On Thu, 2012-02-09 at 17:21 +0200, Antti Palosaari wrote: > On 09.02.2012 17:11, Patrick Boettcher wrote: > > On Thursday 09 February 2012 16:01:12 Antti Palosaari wrote: > >> I have taken radio sniffs from FM capable Realtek DVB-T device. Looks > >> like demodulator ADC samples IF frequency and pass all the sampled > >> data to the application. Application is then responsible for > >> decoding that. Device supports DVB-T, FM and DAB. I can guess both > >> FM and DAB are demodulated by software. > >> > >> Here is 17 second, 83 MB, FM radio sniff: > >> http://palosaari.fi/linux/v4l-dvb/rtl2832u_fm/ > >> Decode it and listen some Finnish speak ;) > >> > >> Could someone help to decode it? I tried GNU Radio, but I failed > >> likely because I didn't have enough knowledge... GNU Radio and > >> Octave or Matlab are way to go. > > > > For someone to decode it, you would need to give more information about > > the format of the stream. Like the sampling frequency, the sample-format > > and then the IF-frequency. > > You can see sampling format easily looking hexdump or open file in > Audacity. It is 8bit unsigned samples, 2 channels (I & Q). > > No knowledge about IF... For good guess is to try some general used IFs. > > Sampling freq can be calculated using sample info and the fact it is > about 17 sec. sample size = 86919168 Bytes, time 17 sec. 2 channels, 1 > byte sample => 2556446,11765 sample/sec (~2.5 MHz!) Randomly checking some of the data with GNUplot, if 2.5 Msps is the sampling rate, then the fastest freq I saw was about 50 kHz. Maybe you have an FM compsite baseband signal: http://en.wikipedia.org/wiki/FM_broadcast#Other_subcarrier_services If you low pass filter with digital filter with (an equivalent to) a 17 kHz cutoff, you may just be left with the mono L+R channel. I am assuming the I&Q channels in the data are 8 bit LPCM so x = (c-128)/128.0 normalizes an usigned byte sample value c in 0 to 255 to a float value x in -1.0 to 1.0. Regards, Andy > > I never did something like myself, but from what I saw in gnuradio there > > should be everything to make a FM-demod based on the data. > > Yes there was a lot of block and those were rather easy to connect using > graphical interface (gnuradio-companion). But I don't know exactly what > block are needed and what are parameters. I used file-sink => > fm-modulator => audio-sink. Likely not enough :i > > Without any earlier experience it is rather challenging. But if there is > someone who have done that earlier using USRP SDR he could likely do it > easier :) > > regards > Antti -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
