Re: [Discuss-gnuradio] Stereo FM receiver

[Dave Emery]

On Mon, Jan 24, 2005 at 07:59:47PM -0800, Matt Ettus wrote:
> 
> > Do you have any good reference on how the digital info is transmitted on
> > the SCA?
> 
> No, but it sounded to me like FSK or AFSK.  Probably 1200 baud.  It may even 
> be
> different depending on the vendor of the system using it.

        FWIW there have been a number of modulations used over the
years.

        Most common are FSK variants:  GMSK, filtered FSK, straight FSK.
Cheap and easy to demodulate with one chip circuits.   1200, 2400, 4800 
9600 and even occasionally higher rates of async type streams are very
common.  Most are at least weakly encrypted.

        Probably the most complex modulation I know of was a partial
response multilevel VSB modulation (forget whether it was 8VSB or 16VSB)
used by a service called Mainstream.

        I believe there has also been occasional use of BPSK, QPSK, and
QAM vector modulations too...

        And at various times over the years stations have used a
straight FM analog SCA subcarrier as the equivalent of an audio voice
frequency data line and put a variety of standard wireline modems on it,
ranging from VFT FSK multitone muxes to modern high speed phone line
modems.   This would be AFSK in traditional ham parlance, though the
modulations involved have varied.

        Last I read the FCC rules there was no real limit on what one
could use provided it fit in the baseband and carrier bandwidth masks
and did not use up more that a small defined percentage of overall
transmitter power.   I am not sure what the move to IBOC (In Band On
Channel) digital audio has done to these rules - probably should look
that up sometime.

        And before analog TV goes away (if it ever finally does) it
should be noted that for many years TV stations have been transmitting
data of various sorts on a subcarrier above the SAP audio on the BSTC
stereo signal on their audio carrier.   Often used for transmitter
control and telemetry and the like...

        Perhaps the most notable thing is that FM SCA signals come and
go in any given market - they are not often all that profitable for the
stations involved and businesses start up hoping to capitalize on some
dream of distributing data via SCA and then fail a few months or years
later and shut down their subcarriers.


> 
> Matt
> 
> 
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-- 
   Dave Emery N1PRE,  address@hidden  DIE Consulting, Weston, Mass 02493