Re: [MMUSIC] How does mmt grouping work with multiple streams of the same type?

[Harald Alvestrand <[e-mail]>]

On 10/12/2012 09:58 AM, Stach, Thomas wrote:
>> >I think we should continue to use RFC 5576 for this purpose:
>> >
>> >a=ssrc:1 <something follows>
> I haven't thought about that, but yes it would work for as well.
> For two audio streams (audio1 w/ PCMU or PCMA, audio2 w/ iLBC) it would look like this?
>
>     a=rtpmap:0 PCMU/8000
>     a=rtpmap:8 PCMA/8000
>     a=rtpmap:97 iLBC/8000
>     a=mmtype: 0 audio
>     a=mmtype: 8 audio
>     a=mmtype: 97 audio
>     a=ssrc:12345 fmtp:0
>     a=ssrc:12345 fmtp:8
>     a=ssrc:67890 fmtp:97
Yes, except that as far as I can tell from RFC 5576, you can't use fmtp: 
without an actual parameter value: "This parameter MUST only be used for 
media
    types for which source-level format parameters have explicitly been
    specified". (section 6.3).

If you need to use a field that's always present, use CNAME.

Note(1): If you only use one payload type for all your audio, you have 
no need to signal which format they're using.

Note(2): In the above, you have ssrc 12345 using both PCMU/8000 and 
PCMA/8000. I don't know if it's intentional or not.

>> >
>> >The suggestion above wouldn't work for the case of two SSRCs
>> >both using the same payload type.
> Well it would if you duplicated the a=mmtype attribute with different ssrc elements in that case.
>       a=mmtype: 0 audio ssrc=1
>       a=mmtype: 0 audio ssrc=2
> Would you do something else for a=ssrc?

Well, a=ssrc *is* something else. The above mixes info about payload 
types and info about SSRCs; I think that's a bad design.