Subject: [AVT] RTCP timestamp units

From: Avinash Bhatia <[email-masked]>
Date: Wed, 20 Aug 2003 18:42:42 -0700

Hi,
    I am facing an issue in inter-stream (audio-video) synchronization 
between two RTP streams. The NTP-RTP timestamp correlation comes in the 
RTCP packets, but the RFC does not specify the units in which the NTP is 
to be specified. This causes a problem in calculating the audio-video 
synchronization.
    As far as I understand, the Synchronization is not possible unless 
the unit of the NTP is known exactly, which can only be known after the 
second RTCP packet arrives (from which you can indirectly judje the NTP 
granularity and hence it's unit). Are there any known solutions to this 
problem ?
Thanks
Avinash

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

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From: Colin Perkins <[email-masked]>
Date: Wed, 20 Aug 2003 17:10:38 +0100

On Thursday, Aug 21, 2003, at 02:42 Europe/London, Avinash Bhatia wrote:
> between two RTP streams. The NTP-RTP timestamp correlation comes in 
> the RTCP packets, but the RFC does not specify the units in which the 
> NTP is to be specified. This causes a problem in calculating the 
> audio-video synchronization.
> the unit of the NTP is known exactly, which can only be known after 
> the second RTCP packet arrives (from which you can indirectly judje 
> the NTP granularity and hence it's unit). Are there any known 
> solutions to this problem ?

RTP makes no guarantee of the accuracy or resolution of the clock the 
sender is using to generate the timestamp. However, the format of the 
timestamp is precisely specified in Section 4 of RFC 3550 (the upper 32 
bits are seconds, the lower 32 bits fractions of a second, since 1 
January 1900).

-- 
Colin Perkins
http://csperkins.org/