multiple audio channels for RTSP and SDP - BUG?

[Kathmandu]

Hello,

First of all a big thank for the work! I like it very much!

Next, maybe the problem I fell upon is known but I could not find any traces of it!

So here is my contribution :

I work on windows XP, and use VLC 0.8.6i, but I don't think the problem is specific to windows.

I have a streamer (a multicast one) that is able to answer to RTSP request or that distribute a SDP file (via FTP).

The SDP file (that is closely the same as thans answer to a RTSP describe request) is the following (IP addresses are masked):

v=0
o=- 1216198939 1216198939 IN IP4 XYZ.A.B.C
s=test
u=rtsp://XYZ.A.B.C/test
c=IN IP4 UVW.Q.R.S/3
b=AS:184
t=0 0
a=control:*
a=range:npt=now-
m=audio 6000 RTP/AVP 97
b=AS:144
b=RR:5400
b=RS:1800
a=rtpmap:97 mpeg4-generic/24000/2
a=fmtp:97 streamType=5;profile-level-id=41;mode=AAC-hbr;sizelength=13;indexlength=3;indexdeltalength=3;config=1190
a=control:trackID=1
a=lang:eng
a=TIAS:128000
a=maxprate:50
m=audio 6002 RTP/AVP 98
b=AS:40
b=RR:1500
b=RS:500
a=rtpmap:98 mpeg4-generic/24000
a=fmtp:98 streamType=5;profile-level-id=40;mode=AAC-hbr;sizelength=13;indexlength=3;indexdeltalength=3;config=1308
a=control:trackID=2
a=lang:fra
a=TIAS:24000
a=maxprate:50

As you can see only 2 audio streams (I can have video but for the sake of shortness ...) XYZ.A.B.C is a unicast address (the one of my streaming equipment) UVW.Q.R.S is the multicast destination of the streams.

When I connect (either by RTSP or using the SDP file) VLC correctly plays the first audio track, and shows me that there is a second.
But when I try to access the second track => no audio at all.

Now the interesting part : If I replace every 98 by 97 in a copy of the SDP file (so having 2 streams with the same dynamic map number, which is not correct (as far as I correctly understood the SDP RFC)) I can play any of the audio track correctly!

It seems to me that 97 is considered to be the specific number for audio whereas it is only one of the dynamic number that can be used to describe a media in the rtpmap.

Please correct me if I am wrong or tell me if VLC will be corrected to support this.

Thanks.

D. HARRANG


PS : except for my hollidays to come, I am ready to help any developper interested in tracking this bug(if it is).

[Rémi Denis-Courmont]

I am afraid that this cannot be supported properly. We'd need the SDP grouping framework or something to associate semantics witht he multiple channels, and that is not supported.

[Kathmandu]

Hello,

Sorry I'm a bit late to answer... Hollidays were great, thanks.

I do not understand your need for SDP grouping. The SDP I sent is consistent : one session (named test), with 3 media, 1 video and 2 audio.

As a matter of fact, when using that SDP file with VLC, it does show there is a second audio track!But it cannot play it!
My guess, as I didn't went through the code, is that the trouble is due to the fact that VLC does not handle correctly the dynamicity of the rtpmap : from 96 to 128 every number can be used to describe a media type. Usually 96 is used for Video and 97 for the corresponding audio if you are streaming a video with one audio. But you could perfectly swap the numbers or use 100 and 105.

I do not know the difficulty to code this kind of behaviour in VLC, but I'm sure this would be more respectful to the RFC.
Anyway, even if I do not have time to do the job myself, I am still willing to help if someone is on it and has questions regarding the problem.


Thanks for your answer or any remark if my understanding is wrong.

Didier.

[Rémi Denis-Courmont]

VLC should have no problem with dynamic payload types. And that's liblive555. not VLC anyway.