#4954: joint stereo indicator missing from mp3 headers -------------------------------------+------------------------------------- Reporter: pico4743 | Type: defect Status: new | Priority: normal Component: avformat | Version: git- Keywords: mp3 | master joint_stereo header | Blocked By: Blocking: | Reproduced by developer: 0 Analyzed by developer: 0 | -------------------------------------+------------------------------------- Summary of the bug: mp3 header files created by ffmpeg do not specify a channel mode of joint_stereo when joint_stereo is performed by libmp3lame. Details: libmp3lame provides a switch (-joint_stereo) to select either joint_stereo or plain stereo. The interface to lame works correctly so that the audio frames are fine, but ffmpeg doesn't use the lame library to write the mp3 header. Instead libavformat/mp3enc.c sets the channel mode to either MONO or STEREO based on how many channels there are. {{{ switch (codec->channels) { case 1: channels = MPA_MONO; break; case 2: channels = MPA_STEREO; break; default: av_log(s, AV_LOG_WARNING, "Unsupported number of channels, " "not writing Xing header.\n"); return -1; } ... header |= channels << 6; }}} MPA_JSTEREO should be used when appropriate. The repair doesn't seem simple, but I'd be willing to help if someone wants to discuss approaches. How to reproduce: {{{ use a version with libmp3lame enabled % ffmpeg -i input.mp3 -joint_stereo 1 output.mp3 The relevant bits of the output.mp3 header will be 0 == MPA_STEREO ffmpeg version -- git master fb4d350 }}} -- Ticket URL: <https://trac.ffmpeg.org/ticket/4954> FFmpeg <https://ffmpeg.org> FFmpeg issue tracker