312 lines
15 KiB
Plaintext
312 lines
15 KiB
Plaintext
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The Boston SCA Report - Weld Pond
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Contents
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SCA Additional Thoughts on SCA Reception by John Sangster
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Eavesdropping On Subcarrier Transmissions by Bob Parnass
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Boston Area SCA Test by Weld Pond
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##############################################################################
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SCA Additional Thoughts on SCA Reception:
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As I recall, the setup described by Bob Parnass (I think that was who it
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was) was to hook a VLF receiver to the output of an ordinary FM receiver
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to pick up SCCA (sic - I always call it that, Sports Car Club of
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America, when it should really be SCA - Subsidiary Communications
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Authorization or something!) transmissions multiplexed on ordinary FM
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broadcasts.
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To understand what is going on, you need to know what the FM station
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actually transmits.
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Let's do it in "top down" fashion. All you computer jocks out there
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should relate nicely to that.
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First of all, the FM station has a "composite audio" input - this is
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just the input on which SOMETHING (consider it a stub subroutine to
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written later) is fed in to the modulator.
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From the FM modulator's point of view, what you put on this input is
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just the MODULATING SIGNAL which you want to Frequency Modulate (FM) the
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station's carrier. Intuitively, you can think of the station as putting
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out a "pure" carrier at frequency F when this modulating signal is zero.
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When it is NONZERO, however, the instantaneous frequency of the
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transmitter is changed. Say the modulating signal value, in volts, or
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whatever, is M. Then the transmitter output frequency is set to F + K*M
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where K is a sensitivity constant which is unimportant except that
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whatever maximum value of M is applied to the FM modulator input should
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result in a maximum "frequency deviation" K*M which is just about the
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maximum that the FCC allows.
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If this station were a plain old-fashioned monaural FM station, all it
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would have to do is feed in the audio signal -- voice, music or funny
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sound effects -- which you want the listeners out there in Radio-Land
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to hear.
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The signal would modulate the carrier, be amplified, fed to the antenna
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and radiated. It would come swooping down into your ordinary, Monaural
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FM receiver. The receiver would say, hmm... The frequency is F + K*M
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I therefore have to subtract F and divide by K to give my loyal owner
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the instantaneous value of the signal waveform, which is M. Voila! Out
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of the FM demodulator comes M in livid high fidelity.
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(I don't think the following information is needed for present purposes,
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but I include it so somebody won't say I am oversimplifying things!!
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Now, even in simple monaural FM there is one trick that we haven't
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mentioned. This is called pre-emphasis and de-emphasis. Very early in
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the game, it was noticed that in FM systems if you fed in no modulation
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at all and listened to the receiver's reconstructed value of M (after it
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did the arithmetic noted above), there was a NOISE output from the
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receiver, even at fairly strong signal levels. The noise was
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particularly noticeable because its amplitude increased with frequency.
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Thus a quite noticeable high-frequency hiss was present on even fairly
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good signals. Somebody then had the bright idea that they should
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effectively "turn down the treble control" at the receiver. A fixed
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frequency compensating network called a "de-emphasis" network was
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designed and standardized to do that. But then the music had its highs
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"de-emphasized", so an "inverse" network called a pre-emphasis network
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was added at the transmitter.
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Next, along came stereo. In order to avoid the wrath of all the owners
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of monaural FM sets, the FCC in its wisdom decreed that a "compatible"
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system would be necessary before they would approve FM stereo. The
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engineers quickly noted that the A+B signal from two microphones gives a
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passable monaural signal (especially if "one-point" miking is used).
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Now the problem was how to get A and B out of A+B. Well, as your high
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school algebra teacher probably taught you, (A+B)+(A-B)=A and
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(A+B)-(A-B)=B. So all they had to do is send (A-B) in some clever way
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and the receiver could reconstruct A and B by "matrixing". The method
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adopted was to "multiplex" this (A-B) signal onto the main carrier by
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using it to modulate a SUBcarrier located at 38 KHz. Double-Sideband
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Suppressed Carrier (DSBSC) modulation was chosen. This gave a "lower
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sideband" extending downward from 38KHz (less 20Hz or so) to 23KHz
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(because the highs were cut off at 15 KHz.) A "pilot carrier" was put
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at 19 KHz which allows the receiver to recover the precise frequency
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phase of the 38 KHz carrier so that recovery of the (A-B) signal could
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proceed. In fact it turns out that this carrier can be used in an even
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more clever way to recover A and B signals directly from the "composite"
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signal ( A+B plus pilot carrier plus A-B ).
