Re: [AVT] Update (-09) of Timed Tex draft (WAS: RE: Comments on draft-ietf-avt-rtp-3gpp-timed-text-08.txt)

[Colin Perkins <csp at csperkins.org>]

Jose,

On 18 Jan 2005, at 14:53, Rey Jose wrote:
...
> >   - Why are type 5 units untimed, rather than being timed
> > with their sending time?
> Because the sample description is strictly NOT a part of the text 
> sample.  The timestamp is used for identifying the units that belong 
> to the same text sample.

Okay - but you give them a timestamp now.

> > It would fit the RTP model better if they were given a timestamp
> > indicating the time at which they become active
>
> Sample descriptions may be needed by several text samples, cannot have 
> two timestamps.  Also with duration is difficult since it is not set 
> in neither files or live streaming.  Further, when decoding a text 
> sample it should be looked for the needed sample descriptions.  If not 
> available, the application decides how to display, if at all.  Most 
> times will be possible, since sample descriptions only relate to 
> formatting and position. Therefore a default formatting could be used 
> for that.  This is also explained in the draft.

I understand this. However, the payload format gives sample 
descriptions a timestamp now, setting it equal to "the current value of 
the RTP timestamp plus one" but ignoring it on reception and assuming 
the sample descriptions become available for use immediately the packet 
is received.

I am suggesting is that this is less than ideal, since a receiver has 
to handle some RTP packets in the stream according to the usual RTP 
timing model, but to ignore that model for some other packets and 
decode them as soon as they arrive. This makes an implicit assumption 
that the sample description is to be used after the time it arrives - 
why not make that explicit, with the RTP timestamp being the time at 
which the sample description becomes available for use?

> > (and arrange the stream such that they cannot apply to packets with 
> > earlier timestamps).
> This can never happen. I mean the server takes care that, the SIDX are 
> correctly synchronised, c.f. section on the dynamic SIDX.  Or maybe I 
> don't get your question?

I understand that it can never happen. Why not make that explicit in 
the use of the RTP timestamp?

> >   - It's not clear why type 1 packets are permitted to
> > contain modifiers? Why not enforce that they contain only text, and 
> > aggregate that text with a type 3 packets as appropriate?
>
> It seems simpler to give it all in one go to the app instead of 
> forcing to paste the parts together for every sample.

This would only be simpler if modifier boxes were never sent separately 
from the text. As it stands, a receiver must support two different ways 
of getting the modifier. If we remove support for type 1 packets, and 
always send either text or modifiers, then a receiver has only one way 
of getting the data. That would reduce the amount of code in the 
receiver.


The other changes in -11 look okay to me.

Colin


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