Reversing an audio file

Question

I'm using AVAssetReader and AVAssetWriter to reverse an audio file. However, the resulting reversed audio is very jerky.

What's the best practice for reversing an audio file?

Any help is much appreciated.

-(void)reverseAudio:(NSURL *)videoURL andVideoAsset:(AVURLAsset *)videoAsset{

AVAssetReader *video2AssetReader = [[AVAssetReader alloc] initWithAsset:videoAsset error:nil];
video2AssetReader.timeRange = CMTimeRangeFromTimeToTime(kCMTimeZero, [videoAsset duration]);


NSArray *audioTracks = [videoAsset tracksWithMediaType:AVMediaTypeAudio];
AVAssetTrack* audioTrack = [audioTracks objectAtIndex:0];

NSDictionary *outputSettingsDict = [[NSDictionary alloc] initWithObjectsAndKeys:
                                    [NSNumber numberWithInt:kAudioFormatLinearPCM],AVFormatIDKey,
                                    [NSNumber numberWithInt:16],AVLinearPCMBitDepthKey,
                                    [NSNumber numberWithBool:NO],AVLinearPCMIsBigEndianKey,
                                    [NSNumber numberWithBool:NO],AVLinearPCMIsFloatKey,
                                    [NSNumber numberWithBool:NO],AVLinearPCMIsNonInterleaved,
                                    nil];

AVAssetReaderTrackOutput *readerAudioTrackOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:audioTrack outputSettings:outputSettingsDict];
[video2AssetReader addOutput:readerAudioTrackOutput];

[video2AssetReader startReading];

// read in the samples
NSMutableArray *audioSamples = [[NSMutableArray alloc] init];

CMSampleBufferRef audioSample;

while((audioSample = [readerAudioTrackOutput copyNextSampleBuffer])){
    [audioSamples addObject:(__bridge id)audioSample];

    CFRelease(audioSample);
}

videoReverseProcess3TotalFrames = audioSamples.count;
NSLog(@"AUDIO SAMPLES COUNT = %f", videoReverseProcess3TotalFrames);

[video2AssetReader cancelReading];

NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
NSString *documentsDirectory = [paths objectAtIndex:0];
NSString *videoPath = [documentsDirectory stringByAppendingPathComponent:@"videoReverseAudioFile.m4a"];
NSError *error = nil;
if([[NSFileManager defaultManager] fileExistsAtPath:videoPath]){
    [[NSFileManager defaultManager] removeItemAtPath:videoPath error:&error];
    if(error){
        NSLog(@"VIDEO DELETE FAILED");
    }
    else{
        NSLog(@"VIDEO DELETED");

    }
}

NSURL *audioExportURL = [[NSURL alloc] initFileURLWithPath:videoPath];

AVAssetWriter *writer = [[AVAssetWriter alloc] initWithURL:audioExportURL fileType:AVFileTypeAppleM4A error:&error];

AudioChannelLayout channelLayout;
memset(&channelLayout, 0, sizeof(AudioChannelLayout));
channelLayout.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;
NSDictionary *audioCompressionSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                          [NSNumber numberWithInt: kAudioFormatMPEG4AAC], AVFormatIDKey,
                                          [NSNumber numberWithFloat:44100.0], AVSampleRateKey,
                                          [NSNumber numberWithInt:2], AVNumberOfChannelsKey,
                                          [NSNumber numberWithInt:128000], AVEncoderBitRateKey,
                                          [NSData dataWithBytes:&channelLayout length:sizeof(AudioChannelLayout)], AVChannelLayoutKey, nil];

AVAssetWriterInput *writerAudioInput;

writerAudioInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeAudio outputSettings:audioCompressionSettings];
writerAudioInput.expectsMediaDataInRealTime = NO;

if([writer canAddInput:writerAudioInput]){
    [writer addInput:writerAudioInput];
}
else{
    NSLog(@"ERROR ADDING AUDIO");
}

[writer startWriting];

CMTime timeStamp = CMSampleBufferGetPresentationTimeStamp((__bridge CMSampleBufferRef)audioSamples[0]);
[writer startSessionAtSourceTime:timeStamp];

while(audioSamples.count > 0){
    if(writer && writerAudioInput.readyForMoreMediaData){
        CMSampleBufferRef audioBufferRef = (__bridge CMSampleBufferRef)audioSamples[audioSamples.count - 1];

        [writerAudioInput appendSampleBuffer:audioBufferRef];

        [audioSamples removeObjectAtIndex:audioSamples.count - 1];
    }
    else{
        [NSThread sleepForTimeInterval:0.2];
    }
}

if(writer.status != AVAssetWriterStatusCancelled){
    [writerAudioInput markAsFinished];
    [writer finishWritingWithCompletionHandler:^{

    }];
}

}

Answer 1

You are not reversing audio, you just reversing audio fragments (buffers) order.

So you have this input: S1, S2, S3, S4 and you produce following output: S4, S3, S2, S1. But inside this fragments you still have original order of frames.

You need to reverse buffer data too.

Update #1

Here is example how you can do this.

