[Tvtime-devel] [ tvtime-Patches-884449 ] vbi clock-signal locking to improve CC decoding

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Patches item #884449, was opened at 2004-01-25 15:31
Message generated for change (Comment added) made by kevko
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Category: Bugfix
Group: Patch against CVS tvtime
Status: Open
Resolution: None
Priority: 5
Submitted By: Kevin Ko (kevko)
Assigned to: Billy Biggs (vektor)
Summary: vbi clock-signal locking to improve CC decoding

Initial Comment:
Re: Bug [ 720072 ] Closed Caption text messed up

The original method used in ccdecode() used some hard
limits to determine the first pair of peaks in the
binary 10 (repeated 7 times) clock-in.  These settings
were not kosher with my cable signal, and so I
experienced, at best, dropped/wrong characters.  Though
more often than not, I was unable to decode XDS or CC data.

I wrote a tool to visualize the data from /dev/vbi?
(code is ugly, but if any wants it, I'll take the time
to clean it up; just email me), and realized that with
my setup, the locking was occurring too early.  

Noisy signals were also hopeless (despite diz's
excellent filter), because the sample threshold would
vary too widely.  

This applies a more general search to latch onto the
VBI clock-in by first optimizing a metric that takes
both distance and signal value into account.  It then
takes advantage of the fixed clock-in VBI freq of
1.006976Mhz (28.64 Mhz sampling freqs for bttv and
philips tuner cards) in conjunction with said metric to
search out the remaining clock-in values.  

An average of the 7 maximum and minimum values is taken
to determine the threshold, the result of which being
more stable than that from a single pair of measurements.

CONCLUSION (in case you want to skip all my noise above):

Things seem to work well with apparently unnoticeable
overhead (relative to the expensive stuff like image
filtering), and top doesn't show much of a difference
on my athlon.  Now, I can decode XDS and captions
without any problems on all supported channels (not
just one), and the resulting output seems relatively
free of errors.

For images of the signal and general illustrations of
my decoding problem/solution:

http://www.princeton.edu/~kko/tvtime/

And as an added bonus, you can see how well diz's
filter works on a noisy signal.  (it's quite good!)  :-)

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>Comment By: Kevin Ko (kevko)
Date: 2004-01-25 19:48

Message:
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Alrighty, I think I've got something a bit better which is
less dependent on these hard coded numbers.  The 'latching'
part remains the same and helps with determining the avg
threshold.  After finding the last minimum, I jump to the
last pair of 11's in the VBI frame using the threshold alone
(the first part of the frame after the clock-in is a bunch
of 0's).  This was originally done with a constant jump, so
this should be better.  (thanks to ntsc-cc's source for
giving me notice of this)

Decoding also occurs over a complete sample cycle rather
than just my randomly chosen range consisting of five
samples surrounding the desired point.  

Anyhow, this is better now: sort of like ntsc-cc for the
decode part, with a clock-in/threshold portion that isn't
based on hard values.

My bad for the extra hassle!  Should have noticed at
first...  ;b

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Comment By: Billy Biggs (vektor)
Date: 2004-01-25 18:05

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You'd better check it out, my setup right now prevents me
from even being able to run tvtime ;-)

My setup and schedule is sort of messed up as I try and
finish/defend my thesis.

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Comment By: Kevin Ko (kevko)
Date: 2004-01-25 18:03

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i'd like to double-check my increment numbers...  or perhaps
you should just try the existing ones?  what i've used
doesn't seem exactly right because of round-off.  

latching to the clock should still be fine, however.

i'll think about it and get back to you. 

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Comment By: Billy Biggs (vektor)
Date: 2004-01-25 15:58

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ahem, i'd check it in if sourceforge weren't fucking
busted...  sigh.

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Comment By: Billy Biggs (vektor)
Date: 2004-01-25 15:39

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Very cool.  Yeah, I was also pretty impressed with diz's
filter, but figured something like missing the clock in
would be breaking stuff, that whole like 'find the first
peak' thing was just broken.  Your code looks awesome, and
your viewer app is a lot like diz's xvbidecode app. :)   
You should drop by #tvtime or #livid on freenode, btw.

I'll apply this now, thanks a ton.

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