Can I use AVAudioEngine to read from a file, process with an audio unit and write to a file, faster than real-time?

Question

I am working on an iOS app that uses AVAudioEngine for various things, including recording audio to a file, applying effects to that audio using audio units, and playing back the audio with the effect applied. I use a tap to also write the output to a file. When this is done it writes to the file in real time as the audio is playing back.

Is it possible to set up an AVAudioEngine graph that reads from a file, processes the sound with an audio unit, and outputs to a file, but faster than real time (ie., as fast as the hardware can process it)? The use case for this would be to output a few minutes of audio with effects applied, and I certainly wouldn't want to wait for a few minutes for it to be processed.

Edit: here's the code that I'm using to set up the AVAudioEngine's graph, and play a sound file:

AVAudioEngine* engine = [[AVAudioEngine alloc] init];

AVAudioPlayerNode* player = [[AVAudioPlayerNode alloc] init];
[engine attachNode:player];

self.player = player;
self.engine = engine;

if (!self.distortionEffect) {
    self.distortionEffect = [[AVAudioUnitDistortion alloc] init];
    [self.engine attachNode:self.distortionEffect];
    [self.engine connect:self.player to:self.distortionEffect format:[self.distortionEffect outputFormatForBus:0]];
    AVAudioMixerNode* mixer = [self.engine mainMixerNode];
    [self.engine connect:self.distortionEffect to:mixer format:[mixer outputFormatForBus:0]];
}

[self.distortionEffect loadFactoryPreset:AVAudioUnitDistortionPresetDrumsBitBrush];

NSError* error;
if (![self.engine startAndReturnError:&error]) {
    NSLog(@"error: %@", error);
} else {
    NSURL* fileURL = [[NSBundle mainBundle] URLForResource:@"test2" withExtension:@"mp3"];
    AVAudioFile* file = [[AVAudioFile alloc] initForReading:fileURL error:&error];

    if (error) {
        NSLog(@"error: %@", error);
    } else {
        [self.player scheduleFile:file atTime:nil completionHandler:nil];
        [self.player play];
    }
}

The above code plays the sound in the test2.mp3 file, with the AVAudioUnitDistortionPresetDrumsBitBrush distortion preset applied, in real time.

I then modified the above code by adding these lines after [self.player play]:

        [self.engine stop];
        [self renderAudioAndWriteToFile];

I modified the renderAudioAndWriteToFile method that Vladimir provided so that instead of allocating a new AVAudioEngine in the first line, it simply uses self.engine that has already been set up.

However, in renderAudioAndWriteToFile, it's logging "Can not render audio unit" because AudioUnitRender is returning a status of kAudioUnitErr_Uninitialized.

Edit 2: I should mention that I'm perfectly happy to convert the AVAudioEngine code I posted to use the C apis if that would make things easier. However, I would want the code to produce the same output as the AVAudioEngine code (including the use of the factory preset shown above).

Answer 1

1. Configure your engine and player node.
2. Call play method for your player node.
3. Pause your engine.
4. Get an audio unit from your AVAudioOutputNode (audioEngine.outputNode) with this method.
5. Render from audio unit with AudioUnitRender in cycle and write audio buffer list to file with Extended Audio File Services.

Example:

Audio engine configuration

- (void)configureAudioEngine {
    self.engine = [[AVAudioEngine alloc] init];
    self.playerNode = [[AVAudioPlayerNode alloc] init];
    [self.engine attachNode:self.playerNode];
    AVAudioUnitDistortion *distortionEffect = [[AVAudioUnitDistortion alloc] init];
    [self.engine attachNode:distortionEffect];
    [self.engine connect:self.playerNode to:distortionEffect format:[distortionEffect outputFormatForBus:0]];
    self.mixer = [self.engine mainMixerNode];
    [self.engine connect:distortionEffect to:self.mixer format:[self.mixer outputFormatForBus:0]];
    [distortionEffect loadFactoryPreset:AVAudioUnitDistortionPresetDrumsBitBrush];
    NSError* error;
    if (![self.engine startAndReturnError:&error])
        NSLog(@"Can't start engine: %@", error);
    else
        [self scheduleFileToPlay];
}

- (void)scheduleFileToPlay {
    NSError* error;
    NSURL *fileURL = [[NSBundle mainBundle] URLForResource:@"filename" withExtension:@"m4a"];
    self.file = [[AVAudioFile alloc] initForReading:fileURL error:&error];
    if (self.file)
        [self.playerNode scheduleFile:self.file atTime:nil completionHandler:nil];
    else
        NSLog(@"Can't read file: %@", error);
}

Rendering methods

- (void)renderAudioAndWriteToFile {
    [self.playerNode play];
    [self.engine pause];
    AVAudioOutputNode *outputNode = self.engine.outputNode;
    AudioStreamBasicDescription const *audioDescription = [outputNode outputFormatForBus:0].streamDescription;
    NSString *path = [self filePath];
    ExtAudioFileRef audioFile = [self createAndSetupExtAudioFileWithASBD:audioDescription andFilePath:path];
    if (!audioFile)
        return;
    AVURLAsset *asset = [AVURLAsset assetWithURL:self.file.url];
    NSTimeInterval duration = CMTimeGetSeconds(asset.duration);
    NSUInteger lengthInFrames = duration * audioDescription->mSampleRate;
    const NSUInteger kBufferLength = 4096;
    AudioBufferList *bufferList = AEAllocateAndInitAudioBufferList(*audioDescription, kBufferLength);
    AudioTimeStamp timeStamp;
    memset (&timeStamp, 0, sizeof(timeStamp));
    timeStamp.mFlags = kAudioTimeStampSampleTimeValid;
    OSStatus status = noErr;
    for (NSUInteger i = kBufferLength; i < lengthInFrames; i += kBufferLength) {
        status = [self renderToBufferList:bufferList writeToFile:audioFile bufferLength:kBufferLength timeStamp:&timeStamp];
        if (status != noErr)
            break;
    }
    if (status == noErr && timeStamp.mSampleTime < lengthInFrames) {
        NSUInteger restBufferLength = (NSUInteger) (lengthInFrames - timeStamp.mSampleTime);
        AudioBufferList *restBufferList = AEAllocateAndInitAudioBufferList(*audioDescription, restBufferLength);
        status = [self renderToBufferList:restBufferList writeToFile:audioFile bufferLength:restBufferLength timeStamp:&timeStamp];
        AEFreeAudioBufferList(restBufferList);
    }
    AEFreeAudioBufferList(bufferList);
    ExtAudioFileDispose(audioFile);
    if (status != noErr)
        NSLog(@"An error has occurred");
    else
        NSLog(@"Finished writing to file at path: %@", path);
}

