SCA1
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6
Well, you may say, that works out nicely, but why not just transmit a separate
Left and separate Right channel, and avoid the math? There are a couple of
reasons why: One, to receive on a Mono receiver, the receiver would have to
detect and demodulate both Left and Right signals and combine them for the
L+R mono channel. The extra cost of adding another demodulator would
unnecessarily raise the cost of simple mono receivers. And, two: signal
bandwidth is limited; there is not enough "room" for an independent (Left -
Right) carrier. In fact, to conserve bandwidth even more, the stereo subcarrier
is transmitted in Double-Side-Band-Suppressed-Carrier (DSBSC) mode. This
signal is located 38 KHz away from the center frequency that the receiver is
tuned to. To recover this audio, a 19 KHz subcarrier (called the "pilot") is
transmitted. In the receiver, this pilot signal is doubled to 38 KHz and is used to
properly demodulate the Left - Right signal.
Have you ever noticed that no matter how expensive the FM receiver is, the top
end of the frequency response is always less than 19 KHz? If it wasn't, while
sitting back enjoying your Hi-Fi, you'd be able to smell your tweeters burning up
as they tried to reproduce the constant 19KHz pilot signal! In addition to all this
activity, there may be another subcarrier, usually 67 KHz away from the center
frequency, containing completely independent audio! Normally your receiver
would effectively filter out this signal, but this is the subcarrier that we are
interested in, and is what our kit is designed to tune in and demodulate.
Why all this talk about stereo and its subcarriers? To give you a little familiarity
with subcarriers and to let you know that there's more out there than expected!
SCA1 CIRCUIT DESCRIPTION
The input is bandpass filtered using L1, L2, and associated components to
isolate the higher frequency subcarriers. The filtered signal is then amplified by
Q1 and applied to the PLL (Phase-Locked-Loop) detector IC chip. The LM565
Phase Lock Loop IC "tracks" the difference between the input signal and its
own internal reference oscillator. This tracking voltage is the recovered audio,
and is low passed by R8-10 and C12-14. A low level audio output is supplied
for an external audio amplifier, or a speaker may be used directly via the
amplified audio output at J3.