Record and play audio with AVAssetWriter

Question

I have reduced this question quite a bit and am hoping for some help.

Basically this class has two methods, one to start recording audio (-recordMode) and the other to play audio (playMode). I currently have this class in a project with a single view controller with two buttons that call the corresponding methods (rec, play). There are no other variables, the class is self-contained.

However it will not play/rec anything and I cannot figure out why. When I try to play the file I get a file size of 0 and an error because you cant init the AVAudioPlayer with a nil reference of course. But I dont understand why the file is empty or why self.outputPath is nil.

.h file

#import <AVFoundation/AVFoundation.h>

@interface MicCommunicator : NSObject<AVCaptureAudioDataOutputSampleBufferDelegate>

@property(nonatomic,retain) NSURL *outputPath;
@property(nonatomic,retain) AVCaptureSession * captureSession;
@property(nonatomic,retain) AVCaptureAudioDataOutput * output;

-(void)beginStreaming;
-(void)playMode;
-(void)recordMode;

@end

.m file:

@implementation MicCommunicator {
    AVAssetWriter *assetWriter;
    AVAssetWriterInput *assetWriterInput;
}

@synthesize captureSession = _captureSession;
@synthesize output = _output;
@synthesize outputPath = _outputPath;

-(id)init {
    if ((self = [super init])) {
        NSArray *searchPaths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
        self.outputPath = [NSURL fileURLWithPath:[[searchPaths objectAtIndex:0] stringByAppendingPathComponent:@"micOutput.output"]];

        AudioChannelLayout acl;
        bzero(&acl, sizeof(acl));
        acl.mChannelLayoutTag = kAudioChannelLayoutTag_Mono; //kAudioChannelLayoutTag_Stereo;
        NSDictionary *audioOutputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                             [NSNumber numberWithInt: kAudioFormatULaw],AVFormatIDKey,        
                                             [NSNumber numberWithFloat:8000.0],AVSampleRateKey,//was 44100.0
                                             [NSData dataWithBytes: &acl length: sizeof( AudioChannelLayout ) ], AVChannelLayoutKey,
                                             [NSNumber numberWithInt:1],AVNumberOfChannelsKey,
                                             [NSNumber numberWithInt:8000.0],AVEncoderBitRateKey,
                                             nil];

        assetWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio outputSettings:audioOutputSettings];
        [assetWriterInput setExpectsMediaDataInRealTime:YES];

        assetWriter = [[AVAssetWriter assetWriterWithURL:_outputPath fileType:AVFileTypeWAVE error:nil] retain];
        [assetWriter addInput:assetWriterInput];
    }
    return self;
}

-(void)dealloc {
    [assetWriter release];
    [super dealloc];
}

//conveniance methods

-(void)playMode
{
    [self stopRecording];

    NSError *error;
    AVAudioPlayer * audioPlayer = [[AVAudioPlayer alloc] initWithContentsOfURL:self.outputPath error:&error];
    audioPlayer.numberOfLoops = -1;

    if (audioPlayer == nil){
        NSLog(@"error: %@",[error description]);        
    }else{ 
        NSLog(@"playing");  
        [audioPlayer play];
    }
}

-(void)recordMode
{
        [self beginStreaming];    
}

-(void)stopRecording
{
    [self.captureSession stopRunning];
    [assetWriterInput markAsFinished];
    [assetWriter  finishWriting];

    NSDictionary *outputFileAttributes = [[NSFileManager defaultManager] attributesOfItemAtPath:[NSString stringWithFormat:@"%@",self.outputPath] error:nil];
    NSLog (@"done. file size is %llu", [outputFileAttributes fileSize]);
}

//starts audio recording
-(void)beginStreaming {
    self.captureSession = [[AVCaptureSession alloc] init];
    AVCaptureDevice *audioCaptureDevice = [AVCaptureDevice defaultDeviceWithMediaType:AVMediaTypeAudio];
    NSError *error = nil;
    AVCaptureDeviceInput *audioInput = [AVCaptureDeviceInput deviceInputWithDevice:audioCaptureDevice error:&error];
    if (audioInput)
        [self.captureSession addInput:audioInput];
    else {
        NSLog(@"No audio input found.");
        return;
    }

    AVCaptureAudioDataOutput *output = [[AVCaptureAudioDataOutput alloc] init];

    dispatch_queue_t outputQueue = dispatch_queue_create("micOutputDispatchQueue", NULL);
    [output setSampleBufferDelegate:self queue:outputQueue];
    dispatch_release(outputQueue);

    [self.captureSession addOutput:output];
    [assetWriter startWriting];
    [self.captureSession startRunning];
}

//callback
-(void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection {
    AudioBufferList audioBufferList;
    NSMutableData *data= [[NSMutableData alloc] init];
    CMBlockBufferRef blockBuffer;
    CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(sampleBuffer, NULL, &audioBufferList, sizeof(audioBufferList), NULL, NULL, 0, &blockBuffer);

    //for (int y = 0; y < audioBufferList.mNumberBuffers; y++) {
    //  AudioBuffer audioBuffer = audioBufferList.mBuffers[y];
    //  Float32 *frame = (Float32*)audioBuffer.mData;
    //          
    //  [data appendBytes:frame length:audioBuffer.mDataByteSize];
    //}

    // append [data bytes] to your NSOutputStream 


    // These two lines write to disk, you may not need this, just providing an example
    [assetWriter startSessionAtSourceTime:CMSampleBufferGetPresentationTimeStamp(sampleBuffer)];
    [assetWriterInput appendSampleBuffer:sampleBuffer];

    CFRelease(blockBuffer);
    blockBuffer=NULL;
    [data release];
}

@end

Answer 1

Per Apple Support:

this is therefore the bug -- the file is created, a number of samples are written successfully then append starts failing for some unknown reason.

It seems that AVAssetWriter fails only with these settings.

AudioQueue is what should be used for ulaw audio