rtl_fm Squelch Questions

Marcus Müller marcus.mueller at ettus.com
Sat Feb 11 14:35:57 UTC 2017

Hi Martin,

never feel ashamed for asking questions. I've learned (support stuff)
that the more at ease you are at potentially missing something, the
easier others feel at answering questions, and the more pleasant the
process is for both sides. And yeah, I've got no doubts you /could/ have
spotted this – but not spotting this in a couple hundred lines of code
is really not a fauxpass nor a sign of anything negative. Instead,
investigating what makes a piece of technology tick or misbehave (in
this very case, more the latter) and writing an extensive email
explaining your question and giving a bit of background – what more
could FOSS projects ask for?!

>  I tend to try ridiculously simple ideas first
> in hopes that one will work. In FM reception, the loudest noise
> occurs when there is no signal or on occasion if there is a
> nearby signal just outside the passband in which case the noise
> squelch is closed more tightly.
Yep! Classical noise-squelch! Direct baseband mixing receiver problem:
You might have a stable pseudo-carrier through LO leakage. Direct
baseband mixer solution: DC block the heck out of your baseband :)

I'll let the others answer your further audio-signal questions; I must
admit that I'm on a train, which isn't necessarily the easiest
environment to diligently write emails :)

Best regards,


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