iOS. Record at 96kHz with USB microphone - ios

Question

I am trying to record at full 96kHz with my RØDE iXY USB microphone.
Recording goes without error and when I launch the app with the mic connected, I see that AVAudioSession is running successfully at 96kHz sample rate.
But if I look at the spectrum it is clear that there is nothing but resample noise above 20kHz:
For comparison this is a spectrum of the same recording using the app bundled with the USB mic (RØDE Rec):
Is there anything else I must do to record at native 96kHz?
Or maybe the RØDE Rec app communicates with the mic with some proprietary protocol over USB and I'm out of luck here?
I included the source code that I use:
static AudioStreamBasicDescription AudioDescription24BitStereo96000 = (AudioStreamBasicDescription) {
.mFormatID = kAudioFormatLinearPCM,
.mFormatFlags = kAudioFormatFlagIsPacked | kAudioFormatFlagIsSignedInteger,
.mChannelsPerFrame = 2,
.mBytesPerPacket = 6,
.mFramesPerPacket = 1,
.mBytesPerFrame = 6,
.mBitsPerChannel = 24,
.mSampleRate = 96000.0
};
- (void)setupAudioSession
{
AVAudioSession *session = [AVAudioSession sharedInstance];
[session setCategory:AVAudioSessionCategoryRecord error:&error];
[session setActive:YES error:&error];
[session setPreferredSampleRate:96000.0f error:&error];
//I got my 96000Hz with the USB mic plugged in!
NSLog(#"sampleRate = %lf", session.sampleRate);
}
- (void)startRecording
{
AudioComponentDescription desc;
desc.componentType = kAudioUnitType_Output;
desc.componentSubType = kAudioUnitSubType_RemoteIO;
desc.componentFlags = 0;
desc.componentFlagsMask = 0;
desc.componentManufacturer = kAudioUnitManufacturer_Apple;
AudioComponent inputComponent = AudioComponentFindNext(NULL, &desc);
AudioComponentInstanceNew(inputComponent, &audioUnit);
AudioUnitScope inputBus = 1;
UInt32 flag = 1;
AudioUnitSetProperty(audioUnit, kAudioOutputUnitProperty_EnableIO, kAudioUnitScope_Input, inputBus, &flag, sizeof(flag));
audioDescription = AudioDescription24BitStereo96000;
AudioUnitSetProperty(audioUnit,
kAudioUnitProperty_StreamFormat,
kAudioUnitScope_Output,
inputBus,
&audioDescription,
sizeof(audioDescription));
AURenderCallbackStruct callbackStruct;
callbackStruct.inputProc = recordingCallback;
callbackStruct.inputProcRefCon = (__bridge void *)(self);
AudioUnitSetProperty(audioUnit,
kAudioOutputUnitProperty_SetInputCallback,
kAudioUnitScope_Global,
inputBus, &callbackStruct,
sizeof(callbackStruct));
AudioOutputUnitStart(audioUnit);
}
static OSStatus recordingCallback(void *inRefCon,
AudioUnitRenderActionFlags *ioActionFlags,
const AudioTimeStamp *inTimeStamp,
UInt32 inBusNumber,
UInt32 inNumberFrames,
AudioBufferList *ioData)
{
AudioBuffer audioBuffer;
audioBuffer.mNumberChannels = 1;
audioBuffer.mDataByteSize = inNumberFrames * audioDescription.mBytesPerFrame;
audioBuffer.mData = malloc( inNumberFrames * audioDescription.mBytesPerFrame );
// Put buffer in a AudioBufferList
AudioBufferList bufferList;
bufferList.mNumberBuffers = 1;
bufferList.mBuffers[0] = audioBuffer;
AudioUnitRender(audioUnit, ioActionFlags, inTimeStamp, inBusNumber, inNumberFrames, &bufferList);
//I then take the samples and write them to WAV file
}

Answer 1

Check the hardware sample rate audio session property with your microphone plugged in. Also check all audio unit function error return values.
RemoteIO may be using a lower input sample rate and then resampling to a 96k stream.