Subject: Re: [AVT] MPEG-4 Payload for RTP [MPEG STREAMING]

From: Dave Singer <[email-masked]>
Date: Tue, 31 Jan 2006 09:23:38 -0800

At 13:35  -0600 30/01/06, Siddhartha Gandhi wrote:
>Consider the following MPEG-4 streaming using RTP, audio ( Payload 
>Type = 97 ) and video ( Payload Type = 96 ) in the trace :
>
>
>
>
>Given,
>     video_time_scale  = 90000 and audio_time_scale = 44100
>     video_payload_type = 96     and  audio_payload_type = 97
>     MPEG-4 video
>         stts = {count, delta} [0] = {5012, 3003}
>
>Hence,  
>     27188 (first video packet) - 24188 (second video packet) = 3000 
>( this matches with the delta above )
>
>3000/90000 =1/30 = 0.03333 sec = 33.33 msec. So two consequtive 
>video packets should have a delay of 33.33 msec.
>
>Two consequtive RTP packets carrying MPEG-4 video payload ( Payload 
>Type = 96) should be 33.33 msec. Infact if we calculate the 
>difference = 5.337154 sec - 5.337140 sec =  0.014 msec != 33.33 msec
>
>Why is this so ?
>
>This time is incremented only after 3 packets containing MPEG-4 
>video payload are sent with M-BIT = true by, (3 * 33.33msec) = 99.99 
>msec
>
>Can anybody tell me the reason as to why Time ( 2nd Column ) is not 
>incremented in accordance with Timestamp (9th column) ?
>
>Is it related to buffering or something else ?

is there composition time re-ordering going on?  what software is the 
transmitter?  you might need to ask them...
-- 
David Singer
Apple Computer/QuickTime

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

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From: Magnus Westerlund <[email-masked]>
Date: Wed, 01 Feb 2006 11:15:39 +0100

Hi,

It looks like you receive two different media streams. Have you checked 
what ports the different packets arrive on?

The most likely explanation is that they are two different streams with 
different RTP timestamp rates, that is why the timestamp looks like it 
jumps back and forth.

Cheers

Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVA/A
----------------------------------------------------------------------
Ericsson AB                | Phone +46 8 4048287
Torshamsgatan 23           | Fax   +46 8 7575550
S-164 80 Stockholm, Sweden | mailto: [email-masked]

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From: Magnus Westerlund <[email-masked]>
Date: Wed, 01 Feb 2006 16:19:23 +0100

> Is it related to buffering or something else ?

This may be some transmission re-scheduling that is done to smooth the 
bit-rate. It can also be the simple problem that the server sends a 
burst of packets each time it transmits and only follow time scale on 
larger scale, not on the finer scale.

cheers

Magnus Westerlund

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From: Dave Singer <[email-masked]>
Date: Wed, 1 Feb 2006 12:12:44 -0800

At 16:19  +0100 1/02/06, Magnus Westerlund wrote:
>This may be some transmission re-scheduling that is done to smooth 
>the bit-rate. It can also be the simple problem that the server 
>sends a burst of packets each time it transmits and only follow time 
>scale on larger scale, not on the finer scale.

that would affect only the actual time of transmission, not the 
timestamp in the packet, no?  the packet timestamp is the 'display' 
time (or at least it's mapped to that)...
-- 
David Singer
Apple Computer/QuickTime