iOS reverse audio through AVAssetWriter

I'm trying to reverse audio in iOS with AVAsset and AVAssetWriter. The following code is working, but the output file is shorter than input. For example, input file has 1:59 duration, but output 1:50 with the same audio content.

- (void)reverse:(AVAsset *)asset
{
AVAssetReader* reader = [[AVAssetReader alloc] initWithAsset:asset error:nil];

AVAssetTrack* audioTrack = [[asset tracksWithMediaType:AVMediaTypeAudio] objectAtIndex:0];

NSMutableDictionary* audioReadSettings = [NSMutableDictionary dictionary];
[audioReadSettings setValue:[NSNumber numberWithInt:kAudioFormatLinearPCM]
                     forKey:AVFormatIDKey];

AVAssetReaderTrackOutput* readerOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:audioTrack outputSettings:audioReadSettings];
[reader addOutput:readerOutput];
[reader startReading];

NSDictionary *outputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                [NSNumber numberWithInt: kAudioFormatMPEG4AAC], AVFormatIDKey,
                                [NSNumber numberWithFloat:44100.0], AVSampleRateKey,
                                [NSNumber numberWithInt:2], AVNumberOfChannelsKey,
                                [NSNumber numberWithInt:128000], AVEncoderBitRateKey,
                                [NSData data], AVChannelLayoutKey,
                                nil];

AVAssetWriterInput *writerInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeAudio
                                                                 outputSettings:outputSettings];

NSString *exportPath = [NSTemporaryDirectory() stringByAppendingPathComponent:@"out.m4a"];

NSURL *exportURL = [NSURL fileURLWithPath:exportPath];
NSError *writerError = nil;
AVAssetWriter *writer = [[AVAssetWriter alloc] initWithURL:exportURL
                                                  fileType:AVFileTypeAppleM4A
                                                     error:&writerError];
[writerInput setExpectsMediaDataInRealTime:NO];
[writer addInput:writerInput];
[writer startWriting];
[writer startSessionAtSourceTime:kCMTimeZero];

CMSampleBufferRef sample = [readerOutput copyNextSampleBuffer];
NSMutableArray *samples = [[NSMutableArray alloc] init];

while (sample != NULL) {

    sample = [readerOutput copyNextSampleBuffer];

    if (sample == NULL)
        continue;

    [samples addObject:(__bridge id)(sample)];
    CFRelease(sample);
}

NSArray* reversedSamples = [[samples reverseObjectEnumerator] allObjects];

for (id reversedSample in reversedSamples) {
    if (writerInput.readyForMoreMediaData)  {
        [writerInput appendSampleBuffer:(__bridge CMSampleBufferRef)(reversedSample)];
    }
    else {
        [NSThread sleepForTimeInterval:0.05];
    }
}

[writerInput markAsFinished];
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
dispatch_async(queue, ^{
    [writer finishWriting];
});
}

---

UPDATE:

If I write samples directly in first while loop - everything is ok (even with writerInput.readyForMoreMediaData checking). In this case result file has exactly the same duration as original. But if I write the same samples from reversed NSArray - the result is shorter.

Print out the size of each buffer in number of samples (through the "reading" readerOuput while loop), and repeat in the "writing" writerInput for-loop. This way you can see all the buffer sizes and see if they add up.

For example, are you missing or skipping a buffer if (writerInput.readyForMoreMediaData) is false, you "sleep", but then proceed to the next reversedSample in reversedSamples (that buffer effectively gets dropped from the writerInput)

UPDATE (based on comments): I found in the code, there are two problems:

1. The output settings is incorrect (the input file is mono (1 channel), but the output settings is configured to be 2 channels. It should be: [NSNumber numberWithInt:1], AVNumberOfChannelsKey. Look at the info on output and input files:

2. The second problem is that you are reversing 643 buffers of 8192 audio samples, instead of reversing the index of each audio sample. To see each buffer, I changed your debugging from looking at the size of each sample to looking at the size of the buffer, which is 8192. So line 76 is now: size_t sampleSize = CMSampleBufferGetNumSamples(sample);

The output looks like:

2015-03-19 22:26:28.171 audioReverse[25012:4901250] Reading [0]: 8192
2015-03-19 22:26:28.172 audioReverse[25012:4901250] Reading [1]: 8192
...
2015-03-19 22:26:28.651 audioReverse[25012:4901250] Reading [640]: 8192
2015-03-19 22:26:28.651 audioReverse[25012:4901250] Reading [641]: 8192
2015-03-19 22:26:28.651 audioReverse[25012:4901250] Reading [642]: 5056


2015-03-19 22:26:28.651 audioReverse[25012:4901250] Writing [0]: 5056
2015-03-19 22:26:28.652 audioReverse[25012:4901250] Writing [1]: 8192
...
2015-03-19 22:26:29.134 audioReverse[25012:4901250] Writing [640]: 8192
2015-03-19 22:26:29.135 audioReverse[25012:4901250] Writing [641]: 8192
2015-03-19 22:26:29.135 audioReverse[25012:4901250] Writing [642]: 8192

This shows that you're reversing the order of each buffer of 8192 samples, but in each buffer the audio is still "facing forward". We can see this in this screen shot I took of a correctly reversed (sample-by-sample) versus your buffer reversal:

I think your current scheme can work if you also reverse each sample each 8192 buffer. I personally would not recommend using NSArray enumerators for signal-processing, but it can work if you operate at the sample-level.