[MPlayer-dev-eng] solution of av desync on capturing with mencoder
jarekczek at poczta.onet.pl
Sun Jun 12 09:57:08 CEST 2011
After years of using mencoder to capture and encode videos I found a way
to achieve a/v synchronization which is better than the current mencoder
solution. Instead of adjusting video I suggest adjusting audio. I have 2
patches to mencoder, which I'll post if I get a positive receive.
Capturing from v4l2 and alsa. The sound card has a different timer than
the video frames timer which results in a skew between audio and video.
Mencoder currently drops or duplicates frames when it detects the time
difference. This solution synchronizes audio with video but has the
1. Duplicated or dropped frames produce jerky videos, especially on
2. Sound card can produce strange fluctuation which forces mencoder to
duplicate and insert frames in a short period which spoils the video
quality even more. If a 1 hour video should have 10 duplicated frames,
instead it has 60 duplicated and 50 inserted frames.
Adjusting sound to video seems more reasonable to me. So we need to
capture with -noskip and scale the audio to match the video duration.
What do we need?
1. Calculation of the audiorate
2. Correction of the initial skew between audio and video. If I turn on
-noskip, then I have initial skew between audio and video of about 0.1s.
With skipping allowed mencoder would fix this.
The user makes the audio adjustment in his script using sox or the
native scaletempo filter.
Patch for 1.
Report audiorate in status line. I suggest introducing -reportaudiorate
switch. The status line could then look like this:
Pos:3569.4s 89234f ( 0%) 25.00fps Trem: 0min 0mb tc:-0.513
r is the audiorate that should be passed to an audio scaler.
After finished encoding a following line is also printed:
duplicated: 2, bad: 0, skipped: 0, v_timer_corr: -0.523, audio_scale:
The audio_scale here is the final audio rate.
Patch for 2.
The switch -noskipafter would allow mencoder to skip frames during the
first 2 seconds of encoding. After 2 seconds it behaves as if -noskip
ADVANTAGES OF A NEW SOLUTION
Clean video without any corrections. No noticeable sound quality loss,
at least using sox. No noticeable a/v desync on my sound card.
Is there a chance for commiting these 2?
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