Re: [AVT] Request for feedback on draft-valin-celt-rtp-profile-01.txt

[Stefan Sayer <stefan.sayer at iptego.com>]

Hello,

o Jean-Marc Valin [05/11/09 22:02]:
> Hi,
>
> We have just updated the CELT RTP profile to take into account the 
> suggestions we got:
> http://www.ietf.org/internet-drafts/draft-valin-celt-rtp-profile-01.txt
>
> Among the changes, we have replaced b:AS= with a bitrate= parameter and 
> only one frame size is now allowed for each dynamic payload. This should 
> solve the problem with early media at the expense of using possibly many 
> payload types. Anything we forgot to address or any new issues?

I realize this has been discussed before and may still be an unsolved 
question, but I still find the description of multiple frames per RTP 
packet in the 01 draft confusing, especially the discrepancy between the 
diagram in "3.  RTP payload format for CELT" and "3.3.  Multiple CELT 
frames in a RTP packet". The first one looks like there would be no 
length bytes, just the plain payload to follow the RTP header. Because 
the length bytes then only appear in the section specific to multiple 
frames in a packet, one might think that they are transmitted only in 
this case. Only later, the Low-Overhead mode is defined, which when used 
may omit the length bytes.

My suggestion is to remove the first diagram (anyway it is clear that 
the payload follows the RTP header), or add the length bytes to it to 
make clear that this is the usual mode of packetization.

In 5.2. Low-Overhead Mode, the text says:
   "One the a=fmtp: parameter low-
   overhead: is defined and contains a single frame size, followed by a
   '/', followed by a comma-separated list of the number of bytes per
   frame for each stream defined in the channel mapping."
but the example gives:
 a=fmtp:97 low-overhead=256/1/86,86,43,30;mapping=2,2,1,1/
      L,R,LR,RR,C,MLFE/ITU-RBS.775-1
What is the extra /1 supposed to mean in 256/1/86,86,43,30 ?
Also, a simpler example (e.g. for mono and stereo streams) could be 
illustrative.

Thanks
Stefan Sayer