[FFmpeg-user] Bad A/V Sync Using ffmpeg on V4L2 Input Device

Leo L. Schwab ewhac at ewhac.org
Mon Mar 24 09:27:00 CET 2014

	I actually contributed to a bug report on this issue a year and a
half ago ( https://trac.ffmpeg.org/ticket/692#comment:15 ).  Basically, the
problem is that using ffmpeg to record from a USB video capture device
leaves the audio and video tracks out of sync.  Depending on which input you
list on the command line first, the sync problem will be different.

	Since contributing to that report, ffmpeg has undergone many
revisions, and the bug report has been updated to say that the problem was
fixed by adding new command line options to the V4L2 indev ("-ts
default|abs|mono2abs").  However, I have no idea what these new options mean
or how they're meant to be used.  I tried again using ffmpeg to record from
the video capture device.  While the audio and video aren't as badly out of
sync as they were 18 months ago, they're still out of sync.

	Clearly, someone thinks this is fixed, leading me to think that I
need more magic on the command line.  Here's my latest incantation:

	ffmpeg  -f alsa -i hw:1  -f v4l2 -i /dev/video0  -acodec libfaac -b:a 128k -vcodec libx264 -b:v 12M -g 1 -preset ultrafast  SYNCTEST.mp4

	Adding various -ts options to the line doesn't seem to accomplish
anything.  The USB video capture device is a Pinnacle Dazzle DVC 100.  I'm
using ffmpeg version 2.1.4.  The console output from a sample capture
session is appended below.

	Thanks in advance for any further insights.


$ ffmpeg  -f alsa -i hw:1  -f v4l2 -i /dev/video0  -acodec libfaac -b:a 128k -vcodec libx264 -b:v 12M -g 1 -preset ultrafast  SYNCTEST.mp4
ffmpeg version 2.1.4 Copyright (c) 2000-2014 the FFmpeg developers
  built on Feb 24 2014 08:21:48 with gcc 4.8 (Debian 4.8.2-16)
  configuration: --prefix=/usr --extra-cflags='-g -O2 -fstack-protector --param=ssp-buffer-size=4 -Wformat -Werror=format-security ' --extra-ldflags='-Wl,-z,relro' --cc='ccache cc' --enable-shared --enable-libmp3lame --enable-gpl --enable-nonfree --enable-libvorbis --enable-pthreads --enable-libfaac --enable-libxvid --enable-postproc --enable-x11grab --enable-libgsm --enable-libtheora --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libx264 --enable-libspeex --enable-nonfree --disable-stripping --enable-libvpx --enable-libschroedinger --disable-encoder=libschroedinger --enable-version3 --enable-libopenjpeg --enable-librtmp --enable-avfilter --enable-libfreetype --enable-libvo-aacenc --disable-decoder=amrnb --enable-libvo-amrwbenc --enable-libaacplus --libdir=/usr/lib/x86_64-linux-gnu --disable-vda --enable-libbluray --enable-libcdio --enable-gnutls --enable-frei0r --enable-openssl --enable-libass --enable-libopus --enable-fontconfig --enable-libpulse --disable-mips32r2 --disable-mipsdspr1 --disable-mipsdspr2 --enable-libvidstab --enable-libzvbi --enable-avresample --enable-libiec61883 --enable-libfdk-aac --enable-vaapi --enable-libdc1394 --disable-altivec --disable-armv5te --disable-armv6 --disable-vis --shlibdir=/usr/lib/x86_64-linux-gnu
  libavutil      52. 48.101 / 52. 48.101
  libavcodec     55. 39.101 / 55. 39.101
  libavformat    55. 19.104 / 55. 19.104
  libavdevice    55.  5.100 / 55.  5.100
  libavfilter     3. 90.100 /  3. 90.100
  libavresample   1.  1.  0 /  1.  1.  0
  libswscale      2.  5.101 /  2.  5.101
  libswresample   0. 17.104 /  0. 17.104
  libpostproc    52.  3.100 / 52.  3.100
Guessed Channel Layout for  Input Stream #0.0 : stereo
Input #0, alsa, from 'hw:1':
  Duration: N/A, start: 1394670862.629223, bitrate: 1536 kb/s
    Stream #0:0: Audio: pcm_s16le, 48000 Hz, stereo, s16, 1536 kb/s
Input #1, video4linux2,v4l2, from '/dev/video0':
  Duration: N/A, start: 575244.424820, bitrate: 165722 kb/s
    Stream #1:0: Video: rawvideo (YUY2 / 0x32595559), yuyv422, 720x480, 165722 kb/s, 29.97 fps, 29.97 tbr, 1000k tbn, 1000k tbc
No pixel format specified, yuv422p for H.264 encoding chosen.
Use -pix_fmt yuv420p for compatibility with outdated media players.
[libx264 @ 0x1d1b0a0] using cpu capabilities: MMX2 SSE2Slow SlowCTZ
[libx264 @ 0x1d1b0a0] profile High 4:2:2 Intra, level 3.0, 4:2:2 8-bit
[libx264 @ 0x1d1b0a0] 264 - core 142 - H.264/MPEG-4 AVC codec - Copyleft 2003-2014 - http://www.videolan.org/x264.html - options: cabac=0 ref=1 deblock=0:0:0 analyse=0:0 me=dia subme=0 psy=1 psy_rd=1.00:0.00 mixed_ref=0 me_range=16 chroma_me=1 trellis=0 8x8dct=0 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=0 threads=3 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=0 weightp=0 keyint=1 keyint_min=1 scenecut=0 intra_refresh=0 rc=abr mbtree=0 bitrate=12000 ratetol=1.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=0
Output #0, mp4, to 'SYNCTEST.mp4':
    encoder         : Lavf55.19.104
    Stream #0:0: Video: h264 (libx264) ([33][0][0][0] / 0x0021), yuv422p, 720x480, q=-1--1, 12000 kb/s, 30k tbn, 29.97 tbc
    Stream #0:1: Audio: aac (libfaac) ([64][0][0][0] / 0x0040), 48000 Hz, stereo, s16, 128 kb/s
Stream mapping:
  Stream #1:0 -> #0:0 (rawvideo -> libx264)
  Stream #0:0 -> #0:1 (pcm_s16le -> libfaac)
Press [q] to stop, [?] for help
frame=  436 fps= 30 q=-1.0 Lsize=     706kB time=00:00:14.91 bitrate= 387.9kbits/s dup=0 drop=4    
video:448kB audio:240kB subtitle:0 global headers:0kB muxing overhead 2.682203%
[libx264 @ 0x1d1b0a0] frame I:436   Avg QP: 0.00  size:  1051
[libx264 @ 0x1d1b0a0] mb I  I16..4: 100.0%  0.0%  0.0%
[libx264 @ 0x1d1b0a0] final ratefactor: -25.57
[libx264 @ 0x1d1b0a0] coded y,uvDC,uvAC intra: 0.0% 0.0% 0.0%
[libx264 @ 0x1d1b0a0] i16 v,h,dc,p: 97%  0%  3%  0%
[libx264 @ 0x1d1b0a0] i8c dc,h,v,p: 100%  0%  0%  0%
[libx264 @ 0x1d1b0a0] kb/s:252.03

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