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Note that all the new junk -- pilot at 19KHz and A-B from 23 to 38 K
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-- are so high in frequency that most people wouldn't hear them, and
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most older monaural FM sets and loudspeakers won't reproduce them
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audibly anyway.
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Well, as if this wasn't bad enough, then along came the SCCA (I warned
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you about my warped sense of humor) and asked for authorization to put
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MUZAK on the air. The obvious thing to do was to put on yet another
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subcarrier, this time at a frequency far enough above the audio so that
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it wouldn't interfere with stereo broadcasting. 67 KHz was chosen as
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the magic frequency. But this time, FM modulation was chosen.
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Remember, all of this stuff is being stacked up in frequency above the
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normal monaural FM broadcast audio in such a way that a normal FM
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receiver won't be affected (much) by it. So before we even go into
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"composite audio" input of our simple FM transmitter, we have a VERY
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"composite" signal indeed. One might diagram it like this:
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weak pilot suppressed just a
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carrier carrier boundary FM
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/ / / sideband
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/ / / sub /
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/ / / car. /
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| | | | | /
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| Normal FM | (A-B) Lower | (A-B) Upper | /||||||||\
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| audio (A+B) | Sideband | Sideband | /||||S|C|A|||||\
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| | | | /|||||S|i|g|n|a|l|||||\
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DC 19KHz 38KHz 53KHz 67KHz
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--monaural
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signal----
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--------stereo composite signal-----------
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-------------full composite (stereo + SCA) signal-------------------
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Now to the main point. The FM transmitter takes this composite audio
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signal as input and FM MODULATES its carrier with it. When this is
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received by an FM receiver, what comes out of the discriminator is the
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composite signal. In a normal monaural FM receiver, this is fed to the
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volume control and thence to the audio amplifier stages, where
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"de-emphasis" is applied as noted above, and finally out to the speakers
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or headphones.
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If it doesn't have FM demodulation available at this frequency, then
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can still use a trick called "slope detection". Tune it above 67 KHz so
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that the 67 KHz carrier falls on the lower slope of the IF selectivity
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curve. I.e. the signal has fallen off about halfway from its peak value
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on the meter. Select AM demodulation, and then tune for the best sound.
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You should get acceptable recovery of the audio. If there is a
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bandwidth switch, experiment with it. Probably the widest bandwidth
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available will work best.
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If you don't have a suitable VLF receiver covering 67 KHz, the simple
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trick with the Phase-Locked Loop device will work just fine. After
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the monaural FM receiver has done most of the work of picking up the
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weak signal at VHF (88 to 108 MHz) and amplifying it and FM demodulating
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it so you have the baseband composite signal available to fool around
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with. What is going on here is that the PLL locks to the 67 KHz signal
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but since that is being FM modulated, it has to work a bit to stay
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locked. The Voltage-Controlled Oscillator has to be pushed above or
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below 67 KHz, its natural frequency, by applying a control voltage to
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the VCO input. All of this is done "automatically" by the design of
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phase-locking circuitry, but YOU can benefit by all this work it is
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doing: the control voltage is precisely a measurement of the
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instantaneous modulation value, M, that YOU want to hear! So you pick
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it off, amplify it, and listen to MUZAK (yuk) to your heart's content.
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From the above rather long-winded description, you can see that the
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is a lot of other junk floating around on the composite signal, namely
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all the stereo stuff below 53 KHz. To make the SCA demodulator work
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and be free of interference from the main channel, it would be well
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put a little selectivity ahead of the SCA demodulation if you are using
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the PLL trick. I haven't experimented with this, but I would think a
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simple high-pass filter to attenuate everything below 53 KHz would do
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fairly well. Of course with the VLF receiver you have all kinds of
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selectivity, so there should be no problem.
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I hope this helps you understand what is going on with SCA. If you
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followed all the details, it should be clear that there is no black
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magic going on, but that once the composite signal has been received, a
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VLF FM receiver (a rather rare beastie) is logically what is needed
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recover the SCA signal. That's why it happens to be possible to string
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together such a seemingly unlikely pair of receivers and get SCA
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broadcasts.
