Subject: RE: [AVT] Transmitting G729ab packets over network

From: "Miao, Kai" <[email-masked]>
Date: Tue, 27 Nov 2001 19:57:33 -0500

Are you talking about the DTX mode of transmission in which only voice
packets are transmitted, ended with a single SID packet, and then followed
by silence (no packets)? 

-----Original Message-----
From: Karthigeyan Vaidyanathan [mailto:[email-masked]]
Sent: Tuesday, November 27, 2001 7:44 PM
To: [email-masked]
Subject: [AVT] Transmitting G729ab packets over network

Hi all,
        I have a doubt regarding transmission of
G729ab encoded speech packet over the network. As you
all know there are three packet lengths in case of
G729ab speech codec viz, 80,15,0 bits. 
        My doubt is  should we transmit the packet
with zero length or should we avoid sending it into
the network?
        In case I avoid sending it, then there is a
discontinuity in the speech and in case I send it then
there is a unnecessary transmission of a the header
with packet lenght zero with no payload. How do I
handle this? It would be great if someone can help me
regarding this. Is there any standard that can be
followed?
Thanks
Karthik

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_______________________________________________
Audio/Video Transport Working Group
[email-masked]
http://www1.ietf.org/mailman/listinfo/avt

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
http://www1.ietf.org/mailman/listinfo/avt

---

From: Karthigeyan Vaidyanathan <[email-masked]>
Date: Wed, 28 Nov 2001 01:21:47 +0000 (GMT)

Yes, thats what I meant....voice packets, followed by
a silence and then atleast 2 no transmission packets
and then could be anything. My doubt was how to
transmit the no trnasmission packets. Or in other
words how to convey to the receive side that there is
a no transmission packet without transmitting
anything. Hope I have made my doubt clear.
Thanks
Karthik

--- "Miao, Kai" <[email-masked]> wrote: > Are you
talking about the DTX mode of transmission
> in which only voice

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---

From: Stephen Casner <[email-masked]>
Date: Tue, 27 Nov 2001 17:32:53 -0800 (PST)

> anything. Hope I have made my doubt clear.

The RTP timestamp advances by the amount of time in the silence, but
the RTP sequence number advances only by one.  That is exactly why
both of those fields are present in the RTP header.
							-- Steve

---

From: Jeff Meunier <[email-masked]>
Date: Tue, 27 Nov 2001 23:36:22 -0600

So, to properly render a stream like this, one has to program logic on the 
playback side that generates noise until the next packet arrives, and 
continue to generate noise (if applicable) if the timestamp indicates that 
the silence interval is still going?

We wondered about this at one point, but is was related to saving g723/g729 
data to file and we just relied on using the non-standard 0 length frames.

FYI, I know in the ITU reference G.723.1 code, it will generate repeated 
silence frames (actually of length one byte) during such intervals. To be 
fully compliant with the RTP profile, I believe these should not be 
transmitted, right? (since they are not a recognized codec format type)

-- Jeff

-- 
Jeff Meunier
[email-masked]

---

From: Simao Campos-Neto <[email-masked]>
Date: Wed, 28 Nov 2001 21:32:44 -0500

That actually is an interesting question...

One should distinguish the "simulation" of the codec operation from the
codec operation itself. The code in the recommendation (either G.729 or
G.723.1) puts out those 'non-transmitted' frames only because it is a
convenient way to convey the non-transmission periods to the decoder.
(It should be noted that the decoder has the Comfort Noise Generation
functionality as part of the relevant Annexes [B in G.729 and A in
G.723.1]). In a real time (interactive) system, those 'voice' frames are
simply not transmitted. In the RTP case, no packets relevant to these
frames are sent [counters updated as indicated by Steve].

In the case of playback of pre-recorded signals, as Jeff mentioned, one
can record the encoded signal with the no-transmit frames so that the
timers can be kept running properly (the player has to provide that
functionality), or alternatively one could record the RTP packets
themselves, which would contain the proper timing information, instead
of the no transmit frames. In signals with very long silences, I guess
that the latter would be more efficient (ie take up less storage space),
while for recorded announcements (where I would expect silences to be
kept to a minimum), the former is probably more efficient.

Cheers,
Simão.
--

-- 
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