Why does AudioOutputUnitStart() failes with return code -500?

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I am trying to develop an app that analyse an audio stream from the microphone, using Core Audio and AVAudioSession on SDK 8.1. Using bits and pieces from apples docs as well as helpful bloggers, I've managed to put together a program which successfully

• Sets properties of an AVAudioSession.


• Activates the AVAudioSession.


• Instantiates and audio component instance.


• Sets properties of the audio unit, eg. a callback function and an AudioStreamDescription.


• Initializes the audio unit.

But the program failes to start the recording - the AudioOutputUnitStart function failes with return code -500. Why is that? [The following code functions as is, if it is saved as ViewController.m and used in a single page xcode template. Even if it is not minimal, I tried to keep it as small as possible].

#import "ViewController.h"
@import AVFoundation;
@import AudioUnit;
#define kInputBus 1
AudioComponentInstance *audioUnit = NULL;
float *convertedSampleBuffer = NULL;
int status = 0;
static OSStatus recordingCallback(void *inRefCon,
                                  AudioUnitRenderActionFlags *ioActionFlags,
                                  const AudioTimeStamp *inTimeStamp,
                                  UInt32 inBusNumber,
                                  UInt32 inNumberFrames,
                                  AudioBufferList *ioData) {
    AudioBufferList *bufferList;
    OSStatus status;
    status = AudioUnitRender(*audioUnit,
                             ioActionFlags,
                             inTimeStamp,
                             inBusNumber,
                             inNumberFrames,
                             bufferList);
    printf("%d", status); printf("%s", " is the return code of AudioUnitRender from the recordingCallback.\n");
    // DoStuffWithTheRecordedAudio(bufferList);
    return noErr;
}
int myAudio() {
    AVAudioSession *mySession = [AVAudioSession sharedInstance];
    [mySession setCategory: AVAudioSessionCategoryRecord error: nil];
    [mySession setMode: AVAudioSessionModeMeasurement error: nil];
    [mySession setPreferredSampleRate:44100 error:nil];
    [mySession setPreferredIOBufferDuration:0.02 error:nil];
    [mySession setActive: YES error: nil];
    audioUnit = (AudioUnit*)malloc(sizeof(AudioUnit));
    AudioComponentDescription desc;
    desc.componentType = kAudioUnitType_Output;
    desc.componentSubType = kAudioUnitSubType_RemoteIO;
    desc.componentManufacturer = kAudioUnitManufacturer_Apple;
    desc.componentFlags = 0;
    desc.componentFlagsMask = 0;
    AudioComponent comp = AudioComponentFindNext(NULL, &desc);
    status = AudioComponentInstanceNew(comp, audioUnit);
    printf("%d", status); printf("%s", " is the return code of Instantiating a new audio component instance.\n");
    UInt32 enable = 1;
    status = AudioUnitSetProperty(*audioUnit, kAudioOutputUnitProperty_EnableIO, kAudioUnitScope_Input, kInputBus, &enable, sizeof(enable));
    printf("%d", status); printf("%s", " is the return code of EnablingIO on the audiounit.\n");
    AudioStreamBasicDescription streamDescription = {0};
    streamDescription.mSampleRate       = 44100;
    streamDescription.mFormatID         = kAudioFormatLinearPCM;
    streamDescription.mFormatFlags      = kAudioFormatFlagIsSignedInteger | kAudioFormatFlagIsPacked;
    streamDescription.mFramesPerPacket  = 1;
    streamDescription.mChannelsPerFrame = 1;
    streamDescription.mBitsPerChannel   = 16;
    streamDescription.mBytesPerPacket   = 2;
    streamDescription.mBytesPerFrame    = 2;
    status = AudioUnitSetProperty(*audioUnit, kAudioUnitProperty_StreamFormat, kAudioUnitScope_Input, 0, &streamDescription, sizeof(streamDescription));
    printf("%d", status); printf("%s", " is the return code of setting the AudioStreamDescription.\n");
    AURenderCallbackStruct callbackStruct;
    callbackStruct.inputProc = recordingCallback;
    callbackStruct.inputProcRefCon = NULL;
    status = AudioUnitSetProperty(*audioUnit,
                                  kAudioOutputUnitProperty_SetInputCallback,
                                  kAudioUnitScope_Global,
                                  kInputBus,
                                  &callbackStruct,
                                  sizeof(callbackStruct));
    printf("%d", status); printf("%s", " is the return code of setting the recording callback on the audiounit\n");
    status = AudioUnitInitialize(*audioUnit);
    printf("%d", status); printf("%s", " is the return code of initializing the audiounit.\n");
    status = AudioOutputUnitStart(*audioUnit);
    printf("%d", status); printf("%s", " is the return code of Starting the audioUnit\n");
    return noErr;
}

@interface ViewController ()
@end
@implementation ViewController
- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.
    myAudio();
    [NSThread sleepForTimeInterval:1];
    exit(0);
}
- (void)didReceiveMemoryWarning {
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}
@end

All printf statements return 0, except the last one which returns -500.

asked 1 min ago

Hans Ekbrand

48

Why does AudioOutputUnitStart() failes with return code -500?