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-John Sangster, W3IK
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jhs at MITRE-Bedford, MA
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##############################################################################
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Subject: update: listening to SCA (subcarrier) transmissions
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From: parnass@ihu1h.UUCP (Bob Parnass, AJ9S)
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Organization: AT&T Bell Laboratories
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Newsgroups: net.ham-radio,net.audio
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Date: 25 Feb 85 17:31:34 GMT
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EAVESDROPPING ON SUBCARRIER TRANSMISSIONS
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A few weeks ago, I posed a question about listening to
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SCA transmissions on the FM commercial broadcast band.
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An article in Monitoring_Times claimed that connecting
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a vlf receiver to an FM broadcast receiver would permit
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SCA detection.
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Necessity being the mother of invention, a way was
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found to confuse the ICOM R71A into tuning below 100
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kHz1, and after receiving substantial inspiration from
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Will Martin, Phil Karn, and others I now can report
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success!
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With the R71A in the FM mode, tuned to 67 kHz, I con-
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nected the ICOM's vlf antenna input through a 0.1 ufd
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capacitor2 to the earphone jack of a $16 General Elec-
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tric AM/FM portable radio, and can now listen in on the
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world of SCA!
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In the first few minutes of tuning around, I've heard
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the Physicians' Network, Muzak, commodity reports, and
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several data transmissions.
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I built a simple SCA interface, consisting of a capaci-
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tor and resistor, into a plastic film canister.
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+-----------------------+
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| AM/FM radio |
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| earphone jack |
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| (ring) (tip) |
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+-----------------------+
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+--/|/|/|/--+
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| 100 ohm |
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| 1 watt |
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| -----
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| ----- 0.1 ufd
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+-----------------------+
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| (gnd) |
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| antenna jack |
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| ICOM R71A |
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+-----------------------+
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Most activity is heard with AM/FM radio tuned to the FM
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broadcast band, and the R71A set to 67 kHz FM. Aside
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from the Muzak, commodity reporting, and the Physici-
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ans' Network, I also heard an announcer reading from
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Popular_Communications on CRIS, the Chicago Radioland
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Information Service. This service carries programming
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of interest to the handicapped.
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With the AM/FM radio tuned to the AM broadcast band,
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and the R71A set to 60 kHz AM, I can hear what seems to
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be stereo subcarriers on AM broadcast stations claiming
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to transmit in AM stereo.
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__________
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1. Parnass, Bob, "Trick the ICOM R71A below 100 kHz", to be
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published in Monitoring_Times.
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2. The capacitor is needed with the GE radio I used to
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block DC.
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===============================================================
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Bob Parnass, Bell Telephone Laboratories - ihnp4!ihu1h!parnass
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##############################################################################
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Boston Area SCA Test
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I have set up a crude SCA listening post. The earphone output of a Sony
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ICF-2010 AM/FM/SW radio is fed into the LW antenna input of an Icom IC-720A
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transceiver. It was very surprizing that the Icom tuned way down to 3 kHz,
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well below the required 67 kHz. The specs said it only would tune as low as
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100 kHz.
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I tuned the Icom to 65 kHz and put it into AM mode. The filter, in AM mode,
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is 3 kHz wide so I wasn't expecting high fidelity. A 14 kHz filter would
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be perfect. The Sony was set to 102.5 MHz.
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I first tried the tape output of the Sony. That didn't work at all. The tape
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output must have a low pass filter on it to eliminate the 19 kHz pilot. I
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next tried the earphone output. I got some noise out of the Icom. It seemed
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to be working. No earphone has a frequency response as high as 19kHz so
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they didn't bother with a filter on the earphone output.
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After fiddling with the Icom tuning a bit and adjusting the volume controls
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on both radios, I definitely heard, you guessed it, MUZAK. The sound quality
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was pretty poor. The Icom doesn't have an FM mode so I used "slope detection."
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This is done by using the AM mode and tuning a bit off frequency. Another
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problem was the lack of a high pass filter to get rid of all the primary
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stereo signal. I could definitely decipher the words someone was saying or
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the song that was playing though.
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There are 3 standard subcarrier frequencies in use: 57 kHz, 67 kHz and 92 Khz.
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I tried all these plus some things in between and found these below from a
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location near Boston, MA.
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57 kHz 67 kHz 92 kHz
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88.9 Data Reading for the blind
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90.9 Chinese
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95.3 Spanish
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96.9 Data
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98.5 Data
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100.7 Data (really at 80? kHz)
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102.5 Muzak Easy Listening
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103.3 Data
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105.7 French
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107.9 Data
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Weld Pond
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