- (void)reverseAudio:(AVURLAsset *)videoAsset {

    AVAssetReader *video2AssetReader = [[AVAssetReader alloc] initWithAsset:videoAsset error:nil];
    video2AssetReader.timeRange = CMTimeRangeFromTimeToTime(kCMTimeZero, [videoAsset duration]);


    NSArray *audioTracks = [videoAsset tracksWithMediaType:AVMediaTypeAudio];
    AVAssetTrack* audioTrack = [audioTracks objectAtIndex:0];

    NSDictionary *outputSettingsDict = [[NSDictionary alloc] initWithObjectsAndKeys:
                                        [NSNumber numberWithInt:kAudioFormatLinearPCM],AVFormatIDKey,
                                        [NSNumber numberWithInt:16],AVLinearPCMBitDepthKey,
                                        [NSNumber numberWithBool:NO],AVLinearPCMIsBigEndianKey,
                                        [NSNumber numberWithBool:NO],AVLinearPCMIsFloatKey,
                                        [NSNumber numberWithBool:NO],AVLinearPCMIsNonInterleaved,
                                        nil];

    AVAssetReaderTrackOutput *readerAudioTrackOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:audioTrack outputSettings:outputSettingsDict];
    [video2AssetReader addOutput:readerAudioTrackOutput];

    [video2AssetReader startReading];

    // read in the samples
    CMTime timeStamp = kCMTimeInvalid;

    NSMutableArray *audioSamples = [[NSMutableArray alloc] init];
    CMSampleBufferRef audioSample;
    while ((audioSample = [readerAudioTrackOutput copyNextSampleBuffer])) {
        [audioSamples addObject:(__bridge id)[self reverseSampleBuffer:audioSample]];

        if (CMTIME_IS_INVALID(timeStamp)) {
            timeStamp = CMSampleBufferGetPresentationTimeStamp(audioSample);
        }

        CFRelease(audioSample);
    }

    NSLog(@"AUDIO SAMPLES COUNT = %d", (int)audioSamples.count);

    [video2AssetReader cancelReading];

    // rest of the code
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *documentsDirectory = [paths objectAtIndex:0];
    NSString *videoPath = [documentsDirectory stringByAppendingPathComponent:@"videoReverseAudioFile.m4a"];
    NSError *error = nil;
    if ([[NSFileManager defaultManager] fileExistsAtPath:videoPath]) {
        [[NSFileManager defaultManager] removeItemAtPath:videoPath error:&error];
        if (error) {
            NSLog(@"VIDEO DELETE FAILED");
        } else {
            NSLog(@"VIDEO DELETED");
        }
    }

    NSURL *audioExportURL = [[NSURL alloc] initFileURLWithPath:videoPath];

    AVAssetWriter *writer = [[AVAssetWriter alloc] initWithURL:audioExportURL fileType:AVFileTypeAppleM4A error:&error];

    AudioChannelLayout channelLayout;
    memset(&channelLayout, 0, sizeof(AudioChannelLayout));
    channelLayout.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;
    NSDictionary *audioCompressionSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                              [NSNumber numberWithInt:kAudioFormatMPEG4AAC], AVFormatIDKey,
                                              [NSNumber numberWithFloat:44100.0], AVSampleRateKey,
                                              [NSNumber numberWithInt:2], AVNumberOfChannelsKey,
                                              [NSNumber numberWithInt:128000], AVEncoderBitRateKey,
                                              [NSData dataWithBytes:&channelLayout length:sizeof(AudioChannelLayout)], AVChannelLayoutKey, nil];

    AVAssetWriterInput *writerAudioInput;

    writerAudioInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeAudio outputSettings:audioCompressionSettings];
    writerAudioInput.expectsMediaDataInRealTime = NO;

    if ([writer canAddInput:writerAudioInput]) {
        [writer addInput:writerAudioInput];
    } else {
        NSLog(@"ERROR ADDING AUDIO");
    }

    [writer startWriting];

    [writer startSessionAtSourceTime:timeStamp];

    while (audioSamples.count > 0) {
        if(writer && writerAudioInput.readyForMoreMediaData) {
            CMSampleBufferRef audioBufferRef = (__bridge CMSampleBufferRef)audioSamples[audioSamples.count - 1];

            [writerAudioInput appendSampleBuffer:audioBufferRef];

            [audioSamples removeObjectAtIndex:audioSamples.count - 1];
        } else {
            [NSThread sleepForTimeInterval:0.2];
        }
    }

    if (writer.status != AVAssetWriterStatusCancelled) {
        [writerAudioInput markAsFinished];
        [writer finishWritingWithCompletionHandler:^{

        }];
    }
}

- (CMSampleBufferRef)reverseSampleBuffer:(CMSampleBufferRef)buffer {
    AudioBufferList list;
    CMBlockBufferRef dataBuffer = NULL;
    // TODO check result code
    CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(buffer,
                                                            NULL,
                                                            &list,
                                                            sizeof(list),
                                                            NULL,
                                                            NULL,
                                                            kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment,
                                                            &dataBuffer);

    CMItemCount numberOfSamples = CMSampleBufferGetNumSamples(buffer);
    for (int i = 0; i < list.mNumberBuffers; i++) {
        SInt16 *samples = (SInt16 *)list.mBuffers[i].mData;
        for (int j = 0; j < numberOfSamples / 2; j++) {
            SInt16 t = samples[j];
            samples[j] = samples[numberOfSamples - 1 - j];
            samples[numberOfSamples - 1 - j] = t;
        }
    }



    CMFormatDescriptionRef format = CMSampleBufferGetFormatDescription(buffer);
    CMSampleBufferRef result = NULL;
    // TODO check result code
    CMSampleBufferCreate(kCFAllocatorDefault, dataBuffer, true, NULL, NULL, format, 0, 0, NULL, 0, NULL, &result);

    return result;
}