How do I use AVAssetWriter to write AAC audio out in ios?

Question

I am using an AVCaptureSession to capture audio and video samples from the devices microphone and camera.

I am then attempting to write the CMSampleBuffers (Using AVAssetWriter and AVAssetWriterInputs) returned via the AVCaptureSessions delegate method

- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:    (CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection

This works fine when my audio AVAssetWriterInput is configured to write the data out in Apple Lossless format (kAudioFormatAppleLossless) but if i try and configure the audio AVAssetWriterInput to use AAC (kAudioFormatMPEG4AAC) it successfully writes video and audio samples for about 500ms then fails with the following error

writer has failed with Error Domain=AVFoundationErrorDomain Code=-11821 "Cannot Decode" UserInfo=0x4b2630 {NSLocalizedFailureReason=The media data could not be decoded. It may be damaged., NSUnderlyingError=0x4ad0f0 "The operation couldn’t be completed. (OSStatus error 560226676.)", NSLocalizedDescription=Cannot Decode}

Here is the code I use to create my AVAssetWriter and AVAssetWriterInputs

NSError *error = nil;
m_VideoCaputurePath = [[NSString stringWithFormat:@"%@/%@.mp4",[UserData getSavePath],[UserData getUniqueFilename]] retain];

if( USE_AAC_AUDIO )
{
    m_audioAndVideoWriter = [[AVAssetWriter alloc] initWithURL:[NSURL fileURLWithPath:m_VideoCaputurePath] fileType:AVFileTypeMPEG4 error:&error];
}
else
{
    m_audioAndVideoWriter = [[AVAssetWriter alloc] initWithURL:[NSURL fileURLWithPath:m_VideoCaputurePath] fileType:AVFileTypeQuickTimeMovie error:&error];
}

//\Configure Video Writer Input
NSDictionary *videoSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                               AVVideoCodecH264, AVVideoCodecKey,
                               [NSNumber numberWithInt:640], AVVideoWidthKey,
                               [NSNumber numberWithInt:480], AVVideoHeightKey,
                               nil];

m_videoWriterInput = [[AVAssetWriterInput
                       assetWriterInputWithMediaType:AVMediaTypeVideo
                       outputSettings:videoSettings] retain];

m_videoWriterInput.expectsMediaDataInRealTime = YES;

//\Configure Audio Writer Input

AudioChannelLayout acl;
bzero(&acl, sizeof(acl));
acl.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;

NSDictionary*  audioOutputSettings;
if( USE_AAC_AUDIO )
{
    audioOutputSettings = [ NSDictionary dictionaryWithObjectsAndKeys: [NSNumber numberWithInt:kAudioFormatMPEG4AAC], AVFormatIDKey,
                                          [ NSNumber numberWithInt: 1 ], AVNumberOfChannelsKey,
                                          [ NSNumber numberWithFloat: 44100.0 ], AVSampleRateKey,
                                          [ NSData dataWithBytes: &acl length: sizeof( acl ) ], AVChannelLayoutKey,
                                          [ NSNumber numberWithInt: 96 ], AVEncoderBitRateKey,
                                          nil];
}
else
{
    audioOutputSettings = [ NSDictionary dictionaryWithObjectsAndKeys:                       
                                          [ NSNumber numberWithInt: kAudioFormatAppleLossless ], AVFormatIDKey,
                                          [ NSNumber numberWithInt: 16 ], AVEncoderBitDepthHintKey,
                                          [ NSNumber numberWithFloat: 44100.0 ], AVSampleRateKey,
                                          [ NSNumber numberWithInt: 1 ], AVNumberOfChannelsKey,                                      
                                          [ NSData dataWithBytes: &acl length: sizeof( acl ) ], AVChannelLayoutKey, nil ];
}

m_audioWriterInput = [[AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeAudio outputSettings:audioOutputSettings] retain];

m_audioWriterInput.expectsMediaDataInRealTime = YES;

//\Add inputs to Write
NSAssert([m_audioAndVideoWriter canAddInput:m_audioWriterInput], @"Cannot write to this type of audio input" );
NSAssert([m_audioAndVideoWriter canAddInput:m_videoWriterInput], @"Cannot write to this type of video input" );

[m_audioAndVideoWriter addInput:m_videoWriterInput];
[m_audioAndVideoWriter addInput:m_audioWriterInput];

Does anyone know how to correctly write audio samples returned from AVCaptureSession using an AVAssetWriterInput configured to write AAC?

Answer 1

I managed to get it to work by changing the AVEncoderBitRateKey parameter passed as the desired output settings from 96 to 64000.

My audio settings for initializing an AVAssetWriterInput capable of writing AAC audio now looks like

    NSDictionary*  audioOutputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                    [ NSNumber numberWithInt: kAudioFormatMPEG4AAC], AVFormatIDKey,
                                    [ NSNumber numberWithInt: 1 ], AVNumberOfChannelsKey,
                                    [ NSNumber numberWithFloat: 44100.0 ], AVSampleRateKey,
                                    [ NSData dataWithBytes: &acl length: sizeof( AudioChannelLayout ) ], AVChannelLayoutKey,
                                    [ NSNumber numberWithInt: 64000 ], AVEncoderBitRateKey,
                                    nil]