled narrowcasting.

The term "broadcast" was coined by early radioengineers from the midwestern United States."Broadcasting", in farming, is one method of spreadingseed using a wide toss of the hand, in a broad cast.

Television and radio programs are distributed throughradio broadcasting or cable, often both simultaneously.

Broadcasting forms a very large segment of massmedia.


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Amplitude Modulation (AM)

Amplitude Modulation

The process of varying the amplitude of a

high-frequency carrier wave in accordance

with the amplitude of the modulating signal


Uses of AM

1. AM broadcast (535 1605 kHz)

2. Citizens Band Radio (27 MHz)

3. Aircraft communications (108 136 MHz)

4. International shortwave (3 30 MHz)

broadcast via sky wave

5. TV picture (using vestigial sideband)


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Limiting condition for AM:

The amplitude of the modulating signal must

not exceed that of the carrier, else

overmodulation(clipping) occurs.


AM Broadcasting Standards (FCC)

Frequency allocation: 535 to 1605 kHz (525 to 1705 kHz)

divided into 106 (130) channels

Channel spacing: 10 kHz (9 kHz)

Permitted channel bandwidth: 30 kHz (maximummodulating signal frequency: 15 kHz)

*note: geographically co-located stations must bespaced at least 3 channels apart for sidebandinterference protection)

Carrier tolerance: 20 Hz

Unmodulated carrier power: 100 W to 50 kW

Type of emission: A3E (double sideband, full carrier)

Intermediate Frequency: 455 kHz


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AM Station PerformanceRequirements (FCC)

Modulation percentage: 85% to 95%

Audio frequency distortion harmonics (AFDH):


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Times of Day in AM Broadcasting

Daytime From local sunrise to local sunset

6 AM to 6 PM (2200 UTC to 1000 UTC)

Nighttime Between local sunset to local sunrise

6 PM to 6 AM (1000 UTC to 2200 UTC)

Experimental period Midnight to local sunrise

12 MN to 6 AM (1600 UTC to 2200 UTC)

Used for experimental purposes in testing and maintainingapparatuses by the licensee, provided that no interference is

caused to other stations maintaining a regular operatingschedule within such a period


3 Important Requirements inBroadcasting

Timing

Programming schedule must be followed (programsshould start and end within the allotted time)

Fidelity

Program material shall not have any reasonabledistortion

Modulation depth The audio signal must modulate the transmitter

properly


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Power Allocations

The transmitter of existing Metro Manila stations maybe located outside of Metro Manila, provided that MetroManila remains within the 80 dBu contour of thetransmitter

The station shall not operate more than 5% and lower

than 10% of its authorized operating power

10 kWAll other areas

50 kWMetro Manila

Maximum Permissible PowerAREA


Classification of Powers

1. Licensed power or authorized operating power That which is actually supplied or fed to the antenna by the

transmitter. Should have a tolerance of -5% or +10%

2. Maximum rated carrier power The maximum power that the transmitter is capable of

supplying to the antenna and still operate satisfactorily

3. Plate input power Product of the voltage and current at the output of the last

radio stage, measured without modulation4. Antenna input power

Product of the square of the antenna current and theantenna resistance at the point where the current ismeasured.


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Two methods of measuring antenna input power

1. Direct method

2. Indirect method

used to determine the output power of FM broadcast stationsand TV aural transmitters

used for AM broadcast stations in emergencies

where: Ia = antenna current with no modulationR

a = impedance or resistance of theantenna where the current is measured

where: Vp = plate voltage of the final amplifier

Ip = plate current of the final amplifierF = power factor correction

aao RIP2

=

FIVP ppo =


AM Antennas and SiteConsiderations

Standard AM broadcast stations use either a singleomnidirecitonal vertical antennas, or multi-element,phased vertical directional arrays

Generally, antennas are erected on flat lands,preferably those having good ground conductivities(e.g. marsh lands)

Earth mat a network of buried wires directly underthe antenna, extending outward from the base, buried

about 6 to 12 inches (15 to 30 cm) below the ground Counterpoise a smaller version of the earth mat

above ground


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Antenna Towers:Obstruction Painting and Lighting

Must be painted with equal-width stripes of aviation (emergency)orange and white, each stripe approximately one-seventh of theheight of the tower, but not over 100 ft (30 m) in width on talltowers. The top and bottom stripes must be orange

To mark the tower at night (sunset to sunrise), towers up to 150 ft(46 m) must have two (2) steady-burning 116-W or 125-W lampsin an aviation red light globe at the top of the tower (beacon)

For towers over 150 ft, the top beacon light consists of 620-W or700-W PS-40 Flashing Code Beacon lamps with aviation redfilters

At half-, third-, quarter-, etc. tower height points (depending onthe height of the tower), flashing 620-W to 700-W beacons areinstalled

Lights should be automatically controlled by a device sensitive to

the night sky. Lights should be inspected at least once a day, or by automatic

indicators


Logs

Log

A listing of the date and time of events,

programs, equipment parameters, tests,

malfunctions, corrections, and other such

information


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Types of Logs inBroadcast Operations

Program Log Contains entries with regard to the nature of the program, its

name and title, start and end times, source, sponsors ofannouncements, duration of advertisements, etc.

