[FFmpeg-trac] #3992(avformat:new): When streaming stereo opus audio over rtp, ffmpeg prints sdp info for a mono stream

[FFmpeg]

#3992: When streaming stereo opus audio over rtp, ffmpeg prints sdp info for a
mono stream
-------------------------------------+-------------------------------------
             Reporter:  CoRoNe       |                     Type:  defect
               Status:  new          |                 Priority:  normal
            Component:  avformat     |                  Version:
             Keywords:  opus rtp     |  unspecified
  sdp mono                           |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 How to reproduce:
 {{{
 ffmpeg -hide_banner -f dshow -channel_layout stereo -i audio="NVIDIA(R)
 nForce(TM)
 Audio" -c:a libopus -ab 96k -f rtp rtp://192.168.178.22:1234
 }}}
 {{{
 Input #0, dshow, from 'audio=NVIDIA(R) nForce(TM) Audio':
   Duration: N/A, start: 8066.233000, bitrate: 1411 kb/s
     Stream #0:0: Audio: pcm_s16le, 44100 Hz, stereo, s16, 1411 kb/s
 Output #0, rtp, to 'rtp://192.168.178.22:1234':
   Metadata:
     encoder         : Lavf56.7.101
     Stream #0:0: Audio: opus (libopus), 48000 Hz, stereo, s16, 96 kb/s
     Metadata:
       encoder         : Lavc56.1.101 libopus
 Stream mapping:
   Stream #0:0 -> #0:0 (pcm_s16le (native) -> opus (libopus))
 SDP:
 v=0
 o=- 0 0 IN IP4 127.0.0.1
 s=No Name
 c=IN IP4 192.168.178.22
 t=0 0
 a=tool:libavformat 56.7.101
 m=audio 1234 RTP/AVP 97
 b=AS:96
 a=rtpmap:97 opus/48000

 Press [q] to stop, [?] for help
 [libopus @ 02f26480] Queue input is backward in time/s
 size=      59kB time=00:00:04.49 bitrate= 106.8kbits/s
 video:0kB audio:56kB subtitle:0kB other streams:0kB global headers:0kB
 muxing
 overhead: 4.765315%
 }}}
 When I copy-paste and save the SDP info to a sdp-file and open it with
 ffplay.exe (or MPC-HC) the stereo opus stream has become mono.
 When I add /2 to the end of the sdp-file, it's stereo again.
 {{{
 a=rtpmap:97 opus/48000/2
 }}}
 I've read the comment in
 [http://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavformat/sdp.c#l600
 libavformat/sdp.c#l600], but I still don't understand why you default to
 mono in this case.

--
Ticket URL: <https://trac.ffmpeg.org/ticket/3992>
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