Subject: RTP inter-media synchronization in "non-MPEG-style" 1-1-1

From: Alagu Periyannan <[email-masked]>
Date: Fri, 27 Jun 1997 12:38:58 -0700

>1-1-1 has always been possible, and that is how PN plans to stream
>container files.  The way that this is done in unicast is that the streams
>are multiplexed using the SSRC, and the SSRC values are enumerated in the
>session description.  It would probably be a good idea to do a more
>rigorous definition of this.  If necessary for multicast, we could have
>separate SSRC and a new field (added via standard RTP mechanism) so that
>the SSRC selection and the stream id don't get mixed up.  A more rigorous
>writeup is needed here.

How is inter-media synchronization done in "non-MPEG-style" 1-1-1?
NOTE: "non-MPEG-style" refers to the scheme described above.

The classic RTP way is to associate the RTCP cname of the
senders (SSRCs) from each RTP session and synchronize them.
All the senders that are part of the same presentation
use the same cname in this scheme.

However, when doing 1-1-1 as explained above there is
only one RTP session. So for this to work the classic
RTP method can not be used for inter-media synchronization.
So one has to rely on just the RTP timestamp for
synchronization. correct?

Do the RTP timestamps come from different spaces for
each media? If yes, then how is the synchronization
between the media established? If no, then this
scheme preclude using different timescales for different
media. Is this a good thing?

---------------------------------------------------
Alagu Periyannan                   [email-masked]

Interactive Multimedia Group
Apple Computer, Inc.

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From: Alagu Periyannan <[email-masked]>
Date: Thu, 3 Jul 1997 08:55:51 -0700

At 4:17 PM -0700 7/2/97, Eric Fleischman wrote:
>This can be done via many approaches. A more obvious one would be in
>reference to a streaming file format. In this approach an RTP dynamic
>payload type value is associated with a particular set of configuration
>information (e.g., the file's header value, SDP contents, etc.) via a
>mechanism outside of the scope of RTP (e.g., RTSP's DESCRIBE method).
>The streaming file format will have its own synchronization method
>(e.g., each media stream may have its own timestamp) which will be used
>for inter-media synchronization. Thus, the RTP timestamp has no
>inter-media synchronization utility in the "normal" 1-1-1 case. Please
>note that the 1-1-1 solely means that these media streams are carried as
>a "unit" (a "presentation" in RTSP terminology) as far as RTP is
>concerned. It does not identify the actual data packetizing approach
>(e.g., interleaved or unique payloads/packet) implemented by the
>presentation.
>

You are describing an MPEG-style solution where the
RTP timestamp is not used, but some other timestamp
inside the RTP payload is used.

However, if you want to use the RTP timestamp
and multiplex multiple media types on a single
RTP session, the choices are to either invent a
new inter-media synchronization scheme or use the
same timescale for all the media types.