szeto@kilroy.jpl.nasa.gov (James T. Szeto) (04/19/91)
After reading the FCC Rules and Regulations concerning the engineering standard for subsidiary communications (SCA), I was wondering if someone would give a clearer understanding on how SCA is added to the FM signal and how it is recovered from the signal. Also, I'm looking for a reference where I may find an example circuit which recovers SCA. Thanx in advance. James T. Szeto | Internet: szeto@kilroy.jpl.nasa.gov Jet Propulsion Laboratory | Phone : (818) 354-2299 Atmospheric and Oceanographic Sciences | Fax : (818) 354-5148 M/S 183-301 | 4800 Oak Grove Drive | Pasadena, CA 91109 |
bill@thd.tv.tek.com (William K. McFadden) (04/20/91)
The SCA carrier is an FM signal (usually 67 kHz) that is added to the
audio of an FM radio station before it is fed to the transmitter. If
the station is stereo, there will also be a 19 kHz pilot tone and a 38
kHz AM suppressed carrier signal that carries the stereo difference
information. All these signals plus the mono audio are mixed together
and fed to the FM transmitter.
To decode these, you first detect the FM signal. The portion from 0 to
15 kHz is lowpass filtered to recover the L+R mono audio. If a 19 kHz
tone is present, a stereo receiver will turn on the 38 kHz DSB detector
to recover L-R, which is combined with L+R in a matrix to decode L and
R. If the receiver has an SCA detector and a 67 kHz carrier is
present, another FM detector demodulates it. Think of an SCA receiver
as having two FM detectors in series.
If you need a better explanation, send email.
For a circuit, try looking at the data sheet for the 565 PLL made by
Signetics, Motorola, National Semiconductor, and others.
--
Bill McFadden Tektronix, Inc. P.O. Box 500 MS 58-639 Beaverton, OR 97077
bill@videovax.tv.tek.com, {hplabs,uw-beaver,decvax}!tektronix!videovax!bill
Phone: (503) 627-6920 "SCUD: Shoots Crooked, Usually Destroyed"system@hale.UUCP (System Administration) (04/21/91)
szeto@kilroy.jpl.nasa.gov (James T. Szeto) writes: > After reading the FCC Rules and Regulations concerning the engineering standa > for subsidiary communications (SCA), I was wondering if someone would give a > clearer understanding on how SCA is added to the FM signal and how it is > recovered from the signal. Also, I'm looking for a reference where I may fin > an example circuit which recovers SCA. > SCA is used to transmit various programming, usally on a subscription basis, services like background music are the most common, but there have been some dabling with data transmission, paging services, etc... The FM signal (without getting into tech) generates two sidebands, which can be modulated to carry additional information, the decoder just adds an offset to the recieved signal to recieve the SCA (also known as MPX for Multiplex.) North Country Radio (PO Box 53, Wykagyl Station, New Rochelle, NY 10804) sells a kit for SCA/MPX decoding for $75 and $2.50 S&H, I believe the company just sells kits that Rudolf Graf and William Sheets come up with, which appear in electronic mags as "Build This" type things. Panaxis Productions (PO Box 130, Paradise, CA 95967-0130) sells one also, and has a much larger catalog, Ernie Wilson even has a kit for SCA Generation if you want to that involved, my catalog is not handy for prices, sorry. ($140 I think for the SCA Generator) This is very basic, note the time if you want to flame for gross lack of technical detail. Future BE in slavery. --------------------------------------=--------------------------------------- System Administrator Hale Telecommunications Public Access system@hale.uucp 619-660-6734 8N1 24 Hours --------------------------------------=--------------------------------------- ------=---------------------------------------
duncan@bolero.ati.com (James R. Duncan) (04/22/91)
In article <1991Apr19.050618.23977@elroy.jpl.nasa.gov> szeto@kilroy.jpl.nasa.gov (James T. Szeto) writes: >After reading the FCC Rules and Regulations concerning the engineering standard >for subsidiary communications (SCA), I was wondering if someone would give a >clearer understanding on how SCA is added to the FM signal and how it is >recovered from the signal. Also, I'm looking for a reference where I may find >an example circuit which recovers SCA. > There's no big technical mystery to "SCA" operation. This is simply a plain vanilla subcarrier addition process. The total stereo program material, which also includes a DSB subcarrier at 38 kHz, is considered to theoretically use up the first 53 kHz of the roughly speaking 100 kHz bandwidth available in the usual FM broadcast carrier. In the remaining space it is common to add one or more subcarriers which can carry data, Muzak, paging, stock market quotes, or whatever. Typically, a broadcaster may elect to add a 67 kHz subcarrier which has been modulated by whatever "SCA" material. That's all there is to it basically; it's just added in with a relatively simple network. Recovery is also quite easy. In this case, one would simply take an FM detector circuit tuned to 67 kHz and attach it to the receiver's detector circuit. The mulitplex or subcarrier process is discussed in many books, probably some ham publications as well. The most convenient FM detector for subcarriers is the PLL, so you would need to find applications notes for these from National Semiconductor, Exar, or especially Signetics. The NS literature is readily available; it even used to be carried by Radio Shack (and maybe still is). e -- KUFX | w ["] | WA6MBV 94.5 |.. |___|_____..duncan@bolero.ati.com | Jim Duncan FM | H | +1 408 297 5977 \_____I_____/ 37 3 10N/121 59 10W ---------------