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Getting available sample rates and bit depths (nominal vs. physical vs. Audio MIDI Setup)
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Getting available sample rates and bit depths (nominal vs. physical vs. Audio MIDI Setup)


  • Subject: Getting available sample rates and bit depths (nominal vs. physical vs. Audio MIDI Setup)
  • From: Gunnar Proppe <email@hidden>
  • Date: Fri, 5 Oct 2007 13:10:20 -0700

I'm attempting to provide the user with pop-up menus for setting the sample rate and bit depth along with the audio output device selection pop-up in a preference panel. I've tried a few approaches and I'm confused by the varying results. Here's some output from my test code:

FireOne (0007)
-- Physical rates: --
	192000 24
	176400 24
	96000 24
	88200 24
-- Virtual rates: --
	192000 32
	176400 32
	96000 32
	88200 32
-- Nominal rates: --
	44100
	48000
	88200
	96000
	176400
	192000

Built-in Output
-- Physical rates: --
	96000 16
	48000 16
	44100 16
	96000 20
	48000 20
	44100 20
-- Virtual rates: --
	96000 32
	48000 32
-- Nominal rates: --
	44100
	48000
	96000

I used AudioDeviceGetProperty(...) with kAudioStreamPropertyAvailablePhysicalFormats , kAudioStreamPropertyAvailableVirtualFormats, kAudioDevicePropertyAvailableNominalSampleRates to obtain these results.

As you can see, the FireOne reports only rates higher than 48000 Hz for physical rates. I get a similar result with another device, a TASCAM US-428 reports only 48000 Hz, not 44000.

Audio MIDI Setup also shows different results: it adds 24 bits to the Built-in Output's bit depth pop-up.

I was hoping to use the results of kAudioStreamPropertyAvailablePhysicalFormats because it pairs the bit depths and sample rates (I'm guessing there are devices that don't offer all sample rates at all bit depths). This is also the only way I've found to get the native bit depths (there's no such thing as kAudioDevicePropertyAvailableBitDepths, right?). Could our devices be responding incorrectly to these requests (the FireOne is using the built-in OS firewire driver)?

It's quite possible I'm doing something boneheaded in my code. If none of this rings a bell I'll provide an excerpt.

What are the definitions of physical, virtual and nominal in this context?

In short, how do I provide the same choices in my GUI as are shown in Audio MIDI Setup?

Thanks,
Gunnar



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