iOS reverse audio through AVAssetWriter

Question

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.

Answer 1

The method described here is implemented in an Xcode project at this link (multi-platform SwiftUI app):

ReverseAudio Xcode Project

It is not sufficient to write the audio samples in the reverse order. The sample data needs to be reversed itself.

In Swift, we create an extension to AVAsset.

The samples must be processed as decompressed samples. To that end create audio reader settings with kAudioFormatLinearPCM:

let kAudioReaderSettings = [
    AVFormatIDKey: Int(kAudioFormatLinearPCM) as AnyObject,
    AVLinearPCMBitDepthKey: 16 as AnyObject,
    AVLinearPCMIsBigEndianKey: false as AnyObject,
    AVLinearPCMIsFloatKey: false as AnyObject,
    AVLinearPCMIsNonInterleaved: false as AnyObject]

Use our AVAsset extension method audioReader:

func audioReader(outputSettings: [String : Any]?) -> (audioTrack:AVAssetTrack?, audioReader:AVAssetReader?, audioReaderOutput:AVAssetReaderTrackOutput?) {
    
    if let audioTrack = self.tracks(withMediaType: .audio).first {
        if let audioReader = try? AVAssetReader(asset: self)  {
            let audioReaderOutput = AVAssetReaderTrackOutput(track: audioTrack, outputSettings: outputSettings)
            return (audioTrack, audioReader, audioReaderOutput)
        }
    }
    
    return (nil, nil, nil)
}

let (_, audioReader, audioReaderOutput) = self.audioReader(outputSettings: kAudioReaderSettings)

to create an audioReader (AVAssetReader) and audioReaderOutput (AVAssetReaderTrackOutput) for reading the audio samples.

We need to keep track of the audio sample:

var audioSamples:[CMSampleBuffer] = []

Now start reading samples.

if audioReader.startReading() {
    while audioReader.status == .reading {
        if let sampleBuffer = audioReaderOutput.copyNextSampleBuffer(){ 
           // process sample                                       
        }
    }
}

Save the audio sample buffer, we need it later when we create the reversed sample:

audioSamples.append(sampleBuffer)

We need an AVAssetWriter:

guard let assetWriter = try? AVAssetWriter(outputURL: destinationURL, fileType: AVFileType.wav) else {
    // error handling
    return
}

The file type is 'wav' because the reversed samples will be written as uncompressed audio format Linear PCM, as follows.

For the assetWriter we specify audio compression settings, and a ‘source format hint’ and can acquire this from an uncompressed sample buffer:

let sampleBuffer = audioSamples[0]
let sourceFormat = CMSampleBufferGetFormatDescription(sampleBuffer)

let audioCompressionSettings = [AVFormatIDKey: kAudioFormatLinearPCM] as [String : Any]

Now we can create the AVAssetWriterInput, add it to the writer and start writing:

let assetWriterInput = AVAssetWriterInput(mediaType: AVMediaType.audio, outputSettings:audioCompressionSettings, sourceFormatHint: sourceFormat)

assetWriter.add(assetWriterInput)

assetWriter.startWriting()
assetWriter.startSession(atSourceTime: CMTime.zero)

Now iterate through the samples, in reverse order, and for each reverse the samples themselves.

We have an extension for CMSampleBuffer that does just that, called ‘reverse’.

Using requestMediaDataWhenReady we do this as follows:

let nbrSamples = audioSamples.count
var index = 0

let serialQueue: DispatchQueue = DispatchQueue(label: "com.limit-point.reverse-audio-queue")
    
assetWriterInput.requestMediaDataWhenReady(on: serialQueue) {
        
    while assetWriterInput.isReadyForMoreMediaData, index < nbrSamples {
        let sampleBuffer = audioSamples[nbrSamples - 1 - index]
            
        if let reversedBuffer = sampleBuffer.reverse(), assetWriterInput.append(reversedBuffer) == true {
            index += 1
        }
        else {
            index = nbrSamples
        }
            
        if index == nbrSamples {
            assetWriterInput.markAsFinished()
            
            finishWriting() // call assetWriter.finishWriting, check assetWriter status, etc.
        }
    }
}

So the last thing to explain is how do you reverse the audio sample in the ‘reverse’ method?

We create an extension to CMSampleBuffer that takes a sample buffer and returns the reversed sample buffer, as an extension on CMSampleBuffer:

func reverse() -> CMSampleBuffer? 

The data that has to be reversed needs to be obtained using the method:

CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer

The CMSampleBuffer header files descibes this method as follows:

“Creates an AudioBufferList containing the data from the CMSampleBuffer, and a CMBlockBuffer which references (and manages the lifetime of) the data in that AudioBufferList.”

