• subcar

    From LU9DCE@21:5/101 to BBSRT on Thu Jul 4 08:40:07 2019
    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.

    -John Sangster, W3IK
    jhs at MITRE-Bedford, MA

    ======================================================================= =======================================================================

    Subject: update: listening to SCA (subcarrier) transmissions
    From: parnass@ihu1h.UUCP (Bob Parnass, AJ9S)
    Organization: AT&T Bell Laboratories
    Newsgroups: net.ham-radio,net.audio
    Date: 25 Feb 85 17:31:34 GMT


    EAVESDROPPING ON SUBCARRIER TRANSMISSIONS


    A few weeks ago, I posed a question about listening to
    SCA transmissions on the FM commercial broadcast band.
    An article in Monitoring_Times claimed that connecting
    a vlf receiver to an FM broadcast receiver would permit
    SCA detection.

    Necessity being the mother of invention, a way was
    found to confuse the ICOM R71A into tuning below 100
    kHz1, and after receiving substantial inspiration from
    Will Martin, Phil Karn, and others I now can report
    success!

    With the R71A in the FM mode, tuned to 67 kHz, I con-
    nected the ICOM's vlf antenna input through a 0.1 ufd
    capacitor2 to the earphone jack of a $16 General Elec-
    tric AM/FM portable radio, and can now listen in on the
    world of SCA!

    In the first few minutes of tuning around, I've heard
    the Physicians' Network, Muzak, commodity reports, and
    several data transmissions.

    I built a simple SCA interface, consisting of a capaci-
    tor and resistor, into a plastic film canister.


    +-----------------------+
    | AM/FM radio |
    | |
    | earphone jack |
    | (ring) (tip) |
    +-----------------------+
    | |
    +--/|/|/|/--+
    | |
    | 100 ohm |
    | 1 watt |
    | |
    | |
    | -----
    | ----- 0.1 ufd
    | |
    +-----------------------+
    | (gnd) |
    | antenna jack |
    | |
    | ICOM R71A |
    +-----------------------+


    Most activity is heard with AM/FM radio tuned to the FM
    broadcast band, and the R71A set to 67 kHz FM. Aside
    from the Muzak, commodity reporting, and the Physici-
    ans' Network, I also heard an announcer reading from
    Popular_Communications on CRIS, the Chicago Radioland
    Information Service. This service carries programming
    of interest to the handicapped.

    With the AM/FM radio tuned to the AM broadcast band,
    and the R71A set to 60 kHz AM, I can hear what seems to
    be stereo subcarriers on AM broadcast stations claiming
    to transmit in AM stereo.

    __________

    1. Parnass, Bob, "Trick the ICOM R71A below 100 kHz", to be
    published in Monitoring_Times.

    2. The capacitor is needed with the GE radio I used to
    block DC.

    ===============================================================
    Bob Parnass, Bell Telephone Laboratories - ihnp4!ihu1h!parnass
    is work it


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