rtl_fm Squelch Questions

Martin McCormick martin.m at suddenlink.net
Fri Feb 10 21:22:21 UTC 2017


	I have been playing with the rtl_fm program and most of
it works amazingly well but I can not seem to get the squelch to
stay open when receiving signals.

	I have not tried to analyze the source yet so I am asking
what principle drives this squelch?

	In the analog world, the best squelches for FM receivers
tend to be noise-driven and use a high-pass filter to filter out
normal audio. When the noise drops below a preset threshold, the
squelch opens.

	For AM receivers, a cheap and easy solution is to monitor
the AGC and open the squelch when there is AGC voltage above a
preset level.

	What I am noticing is that if I set the l value to a
point just above where the noise stops, signals do open the
squelch but even strong signals will not keep it from flickering
on and off constantly.

	If I set the l value any lower, the squelch is always
open so that is not the issue.

	I have tried signals that are absolutely full-quieting,
with and without CTCSS and the squelch opens briefly, closes for
a fraction of a second, opens for another fraction of a second
and randomly flickers on and off for the whole transmission.

	On rare occasions, the squelch opens when the signal
starts, stays open and then closes properly after the carrier
leaves.
Some of these signals are even ever so slightly noisy and I have
heard this situation with and without PL tones or CTCSS so that
doesn't seem to matter.

	Finally, I thought it might have something to do with too
narrow a bandwidth so I increased the sampling rate to 24 K which
made no difference at all.

	If the signal has voice on it, the flickering doesn't
seem to be effected by the words.

	Basically, what is this squelch responding to to keep flapping?

Martin McCormick WB5AGZ


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