How to write a movie with video AND audio using AVAssetWriter?

Question

I want to export a movie with AVAssetWriter and can't figure out how to include video and audio tracks in sync. Exporting only video works fine, but when I add audio the resulting movie looks like this:

First I see the video (without audio), then the video freezes (showing the last image frame until the end) and after some seconds I hear the audio.

I tried some things with CMSampleBufferSetOutputPresentationTimeStamp (subtracting the first CMSampleBufferGetPresentationTimeStamp from the current) for the audio, but it all didn't work and I don't think it is the right direction, since video & audio in the source movie should be in sync anyway...

My setup in short: I create an AVAssetReader and 2 AVAssetReaderTrackOutput (one for video, one for audio) and add them to the AVAssetReader, then I create an AVAssetWriter and 2 AVAssetWriterInput (video & audio) and add them to the AVAssetWriter... I start it all up with:

[assetReader startReading];
[assetWriter startWriting];
[assetWriter startSessionAtSourceTime:kCMTimeZero];

Then I run 2 queues for doing the sample buffer stuff:

dispatch_queue_t queueVideo=dispatch_queue_create("assetVideoWriterQueue", NULL);
[assetWriterVideoInput requestMediaDataWhenReadyOnQueue:queueVideo usingBlock:^
{
     while([assetWriterVideoInput isReadyForMoreMediaData])
     {
         CMSampleBufferRef sampleBuffer=[assetReaderVideoOutput copyNextSampleBuffer];
         if(sampleBuffer)
         {
             [assetWriterVideoInput appendSampleBuffer:sampleBuffer];
             CFRelease(sampleBuffer);
         } else
         {
             [assetWriterVideoInput markAsFinished];
             dispatch_release(queueVideo);
             videoFinished=YES;
             break;
         }
     }
}];

dispatch_queue_t queueAudio=dispatch_queue_create("assetAudioWriterQueue", NULL);
[assetWriterAudioInput requestMediaDataWhenReadyOnQueue:queueAudio usingBlock:^
{
    while([assetWriterAudioInput isReadyForMoreMediaData])
    {
        CMSampleBufferRef sampleBuffer=[assetReaderAudioOutput copyNextSampleBuffer];
        if(sampleBuffer)
        {
            [assetWriterAudioInput appendSampleBuffer:sampleBuffer];
            CFRelease(sampleBuffer);
        } else
        {
            [assetWriterAudioInput markAsFinished];
            dispatch_release(queueAudio);
            audioFinished=YES;
            break;
        }
    }
}];

In the main loop I wait for both queues until they finish:

while(!videoFinished && !audioFinished)
{
    sleep(1);
}
[assetWriter finishWriting];

Furthermore I try to save the resulting file in the library with the following code...

NSURL *url=[[NSURL alloc] initFileURLWithPath:path];
ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
if([library videoAtPathIsCompatibleWithSavedPhotosAlbum:url])
{
    [library writeVideoAtPathToSavedPhotosAlbum:url completionBlock:^(NSURL *assetURL, NSError *error)
     {
         if(error)
             NSLog(@"error=%@",error.localizedDescription);
         else
             NSLog(@"completed...");
     }];
} else
    NSLog(@"error, video not saved...");

[library release];
[url release];

...but I get the error:

Video /Users/cb/Library/Application Support/iPhone Simulator/4.2/Applications/E9865BF9-D190-4912-9248-66768B1AB635/Documents/export.mp4 cannot be saved to the saved photos album: Error Domain=NSOSStatusErrorDomain Code=-12950 "Movie could not be played." UserInfo=0x5e4fb90 {NSLocalizedDescription=Movie could not be played.}

The code works without problems in another program. So something is wrong with the movie...?

Answer 1

-(void)mergeAudioVideo
{

    NSString *videoOutputPath=[_documentsDirectory stringByAppendingPathComponent:@"dummy_video.mp4"];
    NSString *outputFilePath = [_documentsDirectory stringByAppendingPathComponent:@"final_video.mp4"];
    if ([[NSFileManager defaultManager]fileExistsAtPath:outputFilePath])
        [[NSFileManager defaultManager]removeItemAtPath:outputFilePath error:nil];


    NSURL    *outputFileUrl = [NSURL fileURLWithPath:outputFilePath];
    NSString *filePath = [_documentsDirectory stringByAppendingPathComponent:@"newFile.m4a"];
    AVMutableComposition* mixComposition = [AVMutableComposition composition];

    NSURL    *audio_inputFileUrl = [NSURL fileURLWithPath:filePath];
    NSURL    *video_inputFileUrl = [NSURL fileURLWithPath:videoOutputPath];

    CMTime nextClipStartTime = kCMTimeZero;

    AVURLAsset* videoAsset = [[AVURLAsset alloc]initWithURL:video_inputFileUrl options:nil];
    CMTimeRange video_timeRange = CMTimeRangeMake(kCMTimeZero,videoAsset.duration);

    AVMutableCompositionTrack *a_compositionVideoTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeVideo preferredTrackID:kCMPersistentTrackID_Invalid];
    [a_compositionVideoTrack insertTimeRange:video_timeRange ofTrack:[[videoAsset tracksWithMediaType:AVMediaTypeVideo] objectAtIndex:0] atTime:nextClipStartTime error:nil];

    AVURLAsset* audioAsset = [[AVURLAsset alloc]initWithURL:audio_inputFileUrl options:nil];
    CMTimeRange audio_timeRange = CMTimeRangeMake(kCMTimeZero, audioAsset.duration);
    AVMutableCompositionTrack *b_compositionAudioTrack = [mixComposition addMutableTrackWithMediaType:AVMediaTypeAudio preferredTrackID:kCMPersistentTrackID_Invalid];
    [b_compositionAudioTrack insertTimeRange:audio_timeRange ofTrack:[[audioAsset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0] atTime:nextClipStartTime error:nil];

    AVAssetExportSession* _assetExport = [[AVAssetExportSession alloc] initWithAsset:mixComposition presetName:AVAssetExportPresetMediumQuality];
    _assetExport.outputFileType = @"com.apple.quicktime-movie";
    _assetExport.outputURL = outputFileUrl;

    [_assetExport exportAsynchronouslyWithCompletionHandler:
     ^(void ) {
         if (_assetExport.status == AVAssetExportSessionStatusCompleted) {

          //Write Code Here to Continue
         }
         else {
            //Write Fail Code here     
         }
     }
     ];



}

You can use this code to merge audio and video.

Answer 2

It seams that assetWriterAudioInput ignores sample buffer time for audio writing. Do this way.

1) Write video track.

2) When done, mark it finished i.e. [videoWriterInput markAsFinished];

3) do [assetWriter startSessionAtSourceTime:timeRangeStart];

3) instantiate audio reader and start writing audio.