[SciPy-User] finding frequency of wav

[Linda]

Thanks again for you explanation :-) This certainly helps.


On Thu, May 27, 2010 at 17:42, Fabrice Silva <silva at lma.cnrs-mrs.fr> wrote:

> Le jeudi 27 mai 2010 à 16:42 +0200, Linda a écrit :
>
> > The signal I am trying to decode is a DSC transmission that is
> > recorded in a wav file. (Digital Selective Calling, used in marine
> > radio) It is a phase modulated digital signal: '1' is 2100Hz, '0' is
> > 1300 Hz and there's a carrier at 1700Hz. That should be all
> > frequencies involved (apart from noise). Currently I am used
> > generated, clean signals. But probably I should get a clean
> > '10101010'-signal first to try my work on.
> > Since the bitrate is set at 1200bits/sec, the bit length would be
> > samplerate/1200 = 18.4 samples at 22050. I can double the samplerate
> > to 44100, but that still leaves me at only 36.8 samples per chunk.
>
> Then a chuck is what you can consider a stationary signal with a single
> frequency. Due to the bitrate and the sampling frequency, it has only 18
> samples, so that, its Fourier transform is a lobe centered
> on 2100 or 1300Hz whose relative bandwidth is the inverse of the number
> of periods that whould have been observed (neglecting noise).
> Result : width = frequency/#periods = 1300/{almost 1} Hz
> i.e. the very limited number of samples in each sample lead to, in
> frequency domain, a lobe whose width is almost the same as the central
> frequency! because the bitrate and carrier are close...
>
> > If I understand what you say correctly, I would need at least 55 (64)
> samples in each chunk?
> In my previous answer, I only consider a very raw Rayleigth criteria:
> two coding frequencies would at least be spaced by two computed fft
> frequencies. Zero padding can easily deal with this problem. But the
> width of the lobes (linked with the limited number of periods observed)
> may be a lot more tricky to solve. But you can still use estimation
> methods based for example on the spectrum centroïd to determine whether
> the bit is now on 0 or 1.
> >
> >
> > I'm not sure what chunk[3] would have been, I should have used a
> > dotting-signal instead of an unknown message to try this on. I will
> > try this again with more useful data this afternoon.
>
>
>
>
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