The recent de-regulation of broadcast SCA usage has spurred a new interest
in using the SCA for data transmission and paging. SCA stands for "Subsidiary
Communications Authorization", which allows FM broadcast stations to "piggy-
back" up to two additional audio or data channels on the main carrier of the
HOW SCA's ARE ADDED TO THE BROADCAST SIGNAL
SCA's are added to the broadcast FM carrier by a technique called frequency
domain multiplexing which allows the additional subchannels to be separated
from each other and from the main channel by use of specific frequency bands
for each subchannel. In the case of an FM station broadcasting in stereo with
two SCA's, the frequencies used are 67KHz and 92KHz for the two subcarriers.
The main channel and stereophonic information are transmitted in the frequency
band extending from 30Hz to 53KHz while the SCA information is transmitted
above this frequency range in a band extending from 54KHz to 99KHz. The SCA
subcarriers each modulate the main FM carrier by a maximum of 10% of the total
modulation. This means that the effective coverage will not be as great as
the main channel since the SCA does not have full use of the transmitted power.
The sum of all the different components being transmitted is called the
The method of modulation used is frequency modulation of the SCA subcarrier
itself which in turn frequency modulates the main transmitter carrier. This
type of "FM on FM" system is difficult for many people to fully understand.
Think of it this way, the main carrier is deviated a constant plus and minus
7.5KHz (10% of the total) by the 67kHz subcarrier while the 67KHz subcarrier is
itself deviated up to plus and minus 6KHz by the audio or data being fed into
the SCA modulator.
RECEIVING THE SCA
The SCA receiver must first FM demodulate the entire baseband, filter out
the desired subcarrier from the rest of the baseband components, and then FM
demodulate the SCA information. This requires two IF strips and two FM
demodulators. The first IF strip is usually operated at 10.7 MHz with a
bandwidth of about 250 KHz while the second IF strip is located at the SCA
frequency (67 KHz or 92 KHz) with a narrow bandwidth (typically 25KHz or less).
The first FM demodulator is usually a quadrature or pulse counting type while
the second FM demodulator is usually a Phase-Locked-Loop to minimize
interference from the main and stereo channels.
This dual demodulation process places some limitations on the performance
of the SCA receiver. The amplitude and phase response of the IF filters will
have a significant effect on the crosstalk into the SCA from the main and
stereo channels. This type of receiving system is also more susceptible to
multipath than a simple FM system.
The SCA subcarrier being received only has access to a maximum of 10% of
the transmitter power. The additional noise factors introduced by the "FM on
FM" nature of the system cause further degradation of the signal to noise ratio
which may yield a total penalty in coverage of more than 20db when compared to
BRINGING IT ALL TOGETHER
SCA's do offer an important new opportunity to transmit information over
existing FM broadcast facilities. Whether SCA's are used primarily for paging,
data transmission to fixed points, or other uses remains to be seen. In any
case, it is important that each subsystem be individually optimized before the
complete transmission and reception system can perform properly.