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

Nikolay Dimitrov picmaster at mail.bg
Sat Jul 18 10:03:25 UTC 2015


Hi Hayati,

On 07/18/2015 02:02 AM, Hayati Ayguen wrote:
>
> 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?

You need IQ calibration.

When calibrated, spectrum should look much cleaner, signals should
appear only on 1 place of the waterfall/spectrum display, and the LO
carrier (the so-called DC) will be much better suppressed, although not
completely removed (that's why the -dc option exists for the baseband
processing).

Regards,
Nikolay



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