Durata audio errata con ffmpeg

Ho provato a convertire un file m4a in mp3 con ffmpeg, con il seguente command più semplice:

ffmpeg -i in.m4a out.mp3 

Dopo la conversione, la durata sembra strano. L'originale m4a era 4:06. Ma per il nuovo mp3, il Finder di OS X e il rapporto Google Play 09:24, mentre iTunes riport ancora il corretto 4:06.

Penso che ho anche incontrato questo problema diverse volte prima. Perchè è questo? Qualsiasi correzione?

Di seguito è la console output di ffmpeg , nel caso in cui farà qualche aiuto.

 $ffmpeg -i in.m4a out.mp3 ffmpeg version 1.2.1 Copyright (c) 2000-2013 the FFmpeg developers built on Jun 12 2013 13:46:11 with Apple clang version 4.1 (tags/Apple/clang-421.11.66) (based on LLVM 3.1svn) configuration: --prefix=/opt/local --enable-swscale --enable-avfilter --enable-libmp3lame --enable-libvorbis --enable-libopus --enable-libtheora --enable-libschroedinger --enable-libopenjpeg --enable-libmodplug --enable-libvpx --enable-libspeex --enable-libass --enable-libbluray --enable-gnutls --enable-libfreetype --mandir=/opt/local/share/man --enable-shared --enable-pthreads --cc=/usr/bin/clang --arch=x86_64 --enable-yasm --enable-gpl --enable-postproc --enable-libx264 --enable-libxvid libavutil 52. 18.100 / 52. 18.100 libavcodec 54. 92.100 / 54. 92.100 libavformat 54. 63.104 / 54. 63.104 libavdevice 54. 3.103 / 54. 3.103 libavfilter 3. 42.103 / 3. 42.103 libswscale 2. 2.100 / 2. 2.100 libswresample 0. 17.102 / 0. 17.102 libpostproc 52. 2.100 / 52. 2.100 [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fb7a4013000] stream 0, timescale not set [mov,mp4,m4a,3gp,3g2,mj2 @ 0x7fb7a4013000] max_analyze_duration 5000000 reached at 5015510 microseconds Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'in.m4a': Metadata: major_brand : M4A minor_version : 0 compatible_brands: M4A mp42isom creation_time : 2013-06-14 07:36:47 title : 小手拉大手artist : 梁静茹album_artist : 梁静茹album : 亲亲genre : Pop track : 7/12 disc : 1/1 date : 2006 gapless_playback: 0 encoder : iTunes 11.0.4 Duration: 00:04:06.75, start: 0.000000, bitrate: 804 kb/s Stream #0:0(und): Audio: alac (alac / 0x63616C61), 44100 Hz, stereo, s16, 800 kb/s Metadata: creation_time : 2013-06-14 07:36:47 Stream #0:1: Video: png, rgba, 230x230, 90k tbr, 90k tbn, 90k tbc [mp3 @ 0x7fb7a4044200] Frame rate very high for a muxer not efficiently supporting it. Please consider specifying a lower framerate, a different muxer or -vsync 2 Output #0, mp3, to 'out.mp3': Metadata: major_brand : M4A minor_version : 0 compatible_brands: M4A mp42isom gapless_playback: 0 TIT2 : 小手拉大手TPE1 : 梁静茹TPE2 : 梁静茹TALB : 亲亲TCON : Pop TRCK : 7/12 TPOS : 1/1 TDRL : 2006 TSSE : Lavf54.63.104 Stream #0:0: Video: png, rgba, 230x230, q=2-31, 200 kb/s, 90k tbn, 90k tbc Stream #0:1(und): Audio: mp3, 44100 Hz, stereo, s16p Metadata: creation_time : 2013-06-14 07:36:47 Stream mapping: Stream #0:1 -> #0:0 (png -> png) Stream #0:0 -> #0:1 (alac -> libmp3lame) Press [q] to stop, [?] for help frame= 1 fps=0.0 q=0.0 size= 0kB time=00:00:26.54 bitrate= 0.1kbits/sframe= 1 fps=1.0 q=0.0 size= 0kB time=00:00:53.57 bitrate= 0.0kbits/sframe= 1 fps=0.7 q=0.0 size= 0kB time=00:01:19.93 bitrate= 0.0kbits/sframe= 1 fps=0.5 q=0.0 size= 0kB time=00:01:47.07 bitrate= 0.0kbits/sframe= 1 fps=0.4 q=0.0 size= 0kB time=00:02:13.90 bitrate= 0.0kbits/sframe= 1 fps=0.3 q=0.0 size= 0kB time=00:02:40.57 bitrate= 0.0kbits/sframe= 1 fps=0.3 q=0.0 size= 0kB time=00:03:06.01 bitrate= 0.0kbits/sframe= 1 fps=0.2 q=0.0 size= 0kB time=00:03:31.56 bitrate= 0.0kbits/sframe= 1 fps=0.2 q=0.0 size= 0kB time=00:03:57.66 bitrate= 0.0kbits/sframe= 1 fps=0.2 q=0.0 Lsize= 3974kB time=00:04:06.75 bitrate= 131.9kbits/s video:118kB audio:3856kB subtitle:0 global headers:0kB muxing overhead 0.012681% 

Secondo # 2697: Durata dell'output MP3 non corretto in OS X , utilizzando l'opzione -write_xing 0 risolve il problema.