[mythtv-commits] Ticket #6982: FF/Rew speed is way off for HD-PVR recordings

[MythTV]

#6982: FF/Rew speed is way off for HD-PVR recordings
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 Reporter:  gigem             |       Owner:  gigem  
     Type:  defect            |      Status:  new    
 Priority:  minor             |   Milestone:  unknown
Component:  MythTV - General  |     Version:  unknown
 Severity:  medium            |     Mlocked:  0      
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 The actual FF/Rew speed for HD-PVR recordings is much higher than the
 intended speed.   This is actually a long standing problem that affects
 most recordings, but is most apparent with HD-PVR recordings due to their
 exceptionally large keyframe distance.

 When fast forwarding and rewinding, NVP tries to synthesize the desired
 FF/Rew speed by using the actual keyframe distance to calculate how many
 frames to skip forward or backward in a nominal display interval of 200ms.
 The problem is that when all of the encoders were switched to generate by-
 frame position maps, the decoders were also changed to use and report an
 artificial keyframe distance of 1.

 For recordings with typical keyframes distances in the range of 12-15, the
 affect on NVP when calculating the skip count is minor, so I, and
 apparently nobody else, bothered to complain.  For HD-PVR recordings,
 however, the actual keyframe distance is fairly high and causes the
 calculated skip interval to be way too low.  In fact, for modest FF/Rew
 speeds, the calculated skip interval can be less than the actual keyframe
 distance!  The end result is a much higher actual FF/Rew speed than
 intended because the decoder seeks further than NVP expects.

 Unfortunately, the simple fix restoring the actual keyframe distance as
 detected by the decoders doesn't work.  It seems the use of the artificial
 keyframe distance of 1 is too closely intertwined with the by-frame
 position maps and other things.  Suggestions are welcome on how to either
 provide the actual keyframe distance to the FF/Rew code in NVP or untangle
 the by-frame position maps from the keyframe distance.

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Ticket URL: <http://svn.mythtv.org/trac/ticket/6982>
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