Is kAudioFormatFlagIsFloat supported on iPhoneOS?

Question

I am writing an iPhone app that records and plays audio simultaneously using the I/O audio unit as per Apple's recommendations.

I want to apply some sound effects (reverb, etc) on the recorded audio before playing it back. For these effects to work well, I need the samples to be floating point numbers, rather than integers. It seems this should be possible, by creating an AudioStreamBasicDescription with kAudioFormatFlagIsFloat set on mFormatFlags. This is what my code looks like:

AudioStreamBasicDescription streamDescription;

streamDescription.mSampleRate = 44100.0;
streamDescription.mFormatID = kAudioFormatLinearPCM;
streamDescription.mFormatFlags = kAudioFormatFlagIsFloat;
streamDescription.mBitsPerChannel = 32;
streamDescription.mBytesPerFrame = 4;
streamDescription.mBytesPerPacket = 4;
streamDescription.mChannelsPerFrame = 1;
streamDescription.mFramesPerPacket = 1;
streamDescription.mReserved = 0;

OSStatus status;

status = AudioUnitSetProperty(audioUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, 0, &streamDescription, sizeof(streamDescription));
if (status != noErr)
  fprintf(stderr, "AudioUnitSetProperty (kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input) returned status %ld\n", status);

status = AudioUnitSetProperty(audioUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Output, 1, &streamDescription, sizeof(streamDescription));
if (status != noErr)
  fprintf(stderr, "AudioUnitSetProperty (kAudioUnitProperty_StreamFormat, kAudioUnitScope_Output) returned status %ld\n", status);

However, when I run this (on an iPhone 3GS running iPhoneOS 3.1.3), I get this:

AudioUnitSetProperty (kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input) returned error -10868
AudioUnitSetProperty (kAudioUnitProperty_StreamFormat, kAudioUnitScope_Output) returned error -10868

(-10868 is the value of kAudioUnitErr_FormatNotSupported)

I didn't find anything of value in Apple's documentation, apart from a recommendation to stick to 16 bit little-endian integers. However, the aurioTouch example project contains at least some support code related to kAudioFormatFlagIsFloat.

So, is my stream description incorrect, or is kAudioFormatFlagIsFloat simply not supported on iPhoneOS?

Answer 1

From the Core Audio docs:

kAudioFormatFlagIsFloat
  Set for floating point, clear for integer.
  Available in iPhone OS 2.0 and later.
  Declared in CoreAudioTypes.h.

I don't know enough about your stream to comment on its [in]correctness.