Call it as follows, where ‘self’ refers to the CMSampleBuffer we are reversing since this is an extension:

var blockBuffer: CMBlockBuffer? = nil
let audioBufferList: UnsafeMutableAudioBufferListPointer = AudioBufferList.allocate(maximumBuffers: 1)

CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer(
    self,
    bufferListSizeNeededOut: nil,
    bufferListOut: audioBufferList.unsafeMutablePointer,
    bufferListSize: AudioBufferList.sizeInBytes(maximumBuffers: 1),
    blockBufferAllocator: nil,
    blockBufferMemoryAllocator: nil,
    flags: kCMSampleBufferFlag_AudioBufferList_Assure16ByteAlignment,
    blockBufferOut: &blockBuffer
 )

Now you can access the raw data as:

let data: UnsafeMutableRawPointer = audioBufferList.unsafePointer.pointee.mBuffers.mData

Reversing data we need to access the data as an array of ‘samples’ called sampleArray, and is done as follows in Swift:

let samples = data.assumingMemoryBound(to: Int16.self)
        
let sizeofInt16 = MemoryLayout<Int16>.size
let dataSize = audioBufferList.unsafePointer.pointee.mBuffers.mDataByteSize  

let dataCount = Int(dataSize) / sizeofInt16
        
var sampleArray = Array(UnsafeBufferPointer(start: samples, count: dataCount)) as [Int16]

Now reverse the array sampleArray:

sampleArray.reverse()

Using the reversed samples we create a new CMSampleBuffer that contains the reversed samples.

Now we replace the data in the CMBlockBuffer we previously obtained with CMSampleBufferGetAudioBufferListWithRetainedBlockBuffer:

First reassign ‘samples’ using the reversed array:

var status:OSStatus = noErr
        
sampleArray.withUnsafeBytes { sampleArrayPtr in
    if let baseAddress = sampleArrayPtr.baseAddress {
        let bufferPointer: UnsafePointer<Int16> = baseAddress.assumingMemoryBound(to: Int16.self)
        let rawPtr = UnsafeRawPointer(bufferPointer)
                
        status = CMBlockBufferReplaceDataBytes(with: rawPtr, blockBuffer: blockBuffer!, offsetIntoDestination: 0, dataLength: Int(dataSize))
    } 
}

if status != noErr {
    return nil
}

Finally create the new sample buffer using CMSampleBufferCreate. This function needs two arguments we can get from the original sample buffer, namely the formatDescription and numberOfSamples:

let formatDescription = CMSampleBufferGetFormatDescription(self)   
let numberOfSamples = CMSampleBufferGetNumSamples(self)
        
var newBuffer:CMSampleBuffer?
        

Now create the new sample buffer with the reversed blockBuffer:

guard CMSampleBufferCreate(allocator: kCFAllocatorDefault, dataBuffer: blockBuffer, dataReady: true, makeDataReadyCallback: nil, refcon: nil, formatDescription: formatDescription, sampleCount: numberOfSamples, sampleTimingEntryCount: 0, sampleTimingArray: nil, sampleSizeEntryCount: 0, sampleSizeArray: nil, sampleBufferOut: &newBuffer) == noErr else {
    return self
}
        
return newBuffer

And that’s all there is to it!

As a final note the Core Audio and AVFoundation headers provide a lot of useful information, such as CoreAudioTypes.h, CMSampleBuffer.h, and many more.

Answer 2

Complete example for reverse video and audio using Swift 5 into the same asset output, audio processed using above recommendations:

 private func reverseVideo(inURL: URL, outURL: URL, queue: DispatchQueue, _ completionBlock: ((Bool)->Void)?) {
    Log.info("Start reverse video!")
    let asset = AVAsset.init(url: inURL)
    guard
        let reader = try? AVAssetReader.init(asset: asset),
        let videoTrack = asset.tracks(withMediaType: .video).first,
        let audioTrack = asset.tracks(withMediaType: .audio).first

        else {
            assert(false)
            completionBlock?(false)
            return
    }

    let width = videoTrack.naturalSize.width
    let height = videoTrack.naturalSize.height

    // Video reader
    let readerVideoSettings: [String : Any] = [ String(kCVPixelBufferPixelFormatTypeKey) : kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange,]
    let readerVideoOutput = AVAssetReaderTrackOutput.init(track: videoTrack, outputSettings: readerVideoSettings)
    reader.add(readerVideoOutput)

    // Audio reader
    let readerAudioSettings: [String : Any] = [
        AVFormatIDKey: kAudioFormatLinearPCM,
        AVLinearPCMBitDepthKey: 16 ,
        AVLinearPCMIsBigEndianKey: false ,
        AVLinearPCMIsFloatKey: false,]
    let readerAudioOutput = AVAssetReaderTrackOutput.init(track: audioTrack, outputSettings: readerAudioSettings)
    reader.add(readerAudioOutput)

    //Start reading content
    reader.startReading()

    //Reading video samples
    var videoBuffers = [CMSampleBuffer]()
    while let nextBuffer = readerVideoOutput.copyNextSampleBuffer() {
        videoBuffers.append(nextBuffer)
    }

