rtl_fm: degraded demodulation caused by self-introduced DC !?

Hayati Ayguen h_ayguen at web.de
Fri Jul 17 23:02:18 UTC 2015


Hi,

after i got rtl_fm run on Raspi 2 with Debian Jessie .. now i have some
additional noise in the FM demodulated audio!

With a "raw" recording (see attachment) i can see an additional carrier
at the DC frequency of the demodulated output. That corresponds to the
tuned frequency (= 433.25 MHz) parametrized to rtl_fm.
Due to calibration error, the FM carrier has some offset: ~ -1.3 kHz as
visible in screenshot.
The DC carrier does demodulate to some distortion!

Option "-E dc" does not help, cause that removes a DC in the demodulated
output. An additional option to filter DC before demodulation does help
a bit .. but does not solve the problem, which looks to be introduced
earlier ..

I would not have expected such a DC, cause IMHO it's produced whilst
downconversion or filtering.
It's not the RTL dongle's DC, which should be far far away by 1/4 of the
high samplerate.

Someone else seen this problem?
Does anyone have a useful solution?

kind regards,
Hayati

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