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