[ogg-dev] OggPCM2: channel map

[Jean-Marc Valin]

OK, I hadn't understood that you'd also get rid of the global channel
definitions. In that case, you shouldn't use OGG_CHANNEL_FRONT_LEFT like
you were using, but OGG_CHANNEL_STEREO_FRONT_LEFT,
OGG_CHANNEL_QUAD_FRONT_LEFT, OGG_CHANNEL_5_1_FRONT_LEFT, ... I think
that would be a mess, but then again I won't be the primary user of
OggPCM. I suggest everyone interested gives his opinion about this so we
can see where the majority is and/or whether there's a compromise
possible.

	Jean-Marc

Le samedi 19 novembre 2005 à 07:46 -0500, jkoleszar at on2.com a écrit :
> >> True, but remember that the channel map type implied the number of
> >> entries in the table, and also that in this organization you'll always
> >> number the logical channels consequtively since each logical channel
> >> indeed corresponds to an index into the array. If the channel map type
> >> says it's a map for 5.1, there will only be 6 slots in the table no
> >> matter how many channels are actually stored.
> >
> > Not quite, let's say the six first logical channels are the ones used
> > for surround (left, right, back, ...) and you have an ambisonic file
> > (left and right have no meaning) or something like that. You still have
> > a much larger table than you need.
> 
> I think I understand the confusion here, but correct me if I'm wrong. What
> we're proposing is to define the channel map for each supported channel
> group type. The array syntax I used in my last post probably added to the
> confusion - look at it again but without using the assigned values to the
> OGG_CHANNEL_* text. A couple examples:
> 
> Packet 1:
> channel_type = OGG_CHANNEL_MAP_5_1
> channel_map [0 = OGG_CHANNEL_FRONT_LEFT] = 1
> channel_map [1 = OGG_CHANNEL_FRONT_CENTER] = 2
> channel_map [2 = OGG_CHANNEL_FRONT_RIGHT] = 3
> 
> Packet 2:
> channel_type = OGG_CHANNEL_MAP_STEREO
> channel_map [0 = OGG_CHANNEL_FRONT_LEFT] = 1
> channel_map [1 = OGG_CHANNEL_FRONT_RIGHT] = 2
> 
> Packet 3:
> channel_type = OGG_CHANNEL_MAP_UHJ
> channel_map [0 = OGG_CHANNEL_UHJ_SIGMA] = 1
> channel_map [1 = OGG_CHANNEL_UHJ_DELTA] = 2
> 
> From the first two examples, we see that a channel can appear at different
> indexes in the mapping array. The order of channels within the array is
> defined by the channel_type. Under this scheme, the channel id either has
> no numerical value, or has a value defined for each different
> channel_type. To make this more clear, the channels above should be
> renamed:
> OGG_CHANNEL_STEREO_LEFT = 0
> OGG_CHANNEL_STEREO_RIGHT = 1
> OGG_CHANNEL_5_1_FRONT_LEFT = 0
> OGG_CHANNEL_5_1_FRONT_CENTER = 1
> OGG_CHANNEL_5_1_FRONT_RIGHT = 2
> OGG_CHANNEL_UHJ_SIGMA = 0
> OGG_CHANNEL_UHJ_DELTA = 1
> 
> So the number of channels in the map is always equal to the number of
> channels in the group type, not the number of channels in the stream.
> You're right, if the channel id's had a globally unique value (as is
> currently defined), then you'd have to include blank slots for all the
> unused channels as you mentioned, and this wouldn't work.
> 
> Applications should search based on group types, not channel id's - ie, an
> application supports CHANNEL_MAP_STEREO, not CHANNEL_FRONT_LEFT and
> CHANNEL_FRONT_RIGHT. So I don't think defining the channel id's within the
> scope of a group type is too bad.
> 
> 
> > I mean that if we want to describe how to convert from 5.1 to stereo,
> > then you don't want to define the conversion for both [left,right]
> > stereo and for [right,left] stereo. If you have a default, then the
> > conversion is always defined for the default and if you want something
> > else, you can easily swap channels.
> 
> The conversion maps define conversions to logical channels, not physical
> ones. To use a crude example, if you're going from 5.1 to stereo by just
> extracting the FL and FR channels, you'd define a conversion map that says
> stereo left is on channel 0, stereo right is on channel 2. Then if your
> app wants to make (r,l) out of it instead of (l,r) it's free to do so. So
> you would create a stream with Packet 1 above, and Packet 4:
> Packet 4:
> channel_type = OGG_CHANNEL_MAP_STEREO
> channel_map [0 = OGG_CHANNEL_FRONT_LEFT] = 1
> channel_map [1 = OGG_CHANNEL_FRONT_RIGHT] = 3
> 
> Does that help at all, or have I added to the confusion?
> 
> John
> 
> 
> 
> 
>