Re: bus count and channel info confusion
Re: bus count and channel info confusion
- Subject: Re: bus count and channel info confusion
- From: Frank Hoffmann <email@hidden>
- Date: Fri, 16 Jul 2004 17:14:04 +0200
Hi Marc,
4. Audio Effect with any ability to process any number of channels
where the number of input channels has to be equal to the number of
output channels
Input Bus Count: 1, not writeable
Output Bus Count: 1, not writeable
Channel Info: no channel info or {-1, -1}
[edited]
Technically correct, but the preferred way (and way which you should
recognize since it's what most N-N effects do) is to not supply any
Channel Info, it will return an error from GetPropertyInfo() because
the property is not being supported.
You are right. To provide no channel info is also correct and should be
preferred.
8. Stereo Synth (Music Device)
Input Bus Count: 0, not writeable
Output Bus Count: 1, not writeable
Channel Info: no channel info or {0, 2}
[edited]
Correct. * (see footnote)
Yes.
10. Music Device that can output Mono or Stereo and has an optional
side chain input, which can be mono or stereo
[edited]
Input Bus Count: 1, not writeable
Output Bus Count: 1, not writeable
Channel Info: {0, 1}, {0, 2}, {1, 1}, {1, 2}, {2, 1}, {2, 2}
Assuming the input is a "sidechain" and optional, correct. Otherwise
it shouldn't have any of the {0, X} configurations. Also, it may
choose to not support mixed {2, 1} or {1, 2} configurations if the i/o
are tied together in some way. Anyway, depends on the AU, but I could
see either of those possibly being the case.
Yes, I clarified that in the description. The audio side chain input of
most synths is optional, but doesn't have to be.
11. MusicDevice that can configure its Outputs according to user
settings like LinPlug RM IV, Native Instruments Kontakt, Steinberg
Halion 3, etc.
Input Bus Count: 0, not writeable
Output Bus Count: X, not writeable
Channel Info: No ChannelConfig provided. This is a special case
described in IOChannelConfigurations.rtf.
X is the number of Output Busses configured in the GUI of the
plug-in. If the setup reads two stereo and two mono outs, then X
would be 4. X would be 4 as well, if the user has setup four stereo
outs and zero mono outs. The host will read the stream formats of all
busses and does change only the sample rate. For rendering the host
iterates over all output busses and renders each output buffer
seperatly with the channels found in the stream formats.
Question 1: Is it legal to send a property notificaiton to the host,
that the number of busses changed? If yes, then the host should
uninitilaize the Audio Unit, negociate the new setup with the plug-in
and initialize it again?
Question 2: Is it legal to render all channels on bus 0? The above
example with two stereo and two mono busses. Is it legal to setup a
stream format with 6 channels on bus 0? I think no, because the
plug-in can't say which channels are mono or stereo. The host has no
idea about mono and stereo in this case as well.
Are you sure that these scenarios shouldn't be handled in the manner
described in 12?
This is an interesting question. How much freedom is given to the Audio
Units? Is it legal to set up the amount of busses and their stream
configuration in the plug-in GUI?
Aside from this consideration, there are 2 main differences:
1) the plug-in has a fixed bus count and on every bus a fixed stream
format. This is most easily achieved with providing no channel info.
2) in scenario 12 the host decides about the amount of busses. In
scenario 11 the plug-in does.
12. Audio Unit that is able to serve a fixed number of channels
configured in selectable mono or stereo pairs. Example: Native
Instruments Reaktor.
Input Bus Count: X, WRITABLE
Output Bus Count: Y, WRITEABLE
Channel Info: {-N, -M}
N and M are the number of channels the plug-in can serve on input and
ouput busses. The host will set X and Y according to its needs and
set the stream formats for the X and Y busses as required using max N
channels for the input busses and max M channels for the output
busses in total. Rendering will be done for every bus for one time
slice seperate with the number of channels setup in the stream
formats.
Note that it is never legal to use 1 or 2 for N or M.
I think correct.
13. A Music Device with two stereo outs like Korg Wavestation for
example.
Input Bus Count: 0, not writeable
Output Bus Count: 2, not writeable
Channel Info: no channel info or {0,2}
[edited]
Correct. * (see footnote)
* Although, in both of these MusicDevice examples, they also could
simply not support ChannelInfo and instead configure their own
bus(ses) to 0,2 to start with (well, either way, they have to
configure themselves to that to start with). MusicDevices, unlike
effects, are allowed to do that, since not supplying ChannelInfo
should not be interpretted in the same way as with effects, and
instead that signals that only one channel configration is supported,
and that is the configuration that the MD is already set to, and that
configuration can't be changed.
It is maybe unfortunate that Audio Units are allowed to do one thing
several ways, but it is still correct. I agree that to provide no
channel info is the more elegant way here.
Hosts have to be able to handle both ways.
Thanks Marc.
Frank
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