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

[Magnus Westerlund <magnus.westerlund at ericsson.com>]

Colin Perkins wrote:

> Packet 0: Type 1: SIDX=1
>
> Packet 1: Type 5: SIDX=2
>           Type 1: SIDX=2
>
> Packet 2: Type 1: SIDX=2
>
> Packet 3: Type 5: SIDX=2 (Repetition)
>           Type 1: SIDX=2

> > With the proposed timestamp model, the timestamp of the TYPE 5 unit in 
> > packet 3 should have a TS equal to the TS of packet 1, rather than the 
> > TS of packet 3 to indicate that it is useful from the TS time of 
> > packet 1.
>
>
> Why? If you give the type 5 unit sent in packet 3 the timestamp of 
> packet 3, then the behaviour is unchanged from the previous, except that 
> the timing model is used to decode the data.

The reason is that otherwise, you would not care to depacketize packet 3 
until after you have tried to consume packet 2 and failing that due to 
lack of sample description, and moved on. That assumes the quite common 
buffering strategy of sorting packets in timestamp order. Of course it 
might be the fact that 3GPP timed text, just like AMR works best with a 
post depacketized buffering structure. However as this buffer is sorted 
in timestamp order with samples. It would still make most sense to have 
the sample descriptors having a timestamp equal to there first valid 
point. That way they get sorted into the same time as they are first used.

Cheers

Magnus

--

Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVA/A
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Torshamsgatan 23           | Fax   +46 8 7575550
S-164 80 Stockholm, Sweden | mailto: magnus.westerlund at ericsson.com

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