[ogg-dev] OggPCM2: channel map

[Jean-Marc Valin]

> I that this is handled pretty nicely by the "simple map" that Sampo 
> suggested. This is basically the same thing as the "channel map" 
> described on the wiki, but with the (physical,logical) channel pair 
> swapped. So, using the syntax from the wiki:
> channel_type = OGG_CHANNEL_MAP_STEREO
> channel_map [OGG_CHANNEL_FRONT_LEFT] = 1
> channel_map [OGG_CHANNEL_FRONT_RIGHT] = 2

Not sure this is a good idea. Remember that channel_map is just an array
(unless you want to make it a map?). So if you had a OGG_CHANNEL_SPECIAL
with an id of 1000, it would force 1000 entries in the array.

> This is nice because it makes it impossible for somebody to do something 
> stupid like define two channels as carrying FRONT_LEFT. A player can't 
> handle that situation sanely, but handling both FRONT_LEFT and 
> FRONT_RIGHT mapping to channel 0 is easy. Like Sampo said, this does 
> mean having to define (and assign an order to) what channels are in each 
> channel type, but I don't look at that as a bad thing. The other case we 
> were trying to support, of multiple logical streams at different 
> bitrates is still easy:

While it makes it impossible to do something stupid like mapping two
channels to the same target, it makes it easier to not map a channel at
all.

> >One idea I had for this is that there should be a default mapping for
> >all channel mappings and a default channel mapping for the most used
> >number of channels. For example, we would say that unless you include a
> >mapping header, then a 2-channel file is stereo with left channel
> >interleaved before right. I've added the idea at the bottom of
> >http://wiki.xiph.org/index.php/OggPCM2 . Any thoughts on this?
> >
> >  
> >
> I think this is a good idea, but it may be wise to stop at stereo and 
> not provide a preference for any of the 4+ channel formats.

I think a default mapping would be needed just to help for the
conversion header.

	Jean-Marc