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When the FCC established the FM broadcast
system, they assigned each station a channel with a
width of 200 kHz. For example, the station assigned
to 88.1 MHz which is really 88,100 kHz is authorized
to use frequencies from 88,000 to 88,200 kHz.
When a tone of a certain frequency is transmitted over
FM, the bandwidth of the transmitted signal is a minimum
of twice that frequency. Thus, frequencies up to 100
kHz can be transmitted on an FM broadcast station. When
you consider that the average person can only hear frequencies
up to 15 kHz, this leaves a lot of space on the FM transmitter
for other signals.
The frequencies from 19 to 53 kHz are used for transmission
of stereo. Thus, the frequencies from 53 to 100 kHz
are not usable for the transmission of programming.
SCA takes advantage of this extra space by converting
the special programs into frequency bands that center
on 67 and 92 kHz through frequency modulation of those
frequencies.
Because they are well beyond human hearing range, they
have no effect upon reception of normal programming.
At the receiver, the subcarriers are converted back
down to their original frequencies.
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The
diagram above shows the assignment of frequencies
used in an FM transmitter. The main channel is the program
heard with a monaural FM radio.
These frequencies extend from below 25 Hz to 15 kHz. At 19
kHz, is the stereo pilot. The pilot is used to extract the
stereo programming, which exists from 23 to 53 kHz. At 57
kHz is the RDS subcarrier used for data to carry such information
as the station's callsign, and the name of the selection currently
playing. From 60 to 74 kHz is the 67 kHz subcarrier and from
85 to 99 kHz is the 92 kHz subcarrier. Notice that the amplitude
of the subcarriers is very low compared to the main and stereo.
This prevents interference to stations on adjacent channels.
The technical nomenclature for SCA transmissions is: FM subcarrier
on FM carrier, frequency division multiplexing. SCA is a legal
term which is short for Subsidiary Communications Authorization. |
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INFORMATION
RADIO
RECEIVER |
•AM/FM
radio
•67 KHz/92
SCA Receiver
•Flex
Antena For Reception |
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