Re: [AVT] RE: <draft-ietf-avt-rtp-vmr-wb-03.txt>: sampling rate

Qiaobing Xie wrote:
> 
> I'd think that the saving (in terms of reduction to the system design 
> and signaling complexity) by avoiding performing RTP timestamp rate 
> switches (as Magnus pointed out above) far overweighs the ability of 
> using the sampling rate to drive the media clock.
> 

Also, nothing in the case of VMR-WB prevents it from driving the media 
clock. The larger issue is that VMR-WB has built in resampling 
functions, and the fully information carrying sampling rate needed is 
16kHz, not 8. However resampling before the codec, or resampling in the 
codec does not make any difference from the handling, only somewhat in 
regards to quality, due to encoder optimization.

As the native out-put is 16kHz using a 8kHz RTP clock prevents a 
receiver from driving its media clock with the appropriate rate. If a 
receiver is using 8kHz output, then it easier to scale down the rate, 
then to scale it up for the normal receiver behavior.

Thus I see the least problem of driving the media clock when we only 
uses a 16kHz clock, rather then a clock that reflects the encoders 
front-end.

Cheers

Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVA/A
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Ericsson AB                | Phone +46 8 4048287
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S-164 80 Stockholm, Sweden | mailto: magnus.westerlund@ericsson.com

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