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:^{

    }];
}

}

John Tracid · Answer 1 · 2017-06-12T11:39:35.233

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;
}

Hi John. Thank you for your help. How do I go about reversing each buffer? — user1752054, Jun 03 '17 at 20:17
See `CMSampleBufferGetDataBuffer` to get raw buffer and reverse frames directly. — John Tracid, Jun 04 '17 at 06:51
To be honest, I'm not sure how to do that. Could you please help with some code? — user1752054, Jun 04 '17 at 07:22
Thank you for your help, but I'm getting the following exception. *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[AVAssetWriterInput appendSampleBuffer:] Cannot append sample buffer: Input buffer must be in an uncompressed format when outputSettings is not nil' — user1752054, Jun 12 '17 at 06:43

Reversing an audio file

1 Answers1