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Nikolay Dimitrov picmaster at mail.bgHi 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