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

Hayati Ayguen h_ayguen at web.de
Sat Jul 18 08:54:48 UTC 2015

Hi Murat and all others,

what i have hear is definitely NOT the DC spike coming from the analog

Whilst my experiments, trying to get rtl_fm run on Raspi 2, i added an
option to rtl_fm to be verbose ("-v 1"). See

When calling rtl_fm with "-s 24000 -f 433.25M -M raw" i get following
"capture_rate = 42 * 24000 = 1008000"
"capture_freq = freq + capture_rate / 4 = 433502000"

>From capture_rate you see, that the RTL's ADC works at a samplerate of
1.008 MHz. And the RTL is parametrized to 433.502 MHz.
So, the RTL's DC is at 433.502 MHz - far away from the desired 433.25 MHz.
On digital signal preprocessing the rtl_fm mixes the desired frequency
up to baseband 0 Hz and the does low pass filtering and decimation to
the samplerate of 24 kHz.
This result is what i recorded in my previous mail and had a look with
SDR#, the screenshot.

At the moment, I had no look into the source of low pass filter. I
suppose that the DC is introduced here.

kind regards,

Am 18.07.2015 um 08:45 schrieb Murat Tologlu:
> 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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