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List:       linux-video
Subject:    Re: [V4L] Poor cable television reception and dropped frames
From:       Justin Schoeman <justin () suntiger ! ee ! up ! ac ! za>
Date:       2001-07-11 15:56:38
[Download RAW message or body]

Eugene Kuznetsov wrote:
> 
> Hello Justin,
> 
> Wednesday, July 11, 2001, 12:13:21 AM, you wrote:
> 
> JS> "M. Osten" wrote:
> >>
> >> I use vcr to capture television programs with a brooktree tuner/capture
> >> card.  It seems that the channels at the lower end of the spectrum (ie
> >> 2,3,4) have poor reception due to bad in wall cables.  When I do
> >> captures on these channels drop lots of frames in an hour. Channels at
> >> the higher end of the spectrum do fine.
> >>
> >> Machine is a dual P3 933 with 1 gig of ram, and 170 gig of ATA 100
> >> 7200rpm drives.
> >>
> >> Capture thread exiting
> >>          Captured 111504 frames.
> >>              Frames dropped in capture: 652 (0.0%)
> >>              Frames dropped in encoder: 1191 (0.0%)
> >>          Exiting..
> >>          Waiting for write thread
> >>          MP3EncodeR::Close()
> >>          Write thread exiting
> >>          Waiting for audio thread
> >>          Waiting for video thread
> >>          All threads exited
> >>          v4l1: VIDIOCCAPTURE in ::setCapture
> >>          v4l1: VIDIOCSYNC in ::~v4l1if
> >>          v4l1: ioctl VIDIOCCAPTURE in v4l1if::~v4l1if
> >>          Total Unfree 3596428 bytes cnt 49 [0,0]
> 
> JS> If the signal is weak, the video capture card will often miss the
> JS> vertical synchronisation pulse (it gets lost in the background noise).
> JS> When this happens, it detects that too many lines have been received in
> JS> a frame, and does a resynchronisation, effectively dropping a frame.
> JS> The only way around this is to get a better signal.
> 
> If I understand the problem correctly, there is a workaround. I had
> similar problem when capturing from bad-quality VCR tapes. Frame drop
> was much worse than in this case ( 652 out of 110k frames dropped is
> nothing; I was experiencing up to 30-50% drop ). At last I wrote a
> small patch for bttv (http://null.ru/people/hamster/bttv-drop.diff.gz)
> that completely fixed the problem. It was taken against 0.7.39,
> shouldn't be hard to port to newer version.
> 
> --
> Best regards,
>  Eugene
> mailto:divx@euro.ru or sparky@projectmayo.com
> [Team GADGET]  [Team Two Divided By Zero]

This fix works great for VCRs (where the timing drifts slightly), but,
unfortunately, does not help much for weak video signals (where the
timing is OK, but the levels are wrong).

-justin

PS Thanks for that patch, it was a life saver - I just couldn't remember
who submitted it!



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