[gmpi] Re: Topic 7: Audio packaging


Chris wrote:
So what's the downside to agreed-upon ordering / what's the use case for arbitrary channel ordering? Bill, when you said that there are currently different channel orders for 5.1, what was the context (1394 wire, USB wire, IAC, etc.), and if it's only in stream transport protocols, then couldn't that be adapted to GMPI a single convention at the GMPI graph boundary? Seems like that'd remove the need to handle arbitrary ordering within GMPI multichannel audio connections?

Yes, we can decide on one ordering convention per format at the GMPI level. Then the host has to deal with ordering issues when interfacing to the external world.

Chris wrote:
I suppose ideally this could/should be handled with some sort of, as Bill suggests, user-extensible metadata tagging scheme. But given the difficulty we've had so far on Topic 7, I really wonder how to proceed on this.

Some simple ideas:

1) provide an encoding enum along with format enum. This is limited to only a single encoding at a time, and I suppose it might be nicer to have a set of properties that could be defined.

2) provide a "GetPinStreamProperties" function or something that returns a set of properties which can be GMPI published or user defined. The use of stream properties can be ignored of course.

Tim wrote: 
OK, this is going to seriously impact those plugins that want to do things
cross-format in a BIG way.  I don't suspect many, if any, plugins will ever
support these encodings.  You'll have your LtRt reader and that's it. :)

Encoding enums, properties, whatever should be optional and can be ignored. I tend to agree with Tim that their use will probably be limited to specific groups of cooperating plugs and hosts, and likely really pro stuff. But, I think it will be very useful in certain situations.

I agree with Tim that hard encodings (like MP3, AAC, MLP, ADPCM) should be decoded prior to entering the GMPI graph as a PCM sample stream (there remains the possibility of non-PCM media formats in future versions). However, for certain things like matrix encoding (Dolby stereo, Logic 7, CircleSurround), spatial microphone technique (M/S, binaural, soundfield), and spatial playback encoding (crosstalk cancelled, ambisonics), this encoding information is potentially useful. Most examples I can think of are spatial encodings.

Bill









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