[xiph-rtp] Suggested changes to the Vorbis SDP attributes

[Tor-Einar Jarnbjo]

Ralph Giles wrote:

>>b=X-NOM:<nominal bitrate>
>>b=X-MAX:<maximum bitrate>
>>b=X-MIN:<minimum bitrate>
>>    
>>
>
>Just curious; what's the motivation for the name change?
>  
>
The a=fmtp fields are used for format or codec specific values. There's 
no need to use these for bandwidth specifications, as there is a 
designated SDP field (b) to be used for that. SDP uses a bandwidth 
specification in this format:

b=<modifier>:<bandwidth-value>

There are two modifiers defined in the SDP RFC: CT (conference total) 
and AS (application-specific maximum). Neither of these are fitting the 
Vorbis concept of "nominal bitrate", so it would make sense to use the 
extension mechanism for the bandwidth modifier and defien X-NOM, X-MIN 
and X-MAX to be used for the corresponding Vorbis header entries. The 
bandwidth value is given in kbps and not in bps as in the Vorbis header.

>>If not specified, the number of channels defaults to one (RFC 2327, 
>>chapter 6).
>>    
>>
>
>Does RTP have a default mapping for >2 channels? If not, does putting it
>in the rtpmap mean we can use the mapping in the vorbis spec?
>

What do you mean with "mapping"? Channel 0 is left and channel 1 is 
right? That mapping is irrellevant at the point of parsing the session 
description and must be handled somehow by the codec.It might be used by 
a client though to chose between e.g. a 1, 2 or 5 channel stream 
depending on the client's capabilites.

Tor