- (NSString *)filePath {
    NSArray *documentsFolders =
            NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *fileName = [NSString stringWithFormat:@"%@.m4a", [[NSUUID UUID] UUIDString]];
    NSString *path = [documentsFolders[0] stringByAppendingPathComponent:fileName];
    return path;
}

- (ExtAudioFileRef)createAndSetupExtAudioFileWithASBD:(AudioStreamBasicDescription const *)audioDescription
                                          andFilePath:(NSString *)path {
    AudioStreamBasicDescription destinationFormat;
    memset(&destinationFormat, 0, sizeof(destinationFormat));
    destinationFormat.mChannelsPerFrame = audioDescription->mChannelsPerFrame;
    destinationFormat.mSampleRate = audioDescription->mSampleRate;
    destinationFormat.mFormatID = kAudioFormatMPEG4AAC;
    ExtAudioFileRef audioFile;
    OSStatus status = ExtAudioFileCreateWithURL(
            (__bridge CFURLRef) [NSURL fileURLWithPath:path],
            kAudioFileM4AType,
            &destinationFormat,
            NULL,
            kAudioFileFlags_EraseFile,
            &audioFile
    );
    if (status != noErr) {
        NSLog(@"Can not create ext audio file");
        return nil;
    }
    UInt32 codecManufacturer = kAppleSoftwareAudioCodecManufacturer;
    status = ExtAudioFileSetProperty(
            audioFile, kExtAudioFileProperty_CodecManufacturer, sizeof(UInt32), &codecManufacturer
    );
    status = ExtAudioFileSetProperty(
            audioFile, kExtAudioFileProperty_ClientDataFormat, sizeof(AudioStreamBasicDescription), audioDescription
    );
    status = ExtAudioFileWriteAsync(audioFile, 0, NULL);
    if (status != noErr) {
        NSLog(@"Can not setup ext audio file");
        return nil;
    }
    return audioFile;
}

- (OSStatus)renderToBufferList:(AudioBufferList *)bufferList
                   writeToFile:(ExtAudioFileRef)audioFile
                  bufferLength:(NSUInteger)bufferLength
                     timeStamp:(AudioTimeStamp *)timeStamp {
    [self clearBufferList:bufferList];
    AudioUnit outputUnit = self.engine.outputNode.audioUnit;
    OSStatus status = AudioUnitRender(outputUnit, 0, timeStamp, 0, bufferLength, bufferList);
    if (status != noErr) {
        NSLog(@"Can not render audio unit");
        return status;
    }
    timeStamp->mSampleTime += bufferLength;
    status = ExtAudioFileWrite(audioFile, bufferLength, bufferList);
    if (status != noErr)
        NSLog(@"Can not write audio to file");
    return status;
}

- (void)clearBufferList:(AudioBufferList *)bufferList {
    for (int bufferIndex = 0; bufferIndex < bufferList->mNumberBuffers; bufferIndex++) {
        memset(bufferList->mBuffers[bufferIndex].mData, 0, bufferList->mBuffers[bufferIndex].mDataByteSize);
    }
}

I used some functions from this cool framework:

AudioBufferList *AEAllocateAndInitAudioBufferList(AudioStreamBasicDescription audioFormat, int frameCount) {
    int numberOfBuffers = audioFormat.mFormatFlags & kAudioFormatFlagIsNonInterleaved ? audioFormat.mChannelsPerFrame : 1;
    int channelsPerBuffer = audioFormat.mFormatFlags & kAudioFormatFlagIsNonInterleaved ? 1 : audioFormat.mChannelsPerFrame;
    int bytesPerBuffer = audioFormat.mBytesPerFrame * frameCount;
    AudioBufferList *audio = malloc(sizeof(AudioBufferList) + (numberOfBuffers-1)*sizeof(AudioBuffer));
    if ( !audio ) {
        return NULL;
    }
    audio->mNumberBuffers = numberOfBuffers;
    for ( int i=0; i<numberOfBuffers; i++ ) {
        if ( bytesPerBuffer > 0 ) {
            audio->mBuffers[i].mData = calloc(bytesPerBuffer, 1);
            if ( !audio->mBuffers[i].mData ) {
                for ( int j=0; j<i; j++ ) free(audio->mBuffers[j].mData);
                free(audio);
                return NULL;
            }
        } else {
            audio->mBuffers[i].mData = NULL;
        }
        audio->mBuffers[i].mDataByteSize = bytesPerBuffer;
        audio->mBuffers[i].mNumberChannels = channelsPerBuffer;
    }
    return audio;
}

void AEFreeAudioBufferList(AudioBufferList *bufferList ) {
    for ( int i=0; i<bufferList->mNumberBuffers; i++ ) {
        if ( bufferList->mBuffers[i].mData ) free(bufferList->mBuffers[i].mData);
    }
    free(bufferList);
}