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Editorial Team
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Asked: 2026-05-28T04:58:32+00:00

I am trying to convert a flv video to mp4 using a264 , but

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I am trying to convert a flv video to mp4 using a264, but the result video has a out of sync audio/video. The audio seems ok but the video seems slow and therefore out of sync with the audio.

Here is the command I am running:

/opt/local/bin/ffmpeg -i complaint.flv -f mp4 -vcodec libx264 -acodec aac -copyts -strict experimental -fpre /opt/local/share/ffmpeg/libx264-lossless_slow.ffpreset -ab 44k -threads 0 -crf 23 complaint.mp4

And the output: 

Desktop  /opt/local/bin/ffmpeg -i complaint.flv -f mp4 -vcodec libx264 -acodec aac -copyts -strict experimental -fpre /opt/local/share/ffmpeg/libx264-lossless_slow.ffpreset -ab 44k -threads 0 -crf 23 complaint.mp4
ffmpeg version 0.7.8, Copyright (c) 2000-2011 the FFmpeg developers
  built on Jan 12 2012 21:12:26 with gcc 4.2.1 (Apple Inc. build 5666) (dot 3)
  configuration: --prefix=/opt/local --enable-gpl --enable-postproc --enable-swscale --enable-avfilter --enable-libmp3lame --enable-libvorbis --enable-libtheora --enable-libdirac --enable-libschroedinger --enable-libopenjpeg --enable-libxvid --enable-libx264 --enable-libvpx --enable-libspeex --mandir=/opt/local/share/man --enable-shared --enable-pthreads --cc=/usr/bin/gcc-4.2 --arch=x86_64 --enable-yasm
  libavutil    50. 43. 0 / 50. 43. 0
  libavcodec   52.123. 0 / 52.123. 0
  libavformat  52.111. 0 / 52.111. 0
  libavdevice  52.  5. 0 / 52.  5. 0
  libavfilter   1. 80. 0 /  1. 80. 0
  libswscale    0. 14. 1 /  0. 14. 1
  libpostproc  51.  2. 0 / 51.  2. 0
[flv @ 0x7fe57a04ac00] Estimating duration from bitrate, this may be inaccurate
Input #0, flv, from 'complaint.flv':
  Metadata:
    duration        : 7
    creationdate    : Thu Jan 12 23:47:24
    description     : Recorded using VideoRecording example.
  Duration: 00:00:06.61, start: 0.000000, bitrate: N/A
    Stream #0.0: Video: flv, yuv420p, 288x288, 1k tbr, 1k tbn, 1k tbc
    Stream #0.1: Audio: nellymoser, 11025 Hz, mono, s16
File 'complaint.mp4' already exists. Overwrite ? [y/N] y
[buffer @ 0x7fe579c32cc0] w:288 h:288 pixfmt:yuv420p tb:1/1000000 sar:0/1 sws_param:
[libx264 @ 0x7fe57a044600] using cpu capabilities: none!
[libx264 @ 0x7fe57a044600] profile High, level 4.2
[libx264 @ 0x7fe57a044600] 264 - core 118 - H.264/MPEG-4 AVC codec - Copyleft 2003-2011 - http://www.videolan.org/x264.html - options: cabac=1 ref=2 deblock=1:0:0 analyse=0x3:0x33 me=umh subme=6 psy=1 psy_rd=1.00:0.00 mixed_ref=0 me_range=16 chroma_me=1 trellis=0 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=6 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=0 weightp=2 keyint=250 keyint_min=25 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=1.41 aq=1:1.00
Output #0, mp4, to 'complaint.mp4':
  Metadata:
    duration        : 7
    creationdate    : Thu Jan 12 23:47:24
    description     : Recorded using VideoRecording example.
    encoder         : Lavf52.111.0
    Stream #0.0: Video: libx264, yuv420p, 288x288, q=0-69, 200 kb/s, 1k tbn, 1k tbc
    Stream #0.1: Audio: aac, 11025 Hz, mono, s16, 44 kb/s
Stream mapping:
  Stream #0.0 -> #0.0
  Stream #0.1 -> #0.1
Press [q] to stop, [?] for help
frame= 6557 fps=106 q=-1.0 Lsize=     685kB time=00:00:03.62 bitrate=1549.1kbits/s dup=6367 drop=0    
video:615kB audio:17kB global headers:0kB muxing overhead 8.361360%
frame I:27    Avg QP:19.98  size:  8242
[libx264 @ 0x7fe57a044600] frame P:6530  Avg QP:30.71  size:    62
[libx264 @ 0x7fe57a044600] mb I  I16..4: 11.1% 79.3%  9.6%
[libx264 @ 0x7fe57a044600] mb P  I16..4:  0.1%  0.2%  0.0%  P16..4:  1.2%  0.2%  0.3%  0.0%  0.0%    skip:98.0%
[libx264 @ 0x7fe57a044600] 8x8 transform intra:77.8% inter:64.9%
[libx264 @ 0x7fe57a044600] coded y,uvDC,uvAC intra: 75.3% 81.3% 33.3% inter: 0.5% 0.7% 0.1%
[libx264 @ 0x7fe57a044600] i16 v,h,dc,p: 78%  9% 10%  3%
[libx264 @ 0x7fe57a044600] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 35% 11% 32%  3%  4%  5%  2%  5%  4%
[libx264 @ 0x7fe57a044600] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 56%  7%  9%  3%  4%  7%  3%  7%  3%
[libx264 @ 0x7fe57a044600] i8c dc,h,v,p: 45% 15% 34%  6%
[libx264 @ 0x7fe57a044600] Weighted P-Frames: Y:0.0% UV:0.0%
[libx264 @ 0x7fe57a044600] ref P L0: 80.5%  9.5% 10.0%
[libx264 @ 0x7fe57a044600] kb/s:767.20

Is there something I’m missing to make sure the video and audio line up perfectly?

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1 Answer

  1. Editorial Team

    For those who may be trying this, I was better of recording flash in h264 right away, instead of using yuv420p for audio codec. That worked like a charm. Adding the flag -async 1 also helped.

    Update: Check out this bit from the ffmpeg documentation (http://ffmpeg.org/ffmpeg.html):

    -async: Audio sync method. “Stretches/squeezes” the audio stream to match the timestamps, the parameter is the maximum samples per second by which the audio is changed. -async 1 is a special case where only the start of the audio stream is corrected without any later correction. This option has been deprecated. Use the aresample audio filter instead.

    You can read about the aresample filter here: http://ffmpeg.org/trac/ffmpeg/wiki/FilteringGuide

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