Operating Log Contains the technical details of the transmitter during

operation, such as operating parameters (Vp, Ip, Ia, etc.), thetime the transmitter is put on and off the air, the time antennalights are turned on or off, etc.

Maintenance Log Contains the results of transmitter and other equipment tests,

repairs, calibration, checks, etc.


Frequency Modulation (FM)

Frequency Modulation

The process of varying the frequency of a

high-frequency carrier wave in accordance

with the amplitude variations of the

modulating signal


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Uses of FM

1. FM broadcast (88 108 MHz)

2. Television sound

3. Satellite Television (both audio and video)

4. Mobile radio services


FM Broadcasting Standards (FCC)

Frequency allocation: 88 to 108 MHz divided into 100

channels

Channel spacing: 200 kHz

Permitted channel bandwidth: 200 kHz (75 kHzdeviation for maximum modulating frequency of 15 kHz,and a 25 kHz guard band on both sides)

Type of emission: F3E (monoaural)

F8E (stereophonic)

Intermediate Frequency: 10.7 MHz (10.61 to 10.79 MHz)

Pilot Subcarrier Frequency: 19 kHz


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Classes of FM Stations

Class-A Shall have an authorized transmitter power not exceeding 25

kW and an ERP not exceeding 125 kW. Minimum transmitterpower is 10 kW

Class-B Shall have an authorized transmitter power not exceeding 10

kW and an ERP not exceeding 30 kW. Minimum transmitterpower is 1 kW

Class-C A non-commercial, community station having an ERP not

exceeding 1 kW

Class-D Shall have an authorized transmitter power not exceeding 10

W. Used for educational purposes.


FM Antennas and SiteConsiderations

The antenna used in FM broadcasting is a half-wavedipole.

The transmitting antenna location should be chosen sothat line-of-sight can be obtained from the antennaover the general service area.

To provide LOS within the principal area, the antennamust be conveniently above the average terrain.

A tower, elevated structures (buildings) can be used toelevate the antenna

Hills and mountains can also act as natural towers.


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Pre-emphasis and De-emphasis

Since noise sideband power in FM decreasesinversely with the modulating frequency,higher modulating frequencies are moresusceptible to noise than the lower ones.

A method of artificially boosting the affectedfrequencies with respect to a pre-arrangedcurve before transmission to improve noiseimmunity is termed as pre-emphasis.

The compensation at receiver side is called

de-emphasis


The amount of pre/de-emphasis for FM

broadcasting has been standardized as 75s

In the UK, the amount of emphasis is

standardized to 50s.


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Stereophonic FM

Stereo (also stereoscope)

Originally referred to a special photographictechnique used to give the viewer the impression ofobserving a scene in three dimensions

In stereophonic audio, a sound source is recordedfrom two different angles (in this case, the left andright sides). During playback, these signalssimulate the sound to the left and right ears, givingthe illusion of a three-dimensional sound source.


Problems encountered with initial design ofstereophonic FM broadcasts:

1. Original FM broadcasts were monophonic.

The system should be compatible withexisting monophonic receivers

2. The stereo information had to be

transmitted within the 200 kHz bandwidth

allotment


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Block Diagram of a StereophonicTransmission System

MATRIX

FREQ x 2 BMSCA

GENERATOR19 kHz

SUBCARRIER

ADDER

SUM (L+R)

DIFFERENCE (L-R)

LEFT IN

RIGHT IN

50 15 kHz

23 53 kHz

19 kHz

59.5 74.5 kHz

OUT

AUDIO IN


Spectrum of a Stereo FMMultiplex System

0 15 5323 59.5 74.567 kHz19

L+R L-R

StereoSubcarrier

38

SCA


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Some notes on FM Stereo

Why are the L & R signals not sent independently andsimultaneously? So that the system will be compatible with existing monoaural

receivers

Whats the use of the 38 kHz subcarrier? To serve as the carrier for the balance modulation process of the

difference signal

Why 38 kHz? The difference signal must not interfere with the original sum signals

Why suppress the 38 kHz subcarrier? Too much of the total composite modulating signal would be taken

up by the subcarrier voltage

Whats with the 19 kHz pilot subcarrier? It is used as the reference to obtain the difference signal.

Why 19 kHz? Most people cannot hear beyond 15 kHz, much more 19 kHz.


Subsidiary CommunicationsAuthorization (SCA)

The transmission of programs which are of abroadcast nature, but which are of interest primarily tolimited segments of the public wishing to subscribethereto.

Typical applications of SCA Background music, weather, time signals Educational information Talk-back for remote stations Telemetry Facsimile Slow-scan TV

Uses a subcarrier of 67 kHz and is modulated to adepth of 7.5 kHz


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The Broadcast Studio

The studio usually contains equipmentfor program origination.

It is a usual practice to co-locate thestudio and transmitter in a single facility,mostly for economic purposes.


Remote Studio Facilities

In cases where the studio and thetransmitter cannot be located in thesame facility, or it is better for thetransmitter to be located elsewhere(economically or technically), then a

studio-to-transmitter link (STL) may beemployed.

Documents

Part 7a - AM & FM Broadcasting - Handout