joint stereo indicator missing from mp3 headers

[pico4743]

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

[GraniteStateColin]

This problem is still a problem. It's very easy to reproduce per pico4743's explanation above.

Another way to reproduce: Install the current released version of ffmpeg for Windows (don't think the OS matters, but that's what I happened to be using) 3.3.2 as of this post. Convert a file from WAV to MP3 using the '-joint stereo 1' flag. Test the status of the file with Mp3tag -- shows Stereo instead of Joint Stereo. Note that the same LAME library used in Audacity correctly sets the file to Joint Stereo.

In my case, I was banging my head against this before finding this bug report which confirms the problem I was seeing. Basically, the -joint_stereo switch is useless, because anything playing the file thinks it's still in stereo, and NOT joint stereo. Programs like Mp3tag report the output files as Stereo and not as Joint Stereo making this easy to test and validate.