Ramsey Electronics SCA1, Руководство пользователя

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SCA1
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5
FM BROADCASTING AND SUBCARRIERS - SOME BACKGROUND
Before we explain how the SCA decoder works, we first need to know what it is
"looking" for in the received FM signal. (A note to newcomers: don't let this
description scare you. Rest assured that your SCA decoder will perform the
desired task even if you don't fully grasp all the theory that is thrown at you
in the following passages).
Let's discuss what's happening in the Frequency Modulated signal coming from
a commercial transmitter. Look at the diagram above (referred to in
engineering circles as a spectral diagram) to help you understand what we're
about to describe. The signal contains a Left+Right FM modulated carrier -
which is the standard monaural audio signal - and a Left-Right signal which is
used to provide the stereo signal. Follow closely to see how we get separate
Left and Right stereo audio channels, and its not done with mirrors - only
simple algebra!
First, let's see what we have to deal with:
1. A Left+Right signal (called the mono audio channel)
2. A Left-Right signal (called the difference audio channel).
Now let's apply a little algebraic addition:
(L+R) + (L-R) = L+L+R-R = 2L (the R terms cancel)
The Left audio channel.
Now, how about a little algebraic subtraction:
(L+R) - (L-R) = L-L+R-(-R) = 2R (the L terms cancel)
The Right audio channel.
Now, the number "2" in 2R and 2L simply means that the signal is twice the
amplitude (twice as loud) and should not be confused with interpreting it as two
left or right channels.