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Re: Enable matrix mixer inputs without ramping
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Re: Enable matrix mixer inputs without ramping


  • Subject: Re: Enable matrix mixer inputs without ramping
  • From: William Stewart <email@hidden>
  • Date: Mon, 14 Dec 2009 19:36:09 -0800


On Dec 14, 2009, at 1:08 PM, Sean Dougall wrote:

Hi list,

Here's the simplified scenario: Our app routes playback of audio files through matrix mixers, so that we can play back a number of audio files, each channel of audio getting its own input bus on the matrix mixer that feeds the output device.

Problem is, the app doesn't always have a lot of lead time (if any) before an audio file starts playing, because it doesn't have a strict DAW-style timeline. So it needs to be able to enable the input bus immediately. What we're doing is:

		err = AudioUnitSetParameter( _mixerUnit,
									kMatrixMixerParam_Enable,
									kAudioUnitScope_Input,
									bus, 	// bus number getting passed in
									flag,		// YES when audio starts playing, NO when it stops
									0 );

This is resulting in a quick but decidedly non-zero ramp time (on the order of 10 ms),

Yes, the intention here is to ramp in a change to avoid clicks and pops. If you are going to take care of it, then there are a couple of different approaches. I think the most cost effective approach is to have all of your input buses enabled that you think you need, then have the input callbacks do one of two things:



(1) just return an error from the callback
- that should mean that the input is disregarded, and this is the most efficient. If this doesn't work, then: (sometimes error handling code is not quite correct)!
(2) memset the buffer to zero and set the ioSilence flag in the render flags from the callback
- the mixers look at this flag and will not mix in audio that is silent.


which causes playback artifacts in some cases. But we can't just leave all the inputs enabled, because we can't predict ahead of time how many inputs it'll need at any given time (and CPU usage gets really hairy if we leave a generous enough number of inputs open).

I've tried using AudioUnitScheduleParameters() instead, but no love. Is there a sensible way to immediately enable a mixer input?

No really, as the intention of the parameter is to handle clicks and pops (both on enable and disable), so it will ramp regardless of what you do.


Bill

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