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Re: Quick question on configuring an AU for a certain channel config
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Re: Quick question on configuring an AU for a certain channel config

On Dec 5, 2007, at 8:13 PM, tahome izwah wrote:

Now I am left wondering how I declare two of the 4 input channels to
be a side chain. Any ideas or pointers to relevant documentation on
the Apple web site? I managed to download the "Audio Unit Programming
Guide" but it doesn't cover how to set up a side chain in an AU...
:-((

To add to what Bill said, if you're making an effect AU and would rather still inherit from AUEffectBase rather than going down to AUBase (in order to pass the buss count arguments to its constructor), it's also possible to add an extra busses to an AUEffectBase-derived AU. To do that, in your constructor you'll want to first call CreateElements() before doing anything with busses, and then after that you can do this:
SetBusCount(kAudioUnitScope_Input, 2);
Note that, if you're adding additional busses, you'll want to name them for the sake of the host being able to provide an interface to the user for differentiating them. You can do that next something like this:
GetElement(kAudioUnitScope_Input, 0)->SetName(CFSTR("main input"));
GetElement(kAudioUnitScope_Input, 1)->SetName(CFSTR("sidechain input"));
Note that SafeGetElement() can return null, so you should add appropriate safety measures around those calls (which I left out of my quicky example snippet).

Then in order to access that additional element, you'll want to override the AUBase::Render() method and do something like this to start with:

OSStatus status = noErr;
AudioBufferList * sidechainBufferList = NULL;
bool sidechainIsAvailable;
try { sidechainIsAvailable = HasInput(1); }
catch (...) { sidechainIsAvailable = false; }
if (sidechainIsAvailable)
{
AUInputElement * theSidechainInput = GetInput(1);
if (theSidechainInput != NULL)
{
status = theSidechainInput->PullInput(ioActionFlags, inTimeStamp, 1, inNumFramesToProcess);
if (status == noErr)
sidechainBufferList = &(theSidechainInput->GetBufferList());
}
}
if (sidechainBufferList != NULL)
{
// etc...
}

All of the buss handling stuff in the AU SDK is fairly non-obvious in my opinion and kinda difficult to get an overall handle on, so hopefully this helps some to start with...

Sophia

p.s. - The fact that AUBase::HasInput() doesn't really return false for a "no" answer so much as it throws an exception is kinda ridiculous in my opinion; I think I shall go file a bug on that...
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