    //Reading audio samples
    var audioBuffers = [CMSampleBuffer]()
    var timingInfos = [CMSampleTimingInfo]()
    while let nextBuffer = readerAudioOutput.copyNextSampleBuffer() {

        var timingInfo = CMSampleTimingInfo()
        var timingInfoCount = CMItemCount()
        CMSampleBufferGetSampleTimingInfoArray(nextBuffer, entryCount: 0, arrayToFill: &timingInfo, entriesNeededOut: &timingInfoCount)

        let duration = CMSampleBufferGetDuration(nextBuffer)
        let endTime = CMTimeAdd(timingInfo.presentationTimeStamp, duration)
        let newPresentationTime = CMTimeSubtract(duration, endTime)

        timingInfo.presentationTimeStamp = newPresentationTime

        timingInfos.append(timingInfo)
        audioBuffers.append(nextBuffer)
    }

    //Stop reading
    let status = reader.status
    reader.cancelReading()
    guard status == .completed, let firstVideoBuffer = videoBuffers.first, let firstAudioBuffer = audioBuffers.first else {
        assert(false)
        completionBlock?(false)
        return
    }

    //Start video time
    let sessionStartTime = CMSampleBufferGetPresentationTimeStamp(firstVideoBuffer)

    //Writer for video
    let writerVideoSettings: [String:Any] = [
        AVVideoCodecKey : AVVideoCodecType.h264,
        AVVideoWidthKey : width,
        AVVideoHeightKey: height,
    ]
    let writerVideoInput: AVAssetWriterInput
    if let formatDescription = videoTrack.formatDescriptions.last {
        writerVideoInput = AVAssetWriterInput.init(mediaType: .video, outputSettings: writerVideoSettings, sourceFormatHint: (formatDescription as! CMFormatDescription))
    } else {
        writerVideoInput = AVAssetWriterInput.init(mediaType: .video, outputSettings: writerVideoSettings)
    }
    writerVideoInput.transform = videoTrack.preferredTransform
    writerVideoInput.expectsMediaDataInRealTime = false

    //Writer for audio
    let writerAudioSettings: [String:Any] = [
        AVFormatIDKey : kAudioFormatMPEG4AAC,
        AVSampleRateKey : 44100,
        AVNumberOfChannelsKey: 2,
        AVEncoderBitRateKey:128000,
        AVChannelLayoutKey: NSData(),
    ]
    let sourceFormat = CMSampleBufferGetFormatDescription(firstAudioBuffer)
    let writerAudioInput: AVAssetWriterInput = AVAssetWriterInput.init(mediaType: .audio, outputSettings: writerAudioSettings, sourceFormatHint: sourceFormat)
    writerAudioInput.expectsMediaDataInRealTime = true

    guard
        let writer = try? AVAssetWriter.init(url: outURL, fileType: .mp4),
        writer.canAdd(writerVideoInput),
        writer.canAdd(writerAudioInput)
        else {
            assert(false)
            completionBlock?(false)
            return
    }

    let pixelBufferAdaptor = AVAssetWriterInputPixelBufferAdaptor.init(assetWriterInput: writerVideoInput, sourcePixelBufferAttributes: nil)
    let group = DispatchGroup.init()

    group.enter()
    writer.add(writerVideoInput)
    writer.add(writerAudioInput)
    writer.startWriting()
    writer.startSession(atSourceTime: sessionStartTime)

    var videoFinished = false
    var audioFinished = false

    //Write video samples in reverse order
    var currentSample = 0
    writerVideoInput.requestMediaDataWhenReady(on: queue) {
        for i in currentSample..<videoBuffers.count {
            currentSample = i
            if !writerVideoInput.isReadyForMoreMediaData {
                return
            }
            let presentationTime = CMSampleBufferGetPresentationTimeStamp(videoBuffers[i])
            guard let imageBuffer = CMSampleBufferGetImageBuffer(videoBuffers[videoBuffers.count - i - 1]) else {
                Log.info("VideoWriter reverseVideo: warning, could not get imageBuffer from SampleBuffer...")
                continue
            }
            if !pixelBufferAdaptor.append(imageBuffer, withPresentationTime: presentationTime) {
                Log.info("VideoWriter reverseVideo: warning, could not append imageBuffer...")
            }
        }

        // finish write video samples
        writerVideoInput.markAsFinished()
        Log.info("Video writing finished!")
        videoFinished = true
        if(audioFinished){
            group.leave()
        }
    }
    //Write audio samples in reverse order
    let totalAudioSamples = audioBuffers.count
    writerAudioInput.requestMediaDataWhenReady(on: queue) {
        for i in 0..<totalAudioSamples-1 {
            if !writerAudioInput.isReadyForMoreMediaData {
                return
            }
            let audioSample = audioBuffers[totalAudioSamples-1-i]
            let timingInfo = timingInfos[i]
            // reverse samples data using timing info
            if let reversedBuffer = audioSample.reverse(timingInfo: [timingInfo]) {
                // append data
                if writerAudioInput.append(reversedBuffer) == false {
                    break
                }
            }
        }

        // finish
        writerAudioInput.markAsFinished()
        Log.info("Audio writing finished!")
        audioFinished = true
        if(videoFinished){
            group.leave()
        }
    }

    group.notify(queue: queue) {
        writer.finishWriting {
            if writer.status != .completed {
                Log.info("VideoWriter reverse video: error - \(String(describing: writer.error))")
                completionBlock?(false)
            } else {
                Log.info("Ended reverse video!")
                completionBlock?(true)
            }
        }
    }
}

Happy coding!

Answer 3

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.