Subject: [AVT] Re: Request for WG Last-Call for draft-ietf-avt-rtp-3gpp-timed-text-07.txt

From: "Rey Jose" <[email-masked]>
Date: Wed, 1 Dec 2004 15:32:59 +0100

Magnus,

Thanks for the comments.  Below, I don't understand the last part,
between "**": is it meant that a wraparound for the timestamp is needed?

"
19. Section 12.3:

        o the Decoding Time to Sample Box (stts): the 24 least
          significant bits of the "sample_delta" are mapped to the
          field SDUR (Text Sample Duration),

I think this is incorrectly stated, I think one should use as many bits
as existing in the mapping to the SDUR value. Please consider that if
one strictly takes the lower 24, values one can't scale in the mapping,
or do ceiling value with the bits one have. Lets assume that one can
only represent number with 3 digits, and SDUR would be 1023, **then one
could send this sample as one with SDUR=999 and another with
TS=TS_org+999 and SDUR=24.**
"

Thanks,

---------------------------------------------
Jose Rey              mailto:[email-masked]
P R D C G - Panasonic R&D Center Germany GmbH
Phone/Fax: +49(0)6103766-134/166
--------------------------------------------- 

_______________________________________________
Audio/Video Transport Working Group
[email-masked]
https://www1.ietf.org/mailman/listinfo/avt

---

From: Magnus Westerlund <[email-masked]>
Date: Wed, 01 Dec 2004 16:29:34 +0100

Hi Jose,

The comment, is that it is not simply allowed to mask out the lower 24 
bits. Doing a simple masking operation on a 32 bit value that has any 
bits set above would 24 bits would result in that the potentially much 
smaller value is used, rather the complete number. That was what I was 
trying to adress with my example.

Please consider the value 1023, if you have a storage that fits only 3 
digits, any you do a simple masking, then you get the number 23. Which 
is much longer from the real number, than storing 999 would. However to 
get the correct number in this case you would need to use multiple SDUR 
fields that are added together. The for me obvious case of how to solve 
the problem is that one uses multiple copies of the same sample, and 
make the display after each other until the complete time has been 
reached. Thus my example between the "**".

Have I managed to explain the issue sufficiently?

Cheers

Magnus

Rey Jose wrote:

> Magnus,
> 
> Thanks for the comments.  Below, I don't understand the last part, between "**": is it meant that a wraparound for the timestamp is needed?
> 
> "
> 19. Section 12.3:
> 
> 
> I think this is incorrectly stated, I think one should use as many bits as existing in the mapping to the SDUR value. Please consider that if one strictly takes the lower 24, values one can't scale in the mapping, or do ceiling value with the bits one have. Lets assume that one can only represent number with 3 digits, and SDUR would be 1023, **then one could send this sample as one with SDUR=999 and another with TS=TS_org+999 and SDUR=24.**
> "
> 
> Thanks,
> 

-- 

Magnus Westerlund

Multimedia Technologies, Ericsson Research EAB/TVA/A
----------------------------------------------------------------------
Ericsson AB                | Phone +46 8 4048287
Torshamsgatan 23           | Fax   +46 8 7575550
S-164 80 Stockholm, Sweden | mailto: [email-masked]