Play audio file using Audio Units?

Question

I've successfully recorded audio from the microphone into an audio file using Audio Units with the help of openframeworks and this website http://atastypixel.com/blog/using-remoteio-audio-unit.

I want to be able to stream the file back to audio units and play the audio. According to Play an audio file using RemoteIO and Audio Unit I can use ExtAudioFileOpenURL and ExtAudioFileRead. However, how do I play audio data in my buffer?

This is what I currently have:

static OSStatus setupAudioFileRead() {
    //construct the file destination URL
    CFURLRef destinationURL = audioSystemFileURL();
    OSStatus status = ExtAudioFileOpenURL(destinationURL, &audioFileRef);
    CFRelease(destinationURL);

    if (checkStatus(status)) { ofLog(OF_LOG_ERROR, "ofxiPhoneSoundStream: Couldn't open file to read"); return status; }

    while( TRUE ) {
        // Try to fill the buffer to capacity.
        UInt32 framesRead = 8000;
        status = ExtAudioFileRead( audioFileRef, &framesRead, &inputBufferList );

        // error check
        if( checkStatus(status) ) { break; }

        // 0 frames read means EOF.
        if( framesRead == 0 ) { break; }

        //play audio???
    }

    return noErr;
}

Answer 1

From this author: http://atastypixel.com/blog/using-remoteio-audio-unit/, if you scroll down to the PLAYBACK section, try something like this:

static OSStatus playbackCallback(void *inRefCon, 
                                  AudioUnitRenderActionFlags *ioActionFlags, 
                                  const AudioTimeStamp *inTimeStamp, 
                                  UInt32 inBusNumber, 
                                  UInt32 inNumberFrames, 
                                  AudioBufferList *ioData) {    
    // Notes: ioData contains buffers (may be more than one!)
    // Fill them up as much as you can. Remember to set the size value in each buffer to match how
    // much data is in the buffer.

for (int i=0; i < ioData->mNumberBuffers; i++) 
{ 
    AudioBuffer buffer = ioData->mBuffers[i];
    // copy from your whatever buffer data to output buffer
    UInt32 size = min(buffer.mDataByteSize, your buffer.size); 
    memcpy(buffer.mData, your buffer, size);
    buffer.mDataByteSize = size; // indicate how much data we wrote in the buffer

    // To test if your Audio Unit setup is working - comment out the three 
    // lines above and uncomment the for loop below to hear random noise  
    /*
    UInt16 *frameBuffer = buffer.mData;
    for (int j = 0; j < inNumberFrames; j++) {
        frameBuffer[j] = rand();
    }
    */
}
return noErr;
}

If you are only looking for recording from MIC to a file and play it back, the Apple's Speakhere sample is probably much more ready to use.

Answer 2

Basically, 1. Create a RemoteIO unit (See references about how to create RemoteIO);

1. Create a FilePlayer audio unit which is a dedicated audio unit to read an audio file and provide audio data in the file to output units, for example, the RemoteIO unit created in step 1. To actually use the FilePlayer, a lot of settings (specify which file to play, which part of the file to play, etc.) are needed to be done on the it;

2. Set kAudioUnitProperty_SetRenderCallback and kAudioUnitProperty_StreamFormat properties of the RemoteIO unit. The first property is essentially a callback function from which the RemoteIO unit pulls audio data and play it. The second property must be set in accordance to StreamFormat that supported by the FilePlayer. It can be derived from a get-property function invoked on the FilePlayer.

3. Define the callback set in step 3 where the most important thing to do is asking the FilePlayer to render into the buffer provided by the callback for which you will need to invoke AudioUnitRender() on the FilePlayer.

4. Finally start the RemoteIO unit to play the file.

Above is just a preliminary outline of basic things to do to play files using audio units on iOS. You can refer to Chris Adamson and Kevin Avila's Learning Core Audio for details.

Answer 3

It's a relatively simple approach that utilizes the audio unit mentioned in the Tasty Pixel blog. In the recording callback, instead of filling the buffer with data from the microphone, you could fill it with data from the file using ExtAudioFileRead. I'll try and paste an example below. Mind you this will just work for .caf files.

In the start method call an readAudio or initAudioFile function, something that just gets all the info about the file.

- (void) start {
readAudio();
OSStatus status = AudioOutputUnitStart(audioUnit);
checkStatus(status);
}

Now in the readAudio method you initialize the audio file reference as such.

ExtAudioFileRef fileRef;

void readAudio() {
NSString * name = @"AudioFile";
NSString * source = [[NSBundle mainBundle] pathForResource:name ofType:@"caf"];
const char * cString = [source cStringUsingEncoding:NSASCIIStringEncoding];
CFStringRef str = CFStringCreateWithCString(NULL, cString, kCFStringEncodingMacRoman);
CFURLRef inputFileURL = CFURLCreateWithFileSystemPath(kCFAllocatorDefault, str, kCFURLPOSIXPathStyle, false);

AudioFileID fileID;
OSStatus err = AudioFileOpenURL(inputFileURL, kAudioFileReadPermission, 0, &fileID);
CheckError(err, "AudioFileOpenURL");


err = ExtAudioFileOpenURL(inputFileURL, &fileRef);
CheckError(err, "ExtAudioFileOpenURL");

err = ExtAudioFileSetProperty(fileRef, kExtAudioFileProperty_ClientDataFormat, sizeof(AudioStreamBasicDescription), &audioFormat);
CheckError(err, "ExtAudioFileSetProperty");
}

Now that you have the Audio Data at hand, next step is pretty easy. In the recordingCallback read the data from the file instead of the mic.

static OSStatus recordingCallback(void *inRefCon,
                              AudioUnitRenderActionFlags *ioActionFlags,
                              const AudioTimeStamp *inTimeStamp,
                              UInt32 inBusNumber,
                              UInt32 inNumberFrames,
                              AudioBufferList *ioData) {

// Because of the way our audio format (setup below) is chosen:
// we only need 1 buffer, since it is mono
// Samples are 16 bits = 2 bytes.
// 1 frame includes only 1 sample

AudioBuffer buffer;

buffer.mNumberChannels = 1;
buffer.mDataByteSize = inNumberFrames * 2;
buffer.mData = malloc( inNumberFrames * 2 );

// Put buffer in a AudioBufferList
AudioBufferList bufferList;
bufferList.mNumberBuffers = 1;
bufferList.mBuffers[0] = buffer;

// Then:
// Obtain recorded samples
OSStatus err = ExtAudioFileRead(fileRef, &inNumberFrames, &bufferList);

// Now, we have the samples we just read sitting in buffers in bufferList
// Process the new data
[iosAudio processAudio:&bufferList];

// release the malloc'ed data in the buffer we created earlier
free(bufferList.mBuffers[0].mData);

return noErr;
}

This worked for me.