CoreAudio AudioUnitSetProperty always fails to set Sample Rate

c++, core-audio, macos, objective-c

Solution

With the DefaultOuput AudioUnit you only set the input side of the AudioUnit to the format you wish to render. The output side of the unit will match what you specify on the input side but you cannot set it yourself.

Try this after you have set the input stream format and you'll see that you are all set to go...

Float64 outSampleRate = 0.0;
UInt32 size = sizeof(Float64);
AudioUnitGetProperty (gOutputUnit,
                      kAudioUnitProperty_SampleRate,
                      kAudioUnitScope_Output,
                      0,
                      &outSampleRate,
                      &size);
NSLog(@"Output sample rate is now at %f Hz", outSampleRate);

You can also look at the Audio Unit Component Services Reference to see that error code -10865 is kAudioUnitErr_PropertyNotWritable.

Problem

I need to change the output sample rate from 44.1 to 32.0, but it always throws an error, Out: AudioUnitSetProperty-SF=\217\325\377\377, -10865. I don't know why it will let me set it for input, but then not set it for output. My code is: ``` - (void)applicationDidFinishLaunching:(NSNotification *)aNotification { OSStatus MyRenderer(void *inRefCon, AudioUnitRenderActionFlags *ioActionFlags, const AudioTimeStamp *inTimeStamp, UInt32 inBusNumber, UInt32 inNumberFrames, AudioBufferList *ioData){ NSLog(@"Running..."); ioData->mBuffers[0].mDataByteSize = 2048; ioData->mBuffers[0].mData = lbuf; ioData->mBuffers[0].mNumberChannels = 1; return noErr; } void CreateDefaultAU(){ OSStatus err = noErr; // Open the default output unit AudioComponentDescription desc; desc.componentType = kAudioUnitType_Output; desc.componentSubType = kAudioUnitSubType_DefaultOutput; desc.componentFlags = 0; desc.componentFlagsMask = 0; desc.componentManufacturer = 0; AudioComponent comp = AudioComponentFindNext(NULL, &desc); if (comp == NULL) { printf ("FindNextComponent\n"); return; } err = AudioComponentInstanceNew(comp, &gOutputUnit); if (comp == NULL) { printf ("OpenAComponent=%ld\n", err); return; } // Set up a callback function to generate output to the output unit AURenderCallbackStruct input; input.inputProc = MyRenderer; input.inputProcRefCon = NULL; err = AudioUnitSetProperty(gOutputUnit, kAudioUnitProperty_SetRenderCallback, kAudioUnitScope_Input, 0, &input, sizeof(input)); if (err) { printf ("AudioUnitSetProperty-CB=%ld\n", err); return; } AudioStreamBasicDescription streamFormat; streamFormat.mSampleRate = 32000.00; // the sample rate of the audio stream streamFormat.mFormatID = kAudioFormatLinearPCM; // the specific encoding type of audio stream streamFormat.mFormatFlags = kAudioFormatFlagIsSignedInteger;//kAudioFormatFlagsNativeEndian | kAudioFormatFlagIsNonMixable; streamFormat.mFramesPerPacket = 1; streamFormat.mChannelsPerFrame = 1; streamFormat.mBitsPerChannel = 16; streamFormat.mBytesPerPacket = 2; streamFormat.mBytesPerFrame = 2; err = AudioUnitSetProperty(gOutputUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, 0, &streamFormat, sizeof(streamFormat)); if (err) { printf ("In: AudioUnitSetProperty-SF=%4.4s, %ld\n", (char*)&err, err); return; } err = AudioUnitSetProperty(gOutputUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Output, 0, &streamFormat, sizeof(streamFormat)); if (err) { printf ("Out: AudioUnitSetProperty-SF=%4.4s, %ld\n", (char*)&err, err); return; } } void TestDefaultAU(){ OSStatus err = noErr; // Initialize unit err = AudioUnitInitialize(gOutputUnit); if (err) { printf ("AudioUnitInitialize=%ld\n", err); return; } Float64 outSampleRate; UInt32 size = sizeof(Float64); err = AudioUnitGetProperty(gOutputUnit, kAudioUnitProperty_SampleRate, kAudioUnitScope_Output, 0, &outSampleRate, &size); printf("Out srate %f\n",outSampleRate); if (err) { printf ("AudioUnitSetProperty-GF=%4.4s, %ld\n", (char*)&err, err); return; } AudioOutputUnitStart (gOutputUnit); if (err) { printf ("AudioOutputUnitStart=%ld\n", err); return; } AudioUnitReset (gOutputUnit, kAudioUnitScope_Input, 0); } ```

Original source