PL/CTCSS/DCS squelch on rtl_fm?

Nick Foster bistromath at
Mon Dec 9 18:30:05 UTC 2013

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:

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


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