This code to write video+audio through AVAssetWriter and AVAssetWriterInputs is not working. Why?

I've been trying to write a video+audio using AVAssetWriter and AVAssetWriterInputs.

I read multiple posts in this forum of people saying they were able to accomplish that, but it is not working for me. If I just write video then the code is doing its job very well. When I add audio the output file is corrupted and cannot be reproduced.

Here is part of my code:

Setting up AVCaptureVideoDataOutput and AVCaptureAudioDataOutput:

NSError *error = nil;

// Setup the video input
AVCaptureDevice *videoDevice = [AVCaptureDevice defaultDeviceWithMediaType: AVMediaTypeVideo];
// Create a device input with the device and add it to the session.
AVCaptureDeviceInput *videoInput = [AVCaptureDeviceInput deviceInputWithDevice:videoDevice error:&error];
// Setup the video output
_videoOutput = [[AVCaptureVideoDataOutput alloc] init];
_videoOutput.alwaysDiscardsLateVideoFrames = NO;
_videoOutput.videoSettings =
[NSDictionary dictionaryWithObject:
[NSNumber numberWithInt:kCVPixelFormatType_32BGRA] forKey:(id)kCVPixelBufferPixelFormatTypeKey];     

// Setup the audio input
AVCaptureDevice *audioDevice     = [AVCaptureDevice defaultDeviceWithMediaType: AVMediaTypeAudio];
AVCaptureDeviceInput *audioInput = [AVCaptureDeviceInput deviceInputWithDevice:audioDevice error:&error ];     
// Setup the audio output
_audioOutput = [[AVCaptureAudioDataOutput alloc] init];

// Create the session
_capSession = [[AVCaptureSession alloc] init];
[_capSession addInput:videoInput];
[_capSession addInput:audioInput];
[_capSession addOutput:_videoOutput];
[_capSession addOutput:_audioOutput];

_capSession.sessionPreset = AVCaptureSessionPresetLow;     

// Setup the queue
dispatch_queue_t queue = dispatch_queue_create("MyQueue", NULL);
[_videoOutput setSampleBufferDelegate:self queue:queue];
[_audioOutput setSampleBufferDelegate:self queue:queue];
dispatch_release(queue);

Setting up AVAssetWriter and associating both audio and video AVAssetWriterInputs to it:

- (BOOL)setupWriter {
    NSError *error = nil;
    _videoWriter = [[AVAssetWriter alloc] initWithURL:videoURL 
                                             fileType:AVFileTypeQuickTimeMovie
                                                error:&error];
    NSParameterAssert(_videoWriter);


    // Add video input
    NSDictionary *videoCompressionProps = [NSDictionary dictionaryWithObjectsAndKeys:
                                                 [NSNumber numberWithDouble:128.0*1024.0], AVVideoAverageBitRateKey,
                                                        nil ];

    NSDictionary *videoSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                              AVVideoCodecH264, AVVideoCodecKey,
                                              [NSNumber numberWithInt:192], AVVideoWidthKey,
                                              [NSNumber numberWithInt:144], AVVideoHeightKey,
                                              videoCompressionProps, AVVideoCompressionPropertiesKey,
                                              nil];

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

        
    NSParameterAssert(_videoWriterInput);
    _videoWriterInput.expectsMediaDataInRealTime = YES;


    // Add the audio input
    AudioChannelLayout acl;
    bzero( &acl, sizeof(acl));
    acl.mChannelLayoutTag = kAudioChannelLayoutTag_Mono;

        
    NSDictionary* audioOutputSettings = nil;          
    // Both type of audio inputs causes output video file to be corrupted.
    if (NO) {
        // should work from iphone 3GS on and from ipod 3rd generation
        audioOutputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                              [ NSNumber numberWithInt: kAudioFormatMPEG4AAC ], AVFormatIDKey,
                                     [ NSNumber numberWithInt: 1 ], AVNumberOfChannelsKey,
                              [ NSNumber numberWithFloat: 44100.0 ], AVSampleRateKey,
                              [ NSNumber numberWithInt: 64000 ], AVEncoderBitRateKey,
                              [ NSData dataWithBytes: &acl length: sizeof( acl ) ], AVChannelLayoutKey,
                              nil];
    } else {
        // should work on any device requires more space
        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 ];
    } 
        
    _audioWriterInput = [[AVAssetWriterInput 
                            assetWriterInputWithMediaType: AVMediaTypeAudio 
                  outputSettings: audioOutputSettings ] retain];

    _audioWriterInput.expectsMediaDataInRealTime = YES;

