Re: [AVT] I-D ACTION:draft-ietf-avt-rtp-speex-01.txt

[Colin Perkins <csp at csperkins.org>]

Alfred,

On 17 Jun 2007, at 13:40, Alfred E. Heggestad wrote:
...
> > Marker bit "to indicate that the RTP packet contains at least one  
> > complete frame" is unusual for audio (although typical for video).  
> > It would be better used to indicate first packet after a silent  
> > period, as is usual for audio codecs.
>
> fixed in my working copy of -02
>
> I looked in some other RFCs/I-Ds and found this:
>
>   http://www.ietf.org/rfc/rfc4749.txt
>   "The M bit MUST be set to zero in all packets."
>
>   http://www.ietf.org/internet-drafts/draft-ietf-avt-rfc3047- 
> bis-04.txt
>   "The M bit is set to zero."

The usual rules for audio are: if the codec doesn't support silence  
suppression, the marker should always be set to zero; otherwise, set  
the marker bit to one on the first packet after a silent period.

> > Section 4.1.1: "rate" needs to be listed a required parameter,  
> > since the codec supports several sampling rates.
>
> I assume that you mean "rate=sampling rate" here. Not sure how to  
> formulate
> it in the best way, but I ended up with this:
>
>
>       rate: The sampling rate MUST be either 8000 Hz, 16000 Hz, or  
> 32000
>       Hz.
>
>
>
> in other wideband codec specs I found this:
>
>
> http://www.ietf.org/rfc/rfc4749.txt
> "Required parameters: none"
>
> http://www.ietf.org/internet-drafts/draft-ietf-avt-rfc3047-bis-04.txt
>
>   Required parameters:
>
>       bitrate: the data rate for the audio bit stream.  This parameter
>       is mandatory because the bit rate is not signaled within the
>       G.722.1 bit stream.  At the standard G.722.1 bit rates, the  
> value
>       MUST be either 24000 or 32000 at 16 Khz sample rate, and 24000,
>       32000 or 48000 at 32 Khz sample rate.  If using the non-standard
>       bit rates, then it is RECOMMENDED that values in the range 16000
>       to 48000 be used.  Non standard rates MUST have a value that  
> is a
>       multiple of 400 (this maintains octet alignment and does not  
> then
>       require (undefined) padding bits for each frame if not octet
>       aligned).

The current guidelines are in RFC 4855 section 2. You should use  
something like:

Required parameters:
	rate: RTP timestamp clock rate, which is equal to the sampling rate  
in Hz.
	The sampling rate MUST be either 8000, 16000, or 32000.

> > Section 5: the "a=fmtp:" attribute is for media type parameters  
> > only. If you wish to define vbr, cng, and mode parameters, they  
> > must be defined as media type parameters in the IANA  
> > considerations, and then mapped into the "a=fmtp:" line. Section  
> > 6.2 of RFC 4749 is a good example of how to do this.
>
> thanks for this comment. I have moved the definition of the fmtp  
> parameters
> to the "IANA Considerations" section, which is then referenced from  
> the
> "SDP Usage" section.

Not sure what you mean by this?

> unless there are any more comments, I will publish -02 within the next
> few days.

Okay - I'll re-read and comment when -02 is submitted.

Cheers,
--
Colin Perkins
http://csperkins.org/



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