Subject: [rohc] ROHCv2: Default ts_stride?

From: "Kristofer Sandlund" <[email-masked]>
Date: Fri, 14 Sep 2007 12:55:53 +0200

Hi all,

we've done some work on the v2 draft and have come up with
a change that we feel would improve the RTP profiles. As usual, 
object if you think this is a bad change.

- In 3095, the TS_STRIDE has a default value of 1 (no scaling), so that
every RTP context has to explicitly send a new scaling value. To us, it
seems like the default should be the most usual stride we expect to see,

so that those codecs do not have to update the stride explicitly.  
Since voice will be the place where the gain is largest, and it seems 
like the most frequent stride to use will be 160 (e.g. AMR and EVRC
with 8000Hz sampling rate, 20ms frame period), we suggest to set the
initial  value for ts_stride ROHCv2 to 160.
Effects of such a change would be:
  * Codecs such as AMR & EVRC would not have to send ts_stride at all
    (saving about 2 octets * optimistic approach)
  * Codecs with other stride values that want to scale the TS would
    behave exactly as today (i.e. they have to set up the ts_stride)
  * A flow where the compressor would use no scaling (in 3095, it
    does not have to send ts_stride at all) would have to send the 
    ts_stride * OAA

So, IMO a value of 160 will be more frequent than TS with scaling 
factor 1, so I think this is an improvement to the compression ratio.

Objections?

/Kristofer

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From: "Kapoor, Rohit" <[email-masked]>
Date: Fri, 14 Sep 2007 17:59:50 -0700

Kristofer,

I think this is a good idea, even though the gains are kinda small
(since only typically IR packets will see a reduction in size of ~2
octets).

The only question is: assuming WB vocoders are more prevalent in the
future, should we optimize for that (delta Timestamp = 320) or for the
more prevalent NB codecs of today?

Thanks,
Rohit

> -----Original Message-----
> From: Kristofer Sandlund [mailto:[email-masked]]
> Sent: Friday, September 14, 2007 3:56 AM
> To: [email-masked]
> Subject: [rohc] ROHCv2: Default ts_stride?
> 
> Hi all,
>

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From: [email-masked]
Date: Mon, 17 Sep 2007 14:59:57 +0200

Rohit,

I think that NB codecs will remain for a while; and we know that they are 
prevalent today, so it makes sense to use 160 as default value in RoHCv2 - it 
will be useful right away and for some time ahead. Furthermore, this is anyway 
a small optimization as you point out, so I don't think it will be a problem 
for other codecs in the future to send a new value with the IR header.

Cheers,

///Ghyslain