    // add input
    [_videoWriter addInput:_videoWriterInput];
    [_videoWriter addInput:_audioWriterInput];

    return YES;
}

here are functions to start/stop video recording

- (void)startVideoRecording
{
    if (!_isRecording) {
        NSLog(@"start video recording...");
        if (![self setupWriter]) {
             return;
        }
        _isRecording = YES;
    }
}

- (void)stopVideoRecording
{
    if (_isRecording) {
        _isRecording = NO;

        [_videoWriterInput markAsFinished];
        [_videoWriter endSessionAtSourceTime:lastSampleTime];

        [_videoWriter finishWriting];

        NSLog(@"video recording stopped");
    }
}

And finally the CaptureOutput code

- (void)captureOutput:(AVCaptureOutput *)captureOutput
didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer
       fromConnection:(AVCaptureConnection *)connection
{
    if (!CMSampleBufferDataIsReady(sampleBuffer)) {
        NSLog( @"sample buffer is not ready. Skipping sample" );
        return;
    }


    if (_isRecording == YES) {
        lastSampleTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer);
        if (_videoWriter.status != AVAssetWriterStatusWriting ) {
            [_videoWriter startWriting];
            [_videoWriter startSessionAtSourceTime:lastSampleTime];
        }

        if (captureOutput == _videoOutput) {
            [self newVideoSample:sampleBuffer];
        }

        /*
        // If I add audio to the video, then the output file gets corrupted and it cannot be reproduced
        } else {
            [self newAudioSample:sampleBuffer];
        }
    */
    }
}

- (void)newVideoSample:(CMSampleBufferRef)sampleBuffer
{     
    if (_isRecording) {
        if (_videoWriter.status > AVAssetWriterStatusWriting) {
             NSLog(@"Warning: writer status is %d", _videoWriter.status);
             if (_videoWriter.status == AVAssetWriterStatusFailed)
                  NSLog(@"Error: %@", _videoWriter.error);
             return;
        }

        if (![_videoWriterInput appendSampleBuffer:sampleBuffer]) {
             NSLog(@"Unable to write to video input");
        }
    }
}



- (void)newAudioSample:(CMSampleBufferRef)sampleBuffer
{     
    if (_isRecording) {
        if (_videoWriter.status > AVAssetWriterStatusWriting) {
             NSLog(@"Warning: writer status is %d", _videoWriter.status);
             if (_videoWriter.status == AVAssetWriterStatusFailed)
                  NSLog(@"Error: %@", _videoWriter.error);
             return;
        }

        if (![_audioWriterInput appendSampleBuffer:sampleBuffer]) {
             NSLog(@"Unable to write to audio input");
        }
    }
}

I would be very glad if someone could find which is the problem in this code.

In startVideoRecording I call (I assume you are calling this at some point)

[_capSession startRunning] ;

In stopVideoRecording I do not call

[_videoWriterInput markAsFinished];
[_videoWriter endSessionAtSourceTime:lastSampleTime];

The markAsFinished is more for use with the block style pull method. See requestMediaDataWhenReadyOnQueue:usingBlock in AVAssetWriterInput for an explanation. The library should calculate the proper timing for interleaving the buffers.

You do not need to call endSessionAtSrouceTime. The last time stamp in the sample data will be used after the call to

[_videoWriter finishWriting];

I also explicitly check for the type of capture output.

else if( captureOutput == _audioOutput) {
    [self newAudioSample:sampleBuffer]; 
}

Here is what I have. The audio and video come through for me. It is possible I changed something. If this does not work for you then I will post everything I have.

-(void) startVideoRecording
    {
        if( !_isRecording )
         {
            NSLog(@"start video recording...");
            if( ![self setupWriter] ) {
                NSLog(@"Setup Writer Failed") ;
                
                return;
            }
                
            [_capSession startRunning] ;
            _isRecording = YES;
         }
    }

    -(void) stopVideoRecording
    {
        if( _isRecording )
         {
            _isRecording = NO;
            
            [_capSession stopRunning] ;
            
            if(![_videoWriter finishWriting]) { 
                NSLog(@"finishWriting returned NO") ;
            }
            //[_videoWriter endSessionAtSourceTime:lastSampleTime];
            //[_videoWriterInput markAsFinished];
            //[_audioWriterInput markAsFinished];
            
            NSLog(@"video recording stopped");
         }
    }

First, do not use [NSNumber numberWithInt:kCVPixelFormatType_32BGRA], as it is not the native format of the camera. use [NSNumber numberWithInt:kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange]

Also, you should always check before calling startWriting that it isn't already running. You do not need to set session end time, as stopWriting will do that.