[mythtv-commits] Ticket #11328: HD-PVR recordings can have a long initial sequence with no keyframes, confusing ffmpeg

[MythTV]

#11328: HD-PVR recordings can have a long initial sequence with no keyframes,
confusing ffmpeg
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     Reporter:  mythwiki@…            |      Owner:  danielk
         Type:  Bug Report - General  |     Status:  new
     Priority:  minor                 |  Milestone:  unknown
    Component:  MythTV - Recording    |    Version:  Unspecified
     Severity:  low                   |   Keywords:  hd-pvr, ffmpeg,
Ticket locked:  0                     |  scripts
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 I'm speculating a bit about the cause of the problem, but it seems likely
 to be due to a recording having a long time before the first keyframe is
 encountered.  The HD-PVR issues a keyframe every 3 or 4 seconds.  On some,
 but not all recordings, ffmpeg has difficulty reading the file.  This
 manifests itself in two ways:
 - ffprobe returns nonsensical bitrates, causing it to believe that the
 recording is about 1/4 the true length, which leads to problems with
 scripts that use ffprobe to determine the total length of a recording
 - ffmpeg decides that the file doesn't contain pts values, so it makes up
 its own.  However, when ffmpeg creates its own pts for output, it starts
 at a time of 1.0 seconds, rather than the 0.0 seconds that the HD-PVR
 produces in its pts values.  For scripts that need precise control over
 time, this leads to an uncertainty of 1 second in the position of frames.

 I suspect these problems would go away if the HD-PVR recording loop
 discarded frames before the first keyframe.  I don't know if this is a
 practical solution, but it might be helpful to try.

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Ticket URL: <http://code.mythtv.org/trac/ticket/11328>
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