Mark Zenier wrote:
SAP is a narrowband FM subcarrier on the TV Audio Signal. 75 Khz,
I think.

78.67KHz, 5 times horizontal sweep. It's DBX companded.

The other subcarrier ("pro channel") is at 102.27KHz. It has no fixed
purpose but many stations indeed use it for studio=remote purposes.

This is the same as a SCA signal on a FM Audio Broadcast, where
67, 75, and 92 kHz are used for "Elevator Music", talking newspapers
for the blind, and various subscription ethnic radio services.

There's no 75KHz subcarrier for FM but there is a data carrier at 57KHz
on some stations, used for RDS. (which allows some newer car radios to
display the name of the station you're tuned to - and sometimes, the
song being played)

It is theoretically illegal (under the ECPA) to monitor FM SCAs or the
"pro channel" without the station's permission. No, I don't think
there's much chance of getting caught.

--
Doug Smith W9WI
Pleasant View (Nashville), TN EM66
http://www.w9wi.com