Subject: [payload] draft-ietf-payload-rtp-opus

From: "Timothy B. Terriberry" <[email-masked]>
Date: Wed, 21 Aug 2013 18:21:22 -0700

In answering a question about draft-ietf-codec-oggopus [1], I realized 
that we don't describe how DTX is expected to work very well in the 
current payload draft. In the reference implementation, it is 
implemented by continually submitting audio to the encoder, which uses 
its voice activity detection to identify whole frames that can be 
dropped. If voice activity resumes in the middle of a frame, the whole 
frame is transmitted.

I think we should make such behavior required, so that receivers can use 
simple packet loss concealment to fill in the missing data, instead of 
being required to synthesize an arbitrary number of samples.

Therefore I propose we add the following sentence to the second to last 
paragraph of section 3.1.3: "The transmitter MUST drop whole frames 
only, to ensure successive RTP timestamps differ by a multiple of 120 
and to allow the receiver to use whole frames for concealment."

[1] http://www.ietf.org/mail-archive/web/codec/current/msg03030.html

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From: Jean-Marc Valin <[email-masked]>
Date: Thu, 22 Aug 2013 23:16:26 -0400

On 08/21/2013 09:21 PM, Timothy B. Terriberry wrote:
> Therefore I propose we add the following sentence to the second to
> last paragraph of section 3.1.3: "The transmitter MUST drop whole
> frames only, to ensure successive RTP timestamps differ by a
> multiple of 120 and to allow the receiver to use whole frames for
> concealment."

Yes, makes sense. I'll add that.

	Jean-Marc