For transmitters that don't use tone squelch, the best FM squelch is a noise squelch, which detects the "quieting" an FM signal causes. The gr-scanner project (which is incomplete/work-in-progress, don't bug me about functionality yet) has one, albeit written for Gnuradio:

https://github.com/bistromath/gr-scanner/blob/master/python/standard_squelch_ff.py

It's a rewrite of Gnuradio's "standard squelch".

--n

On Mon, Dec 9, 2013 at 10:17 AM, James Sharp <james@fivecats.org> wrote:

On 12/9/2013 7:33 AM, Chris Hanson wrote:

Thanks for the response. I looked at the code and from what I can tell,
it’s just using various ham radio rigs built in CTCSS facilities and
programming them via RS232. Am I missing something?

The app_rpt file has some functions to detect tones in an audio stream that can be used to detect the CTCSS tones. Look through the "tone_detect" function along with the goertzel_* functions.

They're not specifically tagged as functions to decode CTCSS tones, but they should be able to be used as such.

Those functions are expecting to be fed a 16-bit signed linear mono audio stream.