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

Murat Tologlu mtologlu at hotmail.com
Sat Jul 18 06:45:13 UTC 2015


Dear Hayati,

I am glad to see that my Debian-Jessie suggestion helped you to progress.

Regarding your recent question: If I don't understand wrong, This is a well-known problem called as DC Spike which exist more or less in all SDR receivers. . Dr. Wickert of UCCS has an excellent laboratory note on RTL-SDR also explains this nature (see page 7) :  http://www.eas.uccs.edu/wickert/ece4670/lecture_notes/Lab6.pdf You can reach Dr. Wickert's other DSP notes from here: http://www.eas.uccs.edu/wickert/index.shtml 

DC Spike comes from analog front end of the RTL-SDR dongle and we can talk about several solutions. First and the easiest solution is to move the center frequency a bit up and down to avoid interference with DC spike (this is called as offset tuning). Some hardware manufacturer claim that they manufacture better quality thus lower DC spike sdr chips and boards. You may also consider a software "notch filter" to reduce the DC spike !

Kind regards,
Murat 

-----Original Message-----
From: osmocom-sdr [mailto:osmocom-sdr-bounces at lists.osmocom.org] On Behalf Of Hayati Ayguen
Sent: Saturday, July 18, 2015 2:02 AM
To: osmocom-sdr at lists.osmocom.org
Subject: rtl_fm: degraded demodulation caused by self-introduced DC !?


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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