Re: [Discuss-gnuradio] FM SCA

[Tim Pozar]

On Sat, Feb 23, 2002 at 11:39:28AM -0800, Matthew Ettus wrote:
> On Sat, 2002-02-23 at 05:36, Johnathan Corgan wrote:
> > What does an SCA sub-carrier look like?  I am in San Jose; I have a
> > AR8200 with a nifty spectrum analyzer that might help identify other
> > local stations.
> 
> If you are looking at un-demodded spectrum  (i.e. RF on a spec analyzer)
> it looks like a slight bump in the spectrum at 67kHz away from the
> carrier.
> 
> If you are looking at demodded spectrum (i.e. on the audio spectrum
> analyzer), then it looks kind of like a square bump in the spectrum
> centered at 67kHz.  SCA is FM modulated on the main FM.  FM on FM...
> 
> Of course if your receiver filters the demodded FM before you can see
> it, then it will have filtered out anything higher than about 18kHz, and
> you won't see it.

Actually SCA can be at either 67 or 92KHz.  Mono stations will
normally do an SCA at 56Khz.  Typically you will see it at 67Kc but
at one time there were some stations in the Bay Area doing 92Kc.
For instance KOIT (96.5Mc) had both SCAs running in the past.  They
were doing FSK data on those "channels".

KPFA (94.1Mc) has for many years done audio on the 67Kc sub-carrier.
The service was reading for the blind.

SCA was pretty popular at one point for distribution of programming
such as Muzak for stores. But Muzak has moved to other delivery
methods as they have better control and more bandwidth not using
FM stations.

Then there is RDS.  It was pushed pretty heavy by the CES and others
about '94/95.   There may be stations still doing it.  KALW (91.7Mc)
was one of the last hold outs.  Although they may have pulled it
out of their air chain by now.  RDS normally hangs out at 76Kc.

These are the "norms" of SCA.  In effect, you can run anything you
want so long as it meets FCC's 73.319.  About 10 years ago, KFOG
(104.5Mc) was running an SCA (promoted by Seiko) that filled pretty
much everything from 53Kc out with data for a pager system that
would be delivered to a receiver that looked like a watch.  There
was an attempt to roll out another standard called "High Speed Data
System" (HSDS).  It was much like the SCA that ran on KFOG but it
didn't get enough interest.

A very basic description of the composite spectrum and SCA deployment
can be found at:

        http://www.compolinc.com/sca_technology.htm
and
        http://home.tbbs.net/~bhammond/sca.htm

Other useful links...

        FCC R&R Sec. 73.319  FM multiplex subcarrier technical standards:
        
http://frwebgate.access.gpo.gov/cgi-bin/get-cfr.cgi?TITLE=47&PART=73&SECTION=319&TYPE=TEXT&YEAR=2001

Tim
--
  Snail: Tim Pozar / LNS / 1978 45th Ave / San Francisco CA 94116 / USA
               POTS: +1 415 665 3790  Radio: KC6GNJ / KAE6247
"It's a damn poor mind that can only think of one way to spell a word."
                        - Andrew Jackson
"What is wanted is not the will to believe, but the will to find out,
which is the exact opposite." - Bertrand Russell, "Skeptical_Essays"