Group 42-Sells Out! - The Information Archive/Group 42 Sells Out (Group 42) (1996).iso/commun/scanner/sca/sca.txt

Text File | 1996-01-28 | 11KB | 191 lines

Additional Thoughts on SCA Reception:

As I recall, the setup described by Bob Parnass (I think that was who it
was) was to hook a VLF receiver to the output of an ordinary FM receiver
to pick up SCCA (sic - I always call it that, Sports Car Club of
America, when it should really be SCA - Subsidiary Communications
Authorization or something!) transmissions multiplexed on ordinary FM
broadcasts.

To understand what is going on, you need to know what the FM station
actually transmits.

Let's do it in "top down" fashion.  All you computer jocks out there
should relate nicely to that.

First of all, the FM station has a "composite audio" input - this is
just the input on which SOMETHING (consider it a stub subroutine to
written later) is fed in to the modulator.

From the FM modulator's point of view, what you put on this input is
just the MODULATING SIGNAL which you want to Frequency Modulate (FM) the
station's carrier.  Intuitively, you can think of the station as putting
out a "pure" carrier at frequency F when this modulating signal is zero.
When it is NONZERO, however, the instantaneous frequency of the
transmitter is changed.  Say the modulating signal value, in volts, or
whatever, is M.  Then the transmitter output frequency is set to F + K*M
where K is a sensitivity constant which is unimportant except that
whatever maximum value of M is applied to the FM modulator input should
result in a maximum "frequency deviation" K*M which is just about the
maximum that the FCC allows.

If this station were a plain old-fashioned monaural FM station, all it
would have to do is feed in the audio signal -- voice, music or funny
sound effects -- which you want the listeners out there in Radio-Land
to hear.

The signal would modulate the carrier, be amplified, fed to the antenna
and radiated.  It would come swooping down into your ordinary, Monaural
FM receiver.  The receiver would say, hmm...  The frequency is F + K*M
I therefore have to subtract F and divide by K to give my loyal owner
the instantaneous value of the signal waveform, which is M.  Voila!  Out
of the FM demodulator comes M in livid high fidelity.

(I don't think the following information is needed for present purposes,
but I include it so somebody won't say I am oversimplifying things!!
Now, even in simple monaural FM there is one trick that we haven't
mentioned.  This is called pre-emphasis and de-emphasis.  Very early in
the game, it was noticed that in FM systems if you fed in no modulation
at all and listened to the receiver's reconstructed value of M (after it
did the arithmetic noted above), there was a NOISE output from the
receiver, even at fairly strong signal levels.  The noise was
particularly noticeable because its amplitude increased with frequency.
Thus a quite noticeable high-frequency hiss was present on even fairly
good signals.  Somebody then had the bright idea that they should
effectively "turn down the treble control" at the receiver.  A fixed
frequency compensating network called a "de-emphasis" network was
designed and standardized to do that.  But then the music had its highs
"de-emphasized", so an "inverse" network called a pre-emphasis network
was added at the transmitter.

Next, along came stereo.  In order to avoid the wrath of all the owners
of monaural FM sets, the FCC in its wisdom decreed that a "compatible"
system would be necessary before they would approve FM stereo.  The
engineers quickly noted that the A+B signal from two microphones gives a
passable monaural signal (especially if "one-point" miking is used).
Now the problem was how to get A and B out of A+B.  Well, as your high
school algebra teacher probably taught you, (A+B)+(A-B)=A and
(A+B)-(A-B)=B.  So all they had to do is send (A-B) in some clever way
and the receiver could reconstruct A and B by "matrixing".  The method
adopted was to "multiplex" this (A-B) signal onto the main carrier by
using it to modulate a SUBcarrier located at 38 KHz. Double-Sideband
Suppressed Carrier (DSBSC) modulation was chosen.  This gave a "lower
sideband" extending downward from 38KHz (less 20Hz or so) to 23KHz
(because the highs were cut off at 15 KHz.)  A "pilot carrier" was put
at 19 KHz which allows the receiver to recover the precise frequency
phase of the 38 KHz carrier so that recovery of the (A-B) signal could
proceed.  In fact it turns out that this carrier can be used in an even
more clever way to recover A and B signals directly from the "composite"
signal ( A+B plus pilot carrier plus A-B ).

Note that all the new junk -- pilot at 19KHz and A-B from 23 to 38 K
-- are so high in frequency that most people wouldn't hear them, and
most older monaural FM sets and loudspeakers won't reproduce them
audibly anyway.

