I'm trying to remove a superfluous audio track from a movie file. Specifically, this movie on archive.org has the original audio as track 2, and what appears to be a slavic (Russian?) voiceover in track 1. I'd like to get rid of the latter.
I found this question that suggested I should do the following:
ffmpeg -i Benjamín\ dúfa.avi -map 0 -map -0:a:0 -c copy Benjamín\ dúfa\ \(aðeins\ íslenskt\ tal\).avi
But this doesn't work: after removing the first audio track, audio and video are not in sync in the resulting file anymore (when played in VLC). Now, I was able to overcome this problem by also reencoding the remaining audio track along the way, using
ffmpeg -i Benjamín\ dúfa.avi -map 0 -map -0:a:0 -c:v copy Benjamín\ dúfa\ \(aðeins\ íslenskt\ tal\).avi
But while this works, I'd like to understand why copying the audio track instead does not. I suppose it's a somewhat philosophical question --- consider it a matter of intellectual curiosity, combined with an aversion to needless lossy re-encoding.
So if anyone can explain this to me, I'd very grateful. Thanks!
EDIT: as per Gyan's request, here's ffmpeg's output for the first command:
ffmpeg version 4.2 Copyright (c) 2000-2019 the FFmpeg developers
built with gcc 9.1.1 (GCC) 20190807
configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libdav1d --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth --enable-libopenmpt
libavutil 56. 31.100 / 56. 31.100
libavcodec 58. 54.100 / 58. 54.100
libavformat 58. 29.100 / 58. 29.100
libavdevice 58. 8.100 / 58. 8.100
libavfilter 7. 57.100 / 7. 57.100
libswscale 5. 5.100 / 5. 5.100
libswresample 3. 5.100 / 3. 5.100
libpostproc 55. 5.100 / 55. 5.100
Input #0, avi, from 'Benjamín dúfa.avi':
Metadata:
encoder : VirtualDubModRus 1.5.10.2 (build 2542/release)
IAS1 : Islenska
Duration: 01:27:37.00, start: 0.000000, bitrate: 1313 kb/s
Stream #0:0: Video: mpeg4 (Advanced Simple Profile) (XVID / 0x44495658), yuv420p, 720x400 [SAR 1:1 DAR 9:5], 979 kb/s, 25 fps, 25 tbr, 25 tbn, 25 tbc
Stream #0:1: Audio: mp3 (U[0][0][0] / 0x0055), 48000 Hz, stereo, fltp, 192 kb/s
Stream #0:2: Audio: mp3 (U[0][0][0] / 0x0055), 48000 Hz, stereo, fltp, 128 kb/s
Output #0, avi, to 'b.avi':
Metadata:
IAS1 : Islenska
ISFT : Lavf58.29.100
Stream #0:0: Video: mpeg4 (Advanced Simple Profile) (XVID / 0x44495658), yuv420p, 720x400 [SAR 1:1 DAR 9:5], q=2-31, 979 kb/s, 25 fps, 25 tbr, 25 tbn, 25 tbc
Stream #0:1: Audio: mp3 (U[0][0][0] / 0x0055), 48000 Hz, stereo, fltp, 128 kb/s
Stream mapping:
Stream #0:0 -> #0:0 (copy)
Stream #0:2 -> #0:1 (copy)
Press [q] to stop, [?] for help
[avi @ 0000000002bf00c0] Timestamps are unset in a packet for stream 0. This is deprecated and will stop working in the future. Fix your code to set the timestamps properly
frame=131425 fps=7616 q=-1.0 Lsize= 718952kB time=01:27:37.00 bitrate=1120.3kbits/s speed= 305x
video:628523kB audio:82141kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 1.166227%
And also for the second, for comparison:
ffmpeg version 4.2 Copyright (c) 2000-2019 the FFmpeg developers
built with gcc 9.1.1 (GCC) 20190807
configuration: --enable-gpl --enable-version3 --enable-sdl2 --enable-fontconfig --enable-gnutls --enable-iconv --enable-libass --enable-libdav1d --enable-libbluray --enable-libfreetype --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libtheora --enable-libtwolame --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libzimg --enable-lzma --enable-zlib --enable-gmp --enable-libvidstab --enable-libvorbis --enable-libvo-amrwbenc --enable-libmysofa --enable-libspeex --enable-libxvid --enable-libaom --enable-libmfx --enable-amf --enable-ffnvcodec --enable-cuvid --enable-d3d11va --enable-nvenc --enable-nvdec --enable-dxva2 --enable-avisynth --enable-libopenmpt
libavutil 56. 31.100 / 56. 31.100
libavcodec 58. 54.100 / 58. 54.100
libavformat 58. 29.100 / 58. 29.100
libavdevice 58. 8.100 / 58. 8.100
libavfilter 7. 57.100 / 7. 57.100
libswscale 5. 5.100 / 5. 5.100
libswresample 3. 5.100 / 3. 5.100
libpostproc 55. 5.100 / 55. 5.100
Input #0, avi, from 'Benjamín dúfa.avi':
Metadata:
encoder : VirtualDubModRus 1.5.10.2 (build 2542/release)
IAS1 : Islenska
Duration: 01:27:37.00, start: 0.000000, bitrate: 1313 kb/s
Stream #0:0: Video: mpeg4 (Advanced Simple Profile) (XVID / 0x44495658), yuv420p, 720x400 [SAR 1:1 DAR 9:5], 979 kb/s, 25 fps, 25 tbr, 25 tbn, 25 tbc
Stream #0:1: Audio: mp3 (U[0][0][0] / 0x0055), 48000 Hz, stereo, fltp, 192 kb/s
Stream #0:2: Audio: mp3 (U[0][0][0] / 0x0055), 48000 Hz, stereo, fltp, 128 kb/s
Stream mapping:
Stream #0:0 -> #0:0 (copy)
Stream #0:2 -> #0:1 (mp3 (mp3float) -> mp3 (libmp3lame))
Press [q] to stop, [?] for help
Output #0, avi, to 'b2.avi':
Metadata:
IAS1 : Islenska
ISFT : Lavf58.29.100
Stream #0:0: Video: mpeg4 (Advanced Simple Profile) (XVID / 0x44495658), yuv420p, 720x400 [SAR 1:1 DAR 9:5], q=2-31, 979 kb/s, 25 fps, 25 tbr, 25 tbn, 25 tbc
Stream #0:1: Audio: mp3 (libmp3lame) (U[0][0][0] / 0x0055), 48000 Hz, stereo, fltp
Metadata:
encoder : Lavc58.54.100 libmp3lame
[avi @ 00000000004b7440] Timestamps are unset in a packet for stream 0. This is deprecated and will stop working in the future. Fix your code to set the timestamps properly
frame=131425 fps=1090 q=-1.0 Lsize= 718952kB time=01:27:37.00 bitrate=1120.3kbits/s speed=43.6x
video:628523kB audio:82141kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 1.166233%