[mythtv] Sound a second behind video

Yeechang Lee ylee at pobox.com
Thu Mar 13 14:44:31 UTC 2008

[Crossposted to mythtv-dev; please trim as appropriate when replying.]

Michael T. Dean <mtdean at thirdcontact.com> says:
> So, since you and others have found that ffplay doesn't work well
> (and I just verified--in ffplay sound played almost a second before
> the video in the commercials, but was in sync during the show),

I've seen the issue Mark describes, or something like it, since first
building my MythTV box more than two years ago. I only have digital
MPEG-2 sources via FireWire and OTA. Unlike Mark, however, I don't
recall seeing it in SD over FireWire (certainly not in analog-channel
cable recordings; I still haven't watched enough from digital cable
channels to be able to say one way or another). What I *have* seen is
what I described at <URL:http://svn.mythtv.org/trac/ticket/1940> in
June 2006. Contrary to what I wrote there, I have since seen the issue
in pillar- or letterboxed SD commercials in the middle of HD
recordings. I never pursued it further because with commflagging I
just never see enough commercials for it to matter to me.

I suspect the issue is tied in some way to the (very) longstanding
issue at <URL:http://svn.mythtv.org/trac/ticket/799>. Below I list the
various recording types (all MPEG-2 over FireWire and OTA, again) I've

1 Elapsed time according to the internal player equals actual
  recording time. All 720p recordings from ABC and FOX and all/most
  1080i recordings from CBS, The CW, HDNet, and HDNet Movies fit in
  this category. From these sources, all recordings have 29.97 frames
  per second. No playback issues whatsoever.
2 Elapsed time equals about 80% of recorded time. All 1080i recordings
  from the premium-movie channels (HBO, Showtime, etc.) are like this;
  so are recordings from many SD channels. Many comments for #799 from
  me and others describe this issue. Playback is fine, except
  occasionally (perhaps a few times per movie) the video will "speed
  up" for a second or two. Audio does not seem to be obviously
  affected. Skipping back more than a few seconds and playing through
  the portion again will generally (not always) reproduce the problem.
3 Elapsed time equals about 86% of recorded time (e.g., 52-53 minutes
  for a 60-minute recording). Most NBC recordings fit into this
  category. Discussed at
  Similar playback issues as with 2. Also, audio/video sync issues
  during many non-HD commercial breaks. I'm pretty sure it's the same
  problem as . . .
4 Elapsed time is about 30 seconds to two or three minutes below
  recorded time. Most NGC and Discovery HD, and many PBS recordings,
  fit here. I suspect the only difference between this and 4 is that
  NBC airs a lot more commercials.
5 Like 4, but applies to Leno or Conan on NBC. The issue here seems
  reversed; the program itself has proper 29.97fps frames, but
  commercials and other non-HD segments (like a clip from a movie a
  guest is promoting) may not. Audio/video often gets desynced during
  such portions, and the desync will continue when the HD portion
  resumes; pausing/unpausing or skipping around will cure the problem.
6 Elapsed time is about 66-68% of recorded time (e.g., 41-42 minutes
  per hour recording). The newest variant, which I've seen on two
  recent NGC recordings. Playback is fine, but the issue I note for 2
  happens slightly more frequently (still not frequently enough to be
  a serious problem), and audio during these bursts does also get
  speed up, before both return to normal a second or two later.

Note that rebuilding the seektable via 'mythtranscode -k -b' does
nothing to change the situation in any of the above cases. Could the
issues in 2 through 5, as well as #799 in general, be solved--albeit
in a ham-handed way--by ignoring the built-in frame data entirely, and
if necessary generate a new seektable that goes by actual recording
time only?

Frontend:		P4 3.0GHz, 1.5TB software RAID 5 array
Backend:		Quad-core Xeon 1.6GHz, 6.6TB sw RAID 6
Video inputs:		Four high-definition over FireWire/OTA
Accessories:		47" 1080p LCD, 5.1 digital, and MX-600

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