Re: [AVT] Speex payload discussion

[Magnus Westerlund <magnus.westerlund@era.ericsson.se>]

Colin Perkins wrote:

> --> Greg Herlein writes:
>  
> > > The EVRC payload format addresses this by defining a simple format where
> > > only a single frame is included in the packet, and a more complex format
> > > which has a header and multiple frames. The signalling then selects which
> > > of the two formats is used. This may be an acceptable solution here, too.
> > >      
> > Is it *that* bad a thing to just let the signalling define the
> > number of frames in a packet in the first place, the way it's
> > currently defined?
> >    
> No, it's just a little unusual.  If there's no reason why a sender would
> want to change the number of frames per packet, then signaling it out of
> band is entirely appropriate. 
>  
Yes, but there normally exist one very good reason why one wants to 
change the number of frames per packet, congestion control. For most low 
bit-rate codecs the simplest way of reducing the bit-rate is simple to 
reduce the number of packets sent. As the IP/UDP/RTP headers often 
represents a bit-rate  of one or more complete audio streams. By sending 
two instead of one frame per packet the rate can often be reduced by 
30%. However as this trade off often comes at the expense of less 
robustness and increased end-to-end delay one tries to avoid it if 
possible. Therefore it is reasonable to be able to change this on the 
fly. However if the Speex codec contains other mechanisms that are 
useful to change the bit-rate then it might not be necessary.

An alternative way if to actually define different payload types when 
there might be need for different packetizations.

Another point, if you have variable frame length how does one 
distinguish the end of one frame from the beginning of the next? If you 
don't have such a difference it will not be possible to demultiplex at 
the receiver.

Best Regards

Magnus Westerlund 
Multimedia Technologies, Ericsson Research ERA/TVA/A
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Torshamsgatan 23           | Fax   +46 8 7575550
S-164 80 Stockholm, Sweden | mailto: magnus.westerlund@era.ericsson.se



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