Well, as if this wasn't bad enough, then along came the SCCA (I warned
you about my warped sense of humor) and asked for authorization to put
MUZAK on the air.  The obvious thing to do was to put on yet another
subcarrier, this time at a frequency far enough above the audio so that
it wouldn't interfere with stereo broadcasting.  67 KHz was chosen as
the magic frequency.  But this time, FM modulation was chosen.
Remember, all of this stuff is being stacked up in frequency above the
normal monaural FM broadcast audio in such a way that a normal FM
receiver won't be affected (much) by it.  So before we even go into
"composite audio" input of our simple FM transmitter, we have a VERY
"composite" signal indeed.  One might diagram it like this:

                    weak pilot   suppressed    just a
                     carrier      carrier     boundary              FM
                       /            /             /             sideband
                      /            /             /          sub    /
                     /            /             /           car.  /
|             |             |             |            |   /
| Normal FM   | (A-B) Lower | (A-B) Upper |        /||||||||\
| audio (A+B) |  Sideband   |  Sideband   |     /||||S|C|A|||||\
|             |             |             |  /|||||S|i|g|n|a|l|||||\
DC           19KHz        38KHz         53KHz        67KHz
--monaural
    signal----
--------stereo composite signal-----------
-------------full composite (stereo + SCA) signal-------------------



Now to the main point.  The FM transmitter takes this composite audio
signal as input and FM MODULATES its carrier with it.  When this is
received by an FM receiver, what comes out of the discriminator is the
composite signal.  In a normal monaural FM receiver, this is fed to the
volume control and thence to the audio amplifier stages, where
"de-emphasis" is applied as noted above, and finally out to the speakers
or headphones.

The de-emphasis circuit tends to reduce the amplitude of the SCA signal
since it is at a high frequency (53 to 81 KHz, with carrier at 67 KHz).
However, enough gets through that it can still probably be used even at
the speaker jack, since it is an FM signal and the absolute amplitude
isn't too important.  The best way to recover the SCA audio, however,
would be to go in to the FM receiver and tap off at the "top" of the
volume control -- the hot side where the discriminator signal is brought
in.  Bring this signal out to a jack which you can install on the set
for all such nefarious purposes.  In fact, make sure that the
de-emphasis network is not installed ahead of this point, although in my
experience it usually is not.

Anyway, once you have brought out the full composite signal, the SCA
signal is STILL AN FM SIGNAL that needs an FM discriminator to turn
back into normal audio.  It happens to have a carrier frequency of 67
KHz because that is what was used as the carrier at the studio when the
composite audio signal was made up.

So if you have a VLF receiver capable of covering 67 KHz, you can feed
the composite audio signal into its input, which happens to be the
antenna connection.  You can then tune it to 67 KHz and switch it to FM
mode and you should get perfect SCA audio out.

If it doesn't have FM demodulation available at this frequency, then
can still use a trick called "slope detection".  Tune it above 67 KHz so
that the 67 KHz carrier falls on the lower slope of the IF selectivity
curve.  I.e. the signal has fallen off about halfway from its peak value
on the meter.  Select AM demodulation, and then tune for the best sound.
You should get acceptable recovery of the audio.  If there is a
bandwidth switch, experiment with it.  Probably the widest bandwidth
available will work best.

If you don't have a suitable VLF receiver covering 67 KHz, the simple
trick with the Phase-Locked Loop device will work just fine.  After
the monaural FM receiver has done most of the work of picking up the
weak signal at VHF (88 to 108 MHz) and amplifying it and FM demodulating
it so you have the baseband composite signal available to fool around
with.  What is going on here is that the PLL locks to the 67 KHz signal
but since that is being FM modulated, it has to work a bit to stay
locked.  The Voltage-Controlled Oscillator has to be pushed above or
below 67 KHz, its natural frequency, by applying a control voltage to
the VCO input.  All of this is done "automatically" by the design of
phase-locking circuitry, but YOU can benefit by all this work it is
doing: the control voltage is precisely a measurement of the
instantaneous modulation value, M, that YOU want to hear!  So you pick
it off, amplify it, and listen to MUZAK (yuk) to your heart's content.

From the above rather long-winded description, you can see that the
is a lot of other junk floating around on the composite signal, namely
all the stereo stuff below 53 KHz. To make the SCA demodulator work
and be free of interference from the main channel, it would be well
put a little selectivity ahead of the SCA demodulation if you are using
the PLL trick.  I haven't experimented with this, but I would think a
simple high-pass filter to attenuate everything below 53 KHz would do
fairly well.  Of course with the VLF receiver you have all kinds of
selectivity, so there should be no problem.

I hope this helps you understand what is going on with SCA.  If you
followed all the details, it should be clear that there is no black
magic going on, but that once the composite signal has been received, a
VLF FM receiver (a rather rare beastie) is logically what is needed
recover the SCA signal.  That's why it happens to be possible to string
together such a seemingly unlikely pair of receivers and get SCA
broadcasts.