On Wed, Jan 13, 2016 at 05:01:01AM +0300, Andrey Turkin wrote: > Why not use libzvbi's slicer? It should be pretty robust with > less-than-ideal signal. > > Regarding your code - is there a need to calculate a frequency? I did > something similar a while back with VBI from TV signal and in my experience: > > a) this kind of frequency/phase estimation doesn't work that well on a > noisy signal.
> b) there's really no need to estimate frequency - it's a known constant > (well it depends on a standard but in each standard it is a constant) and > generators usually don't deviate from it; you just need to get a good phase > estimation based on, say, local maxima/minima of run-in bits. to calculate the phase the area that has alternating 0/1 values can be correlated with a sin() and a cos(), if you consider the 2 resulting values as x and y coordinates the angle they form from the origin is the phase. (this can be worded simpler using complex numbers) when above is done its essential that a whole multiple of cycles is used the important part is that the used sin(x) and cos(x) vectors are orthogonal, that is sum sin(x) * cos(x) over the x values considered is 0 cutting them off randomls would break that (its also possible to use windowing instead of a exact multiple of cycles) consider that the phase is 0 so our signal input is exactly cos(x) (if we define that as 0°), the dot product of that with cos(x) is "1" and sin(x) is 0 now if our input is shifted by 90deg that is it matches sin(x) then the dot product with a cos(x) vector is 0 and sin(x) is 1 ... (also i for simpliity normalized the values, sum of cos(x)*cos(x) for x from 0 to n would of course be larger than 1 for a larger n) PS: not complaining about calculating the phase by some other means above is just a suggestion [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB While the State exists there can be no freedom; when there is freedom there will be no State. -- Vladimir Lenin
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