Audio sync issue

[burek]

From IRC channel:

<Adys> Trying to record from webcam and keep audio in sync, this has audio running behind by 3-4 seconds
<Adys> input comes from a separate usb mic if that makes any difference

% ffmpeg -f alsa -i pulse -f video4linux2 -s 800x600 -i /dev/video0 -sameq -acodec libmp3lame -ab 96k out.mkv
ffmpeg version N-40213-g9f98a8e Copyright (c) 2000-2012 the FFmpeg developers
  built on Apr 29 2012 16:47:10 with gcc 4.7.0 20120414 (prerelease)
  configuration: --enable-libx264 --enable-nonfree --enable-libmp3lame --enable-libpulse --enable-gpl --disable-outdev=oss
  libavutil      51. 48.100 / 51. 48.100
  libavcodec     54. 17.100 / 54. 17.100
  libavformat    54.  3.100 / 54.  3.100
  libavdevice    53.  4.100 / 53.  4.100
  libavfilter     2. 72.100 /  2. 72.100
  libswscale      2.  1.100 /  2.  1.100
  libswresample   0. 11.100 /  0. 11.100
  libpostproc    52.  0.100 / 52.  0.100
[alsa @ 0x279e100] Estimating duration from bitrate, this may be inaccurate
Input #0, alsa, from 'pulse':                                                                                                                                                                                                               
  Duration: N/A, start: 1335715213.308329, bitrate: 1536 kb/s
    Stream #0:0: Audio: pcm_s16le, 48000 Hz, 2 channels, s16, 1536 kb/s
[video4linux2,v4l2 @ 0x279b3a0] Estimating duration from bitrate, this may be inaccurate
Input #1, video4linux2,v4l2, from '/dev/video0':                                                                                                                                                                                            
  Duration: N/A, start: 67955.437449, bitrate: 153600 kb/s
    Stream #1:0: Video: rawvideo (YUY2 / 0x32595559), yuyv422, 800x600, 153600 kb/s, 20 tbr, 1000k tbn, 20 tbc
[buffer @ 0x27a5100] w:800 h:600 pixfmt:yuyv422 tb:1/1000000 sar:0/1 sws_param:flags=2
[buffersink @ 0x279c9e0] auto-inserting filter 'auto-inserted scale 0' between the filter 'src' and the filter 'out'
[scale @ 0x279d320] w:800 h:600 fmt:yuyv422 sar:0/1 -> w:800 h:600 fmt:yuv422p sar:0/1 flags:0x4
[libx264 @ 0x279a820] using cpu capabilities: MMX2 SSE2Fast SSSE3 FastShuffle SSE4.2
[libx264 @ 0x279a820] profile High 4:2:2, level 3.1, 4:2:2 8-bit
[libx264 @ 0x279a820] 264 - core 120 - H.264/MPEG-4 AVC codec - Copyleft 2003-2011 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=12 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=20 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Guessed Channel Layout for  Input Stream #0.0 : stereo
Output #0, matroska, to 'out.mkv':                                                                                                                                                                                                          
  Metadata:
    encoder         : Lavf54.3.100
    Stream #0:0: Video: h264, yuv422p, 800x600, q=-1--1, 1k tbn, 20 tbc
    Stream #0:1: Audio: mp3, 48000 Hz, stereo, s16, 96 kb/s
Stream mapping:
  Stream #1:0 -> #0:0 (rawvideo -> libx264)
  Stream #0:0 -> #0:1 (pcm_s16le -> libmp3lame)
Press [q] to stop, [?] for help
Input stream #0:0 frame changed from rate:48000 fmt:s16 ch:2 chl:0x0 to rate:48000 fmt:s16 ch:2 chl:0x3
frame=   65 fps= 13 q=-1.0 Lsize=     871kB time=00:00:04.40 bitrate=1622.4kbits/s    
video:782kB audio:87kB global headers:0kB muxing overhead 0.307903%
[libx264 @ 0x279a820] frame I:1     Avg QP:23.90  size: 34621
[libx264 @ 0x279a820] frame P:31    Avg QP:23.35  size: 17539
[libx264 @ 0x279a820] frame B:33    Avg QP:25.02  size:  6721
[libx264 @ 0x279a820] consecutive B-frames:  7.7% 64.6% 27.7%  0.0%
[libx264 @ 0x279a820] mb I  I16..4: 18.5% 60.6% 20.9%
[libx264 @ 0x279a820] mb P  I16..4:  1.7%  7.0%  1.4%  P16..4: 48.3% 18.1% 13.3%  0.0%  0.0%    skip:10.0%
[libx264 @ 0x279a820] mb B  I16..4:  0.3%  1.8%  0.3%  B16..8: 38.5%  4.8%  1.2%  direct: 7.8%  skip:45.3%  L0:41.9% L1:49.5% BI: 8.7%
[libx264 @ 0x279a820] 8x8 transform intra:67.8% inter:73.1%
[libx264 @ 0x279a820] coded y,uvDC,uvAC intra: 65.6% 95.6% 80.9% inter: 23.1% 58.6% 16.7%
[libx264 @ 0x279a820] i16 v,h,dc,p: 17% 11%  2% 69%
[libx264 @ 0x279a820] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 23% 12% 12%  8%  8% 11%  7% 11%  9%
[libx264 @ 0x279a820] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 25% 15% 12%  8%  9% 10%  7%  8%  6%
[libx264 @ 0x279a820] i8c dc,h,v,p: 45% 11% 24% 21%
[libx264 @ 0x279a820] Weighted P-Frames: Y:3.2% UV:3.2%
[libx264 @ 0x279a820] ref P L0: 53.4% 12.1% 22.5% 12.1%  0.0%
[libx264 @ 0x279a820] ref B L0: 73.9% 24.6%  1.5%
[libx264 @ 0x279a820] ref B L1: 95.1%  4.9%
[libx264 @ 0x279a820] kb/s:1422.43

<burek> try lowering your video resolution to 320x240 just to see if it works
<Adys> i get some weird speedup and lots of PTS 4254653, next:4716287 invalid dropping st:0

<burek> try: ffmpeg -f video4linux2 -s 320x240 -r 25 -i /dev/video0 -f alsa -i pulse -sameq -acodec libmp3lame -ab 96k out.mkv
<Adys> no errors but sound still lagging about 1 second behind

<Adys> using mp4 solves it, even works in 1280x720

So, there is some kind of bug, when using alsa+video4linux2, which might be investigated a little bit, since a lot of people have reported syncing issues when capturing inputs from webcams (using video4linux2) + alsa audio.

[burek]

btw, you could see there are several other people that reported syncing issues when using video4linux2 + alsa, by looking in irc logs, if you will:  http://ffmpeg.gusari.org/irclogs/

[cehoyos]

Isn't this a duplicate of ticket #692?

[burek]

I'm not sure if it is. I just posted it as a new issue, because it might not be related, since the guy solved his issue by choosing the mp4 output, which I didn't test in case of ticket 692. Also, he didn't have time/will to post the bug report, so I just copied the info from the irc channel, if anyone decides to test it himself.

[cehoyos]

I am closing this ticket because it does not contain any additional information over ticket #692 and no further tests are possible.