Re: kAudioUnitProperty_BusCount
Re: kAudioUnitProperty_BusCount
- Subject: Re: kAudioUnitProperty_BusCount
- From: Marc Poirier <email@hidden>
- Date: Mon, 16 Feb 2004 00:31:15 +0100 (CET)
Hi Seb. There was a bunch of discussion/clarification of this recently on
the list (1 or 2 months ago, I think), so you may want to check the
archives, but also I'll try to tell what I know...
For one thing, the bus count usually is not writable. You need to check
using GetPropertyInfo. So in most cases, you can't set the bus count.
The next important thing to realize is that SupportedNumChannels goes for
all busses for effects. So if the effect has 1,2 and 2,2 as its channel
infos and it has 3 busses, then all 3 busses can do 1,2 or 2,2, and
moreover when you set the channel count, it is set for all busses, not
just for 1 or 2 of them.
For an instrument, there is an exception which is that they can not
support the SupportedNumChannels property and simply configure their own
stream format in the single format that they will support, which can
include different channel counts on different busses. For these, you will
not be changing the channel count at all.
Also, just to explain, the concept of busses are that they have
conceptually separate sets of (1 or more) audio channels. For example, a
ducking compressor would have its audio bus and then modulator bus. It
may be able to do any number of channels on each bus, so long as the
channel count is equal all accross. Or a vocoder would have 1 bus for
modulator and 1 for carrier. A sampler may support different stereo
output pairs as separate busses. These are just a few examples.
Okay, I decided to search the archives a bit and found this message, which
I think is the especially useful one that I was thinking of:
http://lists.apple.com/archives/coreaudio-api/2003/Oct/24/channelsbussesandioconfi.txt
I hope that helped. I'm not a real expert on this as I haven't tinkered
with multi-bus stuff myself yet, I'm just trying to remember what I've
read in the various messages here, so if I got anything wrong, please
someone correct me...
Marc
On Mon, 2 Feb 2004, seb_plogue wrote:
>
Hi,
>
>
I'm posting here as I can't seem to find any
>
clear explanation/answer on this:
>
>
What does the host do with the kAudioUnitProperty_BusCount
>
property?
>
>
I wish to present the user with the available
>
I/O configurations for each AudioUnit so he/she
>
can select the wanted one. I do this by getting
>
the SupportedNumChannels property, if it returns "fixed"
>
I/O, I add those as available, if it returns "flexible"
>
or it's agnostic, I test different configurations
>
by setting the StreamFormat property.
>
>
That seems to be working fine in most cases
>
but I see some AUs reporting having 8 buses
>
and configs of [0,2],[0,8] or [0,2],[0,16].
>
>
Whether I set the StreamFormat to 2 channels or 8/16, it
>
still returns 8 buses. Does that mean the host has to
>
figure out that if there's 8 buses and 16 channels, it should
>
be viewed as 8 stereo buses and when using the 2 channels
>
config, the host should be setting the BusCount to 1?
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