Part 7a - AM & FM Broadcasting - Handout
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Transcript of Part 7a - AM & FM Broadcasting - Handout
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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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ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 3
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
ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 4
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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ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 5
Limiting condition for AM:
The amplitude of the modulating signal must
not exceed that of the carrier, else
overmodulation(clipping) occurs.
ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 6
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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ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 7
AM Station PerformanceRequirements (FCC)
Modulation percentage: 85% to 95%
Audio frequency distortion harmonics (AFDH):
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ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 9
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
ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 10
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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ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 11
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
ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 12
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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ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 13
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 =
ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 14
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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ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 15
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
ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 16
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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ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 17
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.
ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 18
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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ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 19
Uses of FM
1. FM broadcast (88 108 MHz)
2. Television sound
3. Satellite Television (both audio and video)
4. Mobile radio services
ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 20
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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ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 21
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.
ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 22
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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ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 23
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
ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 24
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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ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 25
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.
ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 26
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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ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 27
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
ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 28
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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ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 29
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.
ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 30
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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ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 31
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.
ECE 211 - Broadcast Engineering &Acoustics (AM & FM Broadcasting) 32
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.