Subject: Re: [AVT] Last Call: RTP: A Transport Protocol for Real-Time Applications to Draft Standard

From: Stephen Casner <[email-masked]>
Date: Sun, 17 Feb 2002 16:25:22 -0800 (PST)

This is a Last Call comment on:

>  o RTP Profile for Audio and Video Conferences with Minimal Control
>      <draft-ietf-avt-profile-new-12.txt>
>
>    This document is intended to replace RFC1890, currently a Proposed
>    Standard.

It was recently discovered that RFC1890 contains an ambiguity that
remains in draft-ietf-avt-profile-new-12.txt.  The table in Section
4.1 of the draft that specifies the convention for the ordering of
audio channels contains two possible orderings for four channels:

   channels  description   channel
                              1   2   3   4   5   6
   __________________________________________________
   2         stereo           l   r
   3                          l   r   c
   4         quadrophonic     Fl  Fr  Rl  Rr
   4                          l   c   r   S
   5                          Fl  Fr  Fc  Sl  Sr
   6                          l   lc  c   r   rc  S

This is ambiguous because the ordering is selected implicity by the
number of channels.  It must be that nobody has used 4-channel audio
with RTP since the issue has not arisen before.  We propose to just
eliminate the "quadrophonic" order for the revision of the profile
and add the following note:

   Note: RFC 1890 defined two conventions for the ordering of four
   audio channels.  Since the ordering is indicated implicitly by the
   number of channels, this was ambiguous.  In this revision, the
   order described as "quadrophonic" has been eliminated to remove the
   ambiguity.  This choice was based on the observation that
   quadrophonic consumer audio format did not become popular whereas
   surround-sound subsequently has.

The channel-order parameter specified in RFC3190 could be used to
define a quadrophonic convention in the future if needed.

							-- Steve