[xrblock] Multiple channels in RTP streams

[Alan Clark <[e-mail]>]

On the question of multiple channels in RTP streams - it may be helpful to
have some practical examples to use for reference and as examples.  This may
also answer one question in the XRBLOCK session which related to the
numbering of the channels in the case of multichannel audio.

(i) Multichannel Audio
RFC3551 defines an RTP profile for Audio and Video Conferences.  Section 4
describes the convention for multichannel sample based audio encodings -
which is to interleave samples using the orderings

Channels        1    2    3    4    5    6
2                      L    R
3                      L    R    C
4                      L    C    R    S
5                      FL  FR  FC  SL  SR
6                      L    LC   C   R    RC    S

This is generally consistent with the AIFF-C format, which is widely used
for multichannel audio - and uses this interleaved approach.  Note that this
does define an ordering, and hence channels 1, 2, 3, 4.. can be interpreted
in terms of Left, RIght etc.  Other IETF RFC¹s that relate to multichannel
audio include RFC3190, RFC4184, RFC5691 .....

(ii) MPEG Transport Streams
An MPEG Transport stream comprises multiple channels of video and/or audio
carried in MPEG Transport packets, with each media stream identified by PID.
A typical single program stream might have, for example, one video stream
and two audio streams (for two languages) plus additional streams for
captions, contents (PAT/PMT).  This MPEG Transport Stream is carried in a
single RTP session.  A typical use case would require the reporting of a
Video MOS for the video stream and an Audio MOS for each of the audio
streams.
 
(iii) Multi-view Video
Multi-view video generally covers 3D, multiple viewpoint video and other
similar applications.  Work on this has been performed within MPEG/VCEG as
extensions to H.264 and an RTP payload format was proposed in
draft-ietf-avt-rtp-mvc.

Best Regards

Alan