Subject: [AVT] Synchronization question abt RFC 2250

From: Roy <[email-masked]>
Date: Mon, 18 Aug 2008 09:58:20 +0800

Dear AVT experts,
 
A question came across to me when I go through RFC 2250, would some of you
can help me?
 
According to RFC 3550, RTP timestamp is defined as sample time of each unit.
It also reflacts the presentation time. 
But for some of payload type (e.g. mpeg1/mpeg2 vedio, RFC2250 ),  RTP
timestamp is not the presentation timestamp any more.

RFC 2250, Page 4:
The RTP timestamp will not be passed to the MPEG
   decoder.  This use of the timestamp is somewhat different than
   normally is the case in RTP, in that it is not considered to be the
   media display or presentation timestamp. The primary purposes of the
   RTP timestamp will be to estimate and reduce any network-induced
   jitter and to synchronize relative time drift between the transmitter
   and receiver.

So my question is that, how to synchronize a mpeg1/mpeg2 video RTP stream
(according to RFC 2250) with a normal RTP stream?
Mechanism provided by RTP protocol may not help in this case.
 
Best regards,
Roy
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---

From: Ross Finlayson <[email-masked]>
Date: Sun, 17 Aug 2008 19:21:29 -0700

>According to RFC 3550, RTP timestamp is defined as sample time of 
>each unit. It also reflacts the presentation time. 
>But for some of payload type (e.g. mpeg1/mpeg2 vedio, RFC2250 ), 
>RTP timestamp is not the presentation timestamp any more.
>
>RFC 2250, Page 4:

That paragraph refers only to the encapsulation of MPEG System and 
Transport Streams - *not* Elementary Streams - over RTP.  For MPEG-1 
or 2 Audio or Video Elementary Streams - described later in RFC 2250 
- the RTP timestamp *does* correspond to presentation time, as usual.
-- 

Ross Finlayson
Live Networks, Inc.
http://www.live555.com/

---

From: "Greg Herlein" <[email-masked]>
Date: Mon, 18 Aug 2008 19:31:43 -0700

It's a good question though.  I've seen several different
implementations of RTP encapsulation of MPEG2 transport streams.  One
in particular tries to interpolate the relative PTS of the data in the
RTP packet (this was a commercial implementation).  I'm interested in
what is considered to be 'standard' in this respect.

As a separate question I'd like to ask the old-timers what the
original thinking around this was.  It 'breaks' the concept of layered
separation - the idea that a lower level of protocol is relatively
insulated from the details of the payload.  This need to stamp
PTS-derived RTP timestamps is one argument I get from some vendors
*against* RTP (scaling costs) - they argue that TS-over-UDP is vastly
simpler.  Without the PTS-derived RTP timestamp RTP is not all that
much more complicated than using raw UDP.

<note:  I already grok the advantages of RTP so please no flames - I
was stating the arguments of others>

Greg

On Sun, Aug 17, 2008 at 7:21 PM, Ross Finlayson <[email-masked]> wrote:
> According to RFC 3550, RTP timestamp is defined as sample time of each
> unit. It also reflacts the presentation time.
>
> That paragraph refers only to the encapsulation of MPEG System and Transport
> Streams - *not* Elementary Streams - over RTP.  For MPEG-1 or 2 Audio or
> Video Elementary Streams - described later in RFC 2250 - the RTP timestamp
> *does* correspond to presentation time, as usual.
>
> --
>
> Ross Finlayson
> Live Networks, Inc.
> http://www.live555.com/