Subject: RE: [AVT] One more doubt regarding Jitter Buffer

From: "Desai Sunil Kumar" <[email-masked]>
Date: Thu, 12 Jul 2001 19:46:54 +0530

Hi Colin,
        Is the playing of silence packets job of RTP stack or application.
Suppose if it is the job of RTP stack, what will be the payload. 
	
	One more thing i want to know is whether the palyout buffer will be
sitting at application level or at RTP stack level & what exactly do u mean
by playout buffer?.

Reg & Thanks
Sunil Desai

-----Original Message-----
From: Colin Perkins [mailto:[email-masked]]
Sent: Thursday, July 12, 2001 7:11 PM
To: Desai Sunil Kumar
Cc: [email-masked]; [email-masked]
Subject: Re: [AVT] One more doubt regarding Jitter Buffer 

--> "Desai Sunil Kumar" writes:
>Hi Anand,
>         U mean to say the Audio/Video Application will be maintaining the
>timer & periodically it will be reading the data
>from Jitter Buffer. In that case i have 2 doubts.
>
>	1) If there is no data in Jitter buffer(more related to silence
>suppression i believe).

The simplest approach is to play silence for the duration of the gap in the
playout buffer. You may also want to insert some form of appropriate noise,
generated locally.

>	2) If there is data in Jitter Buffer, but application has not read &
>a new packet has come should i overwrite.

You will have to discard some data, yes.

>	Regarding the Silence suppression the RFC 1890 says that the marker
>bit in first packet should be set to indicate the begining of the talkspurt.
>But how will know when the silence period has started. I read in some CNG
>related AVT draft which says the SID(Silence Descriptor) frames are sent &
>there is a payload type of 13 for CNG Payload. If u have any idea pl let me
>know.

You will know because no packets arrive, and there will be a gap in the
playout buffer (some codecs may also send comfort noise).

Colin

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Audio/Video Transport Working Group
[email-masked]
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---

From: Colin Perkins <[email-masked]>
Date: Thu, 12 Jul 2001 10:52:39 -0400

There is no single answer to your questions, it depends on how closely 
your application and RTP stack are tied together and what you consider 
to be the boundary between them.

In our implementation <http://www-mice.cs.ucl.ac.uk/multimedia/software/rat>
the functions you ask about are both part of the application, with the RTP
stack simply handing off packets as they are received. 

That is not the only possible design, and others may have made different
choices. For example, I can envisage a design where the jitter buffer is
hidden within the RTP stack, and packets are passed to the application 
with the correct timing and with markers for lost packets.

Colin