[AVT] timed text RTP payload

Remarks & questions

[
        2.  Transmit the text sample size, sample duration and sample
   description index in-band.  In 3GP format this information is
   included in the header part.  In RTP it is important to transmit it
   in-band because this information might change from sample to sample.
]

I think you should explicit that it is not a dangerous thing to do because
this information affects only the data in the packet
so when you loose a packet you do not loose the context...

it is rather obviously true for size and duration,
but less obvious for description index, i.e. as you
mention it in the section 1.2 a timed text stream
may transport samples using several different "descriptions"
(i.e. configurations) and it is therefore required
to indicate for each sample the "number" (index) of the
corresponding description.

BTW the sentence "In 3GP format this information is
   included in the header part" is misleading in this respect...

**************

[

      4.  Enable the multiplexing of text samples in one single RTP
   packet.  In a mobile communication environment a typical text sample
   size is around 100 bytes.  Thus, multiplexing several text samples
   makes the transport over RTP more efficient.
]

I think you should replace "multiplex" with "aggregation",
multiplex hints at something much more "drastic" than what you
want to achieve i.e. mixing several different streams together,
which comes with a whole lot of difficult issues etc :-)

**************

Questions

*  How does a receiver know if the first element of the payload is a FHDR or a THDR ?

* Figure 3.2 has two cases, both are 32 bits aligned, is it a requirement ?
otherwise how is the header aligned (with padding ?)

* section 4 *recommends* redundancy, I would leave that more open ...
anyway would you consider the possibility that the payload format itself
would enable redundancy by simply allowing "repeat"
(of, say some highly valuable "sentences" ?)

regards,

Philippe Gentric
Software Architect
Philips MP4Net
philippe.gentric@philips.com
http://www.platform4.philips.com

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