[FFmpeg-devel] [PATCH 2/3] lavd: add teletext quantizer
Michael Niedermayer
michael at niedermayer.cc
Wed Jan 13 20:32:31 CET 2016
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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