On Fri, 2012-02-10 at 20:29 -0600, David Hagood wrote:
> On Fri, 2012-02-10 at 21:08 -0500, Andy Walls wrote:
> >
> > 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.
> How'd you analyze the data - assume it was baseband I/Q and do an FFT?
> If so, and if this was digitized baseband, you should have seen the FM
> stereo pilot tone at 19kHz.
Well, I was examining the data in the time-domain using 'od' to decode
the bytes to text and using gnuplot to visualize. I looked for "high"
frequency contant envelope cycles. Pretty lame, I know.
I did this in octave this morning and noted a carrier/pilot of some
sort:
fid = fopen('rtl2832u_fm_sample_FIXED.bin');
N=2^20;
fseek(fid, N*10, SEEK_SET);
[v, count] = fread(fid,[2,N],'unsigned char');
v = (v-128)/128;
w = v(1,:) + j*v(2,:);
f = fft(v(1,:),N);
fw = fft(v(1,:),N);
Fs = 2.5e6;
freqs = [1:(N/24)-1]*Fs/N;
bins = [2:N/24];
plot(freqs, abs(f(bins))/N, '.');
plot(freqs, abs(fw(bins))/N, '.');
[m,mi] = max(abs(fw)/N);
freqs(mi)
ans = 9534.4
[m,mi] = max(abs(f)/N);
freqs(mi)
ans = 9534.4
ans*2/1000
ans = 19.069
I must have Fs wrong.
Anyway not much happens in the data above 100 kHz, but to see the whol
spectrum:
bins = [2:N/2];
freqs = [1:(N/2)-1]*Fs/N;
semilogy(freqs, abs(f(bins))/N, '.');
semilogy(freqs, abs(fw(bins))/N, '.');
But it appears someone has now already solved the decoding problem.
Regards,
Andy
>
> If it's digitized IF, you should be able to run it through a rectangular
> to polar conversion (compute mag = I^2+Q^2 and phase = arctan(I/Q) (use
> a proper 4 quadrant arctan), then compute frequency by the delta between
> the phase samples. Mag should be constant, frequency would then be your
> audio.
>
>
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