[Bug 751031] New: discover: report confusing number of channels when demuxer and decoder disagree

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=751031

            Bug ID: 751031
           Summary: discover: report confusing number of channels when
                    demuxer and decoder disagree
    Classification: Platform
           Product: GStreamer
           Version: git master
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: Normal
         Component: gst-plugins-base
          Assignee: gstreamer-bugs at lists.freedesktop.org
          Reporter: gdesmott at gnome.org
        QA Contact: gstreamer-bugs at lists.freedesktop.org
     GNOME version: ---

I wanted to watch the GoT season finale in surround and so was looking for a
release containing the 5.1 audio track. Unfortunately they were all reported as
"stereo" according to nautilus file's info screen so I finally decided to watch
game.of.thrones.s05e10.720p.hdtv.x264-0sec.mkv anyway. To my surprise, the
audio was actually in 5.1 and I've been able to enjoy the death of my favorite
characters with a great audio experience!

I took a second look at the media file and totem's media info screen was
actually reporting the audio track with 6 channels. mkvinfo agrees about it as
well as you can see:

| + A track
|  + Track number: 2 (track ID for mkvmerge & mkvextract: 1)
|  + Track UID: 2
|  + Lacing flag: 0
|  + Language: und
|  + Default flag: 0
|  + Codec ID: A_AC3
|  + Track type: audio
|  + Audio track
|   + Channels: 6
|   + Sampling frequency: 48000


but gst-discover seems confused about it. As you can see the caps reports 6
channels but the "Channels" field only 2:

    audio: audio/x-ac3, framed=(boolean)true, channels=(int)6, rate=(int)48000
      Tags:
        container format: Matroska
        encoder: Lavf56.36.100
        audio codec: AC-3 (ATSC A/52)
        language code: und
        bitrate: 384000

      Codec:
        audio/x-ac3, framed=(boolean)true, channels=(int)6, rate=(int)48000
      Additional info:
        None
      Stream ID:
d07ab0288a090e209bf7825305b2589cd3dcdbdded97dc2dc67c3238d7c529d3/002
      Language: und
      Channels: 2
      Sample rate: 48000
      Depth: 32
      Bitrate: 384000
      Max bitrate: 0


I did some digging and it seems that the demuxer detects 6 channels (hence the
caps and mkvinfo being correct) but the first audio frame(s) are actually
stereo!

0:00:00.239137410 32334 0x7f6ba40cf680 INFO                  a52dec
gsta52dec.c:432:gst_a52dec_reneg:<a52dec0> reneg channels:2 rate:48000
...
0:00:00.291923950 32334 0x7f6ba40cf680 DEBUG                 a52dec
gsta52dec.c:508:gst_a52dec_handle_frame:<a52dec0> stream channel flags changed,
marking update
0:00:00.293877646 32334 0x7f6ba40cf680 INFO                  a52dec
gsta52dec.c:432:gst_a52dec_reneg:<a52dec0> reneg channels:6 rate:48000

I'm not sure what's the proper answser here. I guess it makes sense to trust
the audio decoder over the demuxer and we can't really read the whole file when
discovering. On the other hand, it seems weird that the number of channels
reported by GstDiscover doesn't match the one displayed in caps.

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