How to write array of float values to audio file in Core Audio?

Question

I am trying to implement linear convolution using Core Audio, I have the algorithm implemented and working, but I am trying to write the output of this into a .wav audio file. Here is the code for the algorithm...

    //Create array containing output of convolution (size of array1 + array2 - 1)
float *COutput;

COutput = (float *)malloc(((size1+size2)-1)* sizeof(float));

int sizeOutput = ((size1 + size2)-1);


//Convolution Algorithm!!!

for (i=0; i<sizeOutput; i++) {

    COutput[i]=0;

    for (j=0; j<sizeCArray1; j++) {

        if (((i-j)+1) > 0) {
            COutput[i] += CArray1[i - j] * CArray2[j];
        }

    }

}

I need to write the float values within COutput (a standard array of floats) into an audio file. Am I right in assuming I need to send these float values to an AudioBuffer within an AudioBufferList initially? Or is there a simple way of doing this?

Many thanks for any help or guidance!

Answer 1

The free DiracLE time stretching library ( http://dirac.dspdimension.com ) has utility code that converts ABLs (AudioBufferLists) into float arrays and vice-versa as part of their example code. Check out their EAFRead and EAFWrite classes, they're exactly what you're looking for.

Answer 2

Yes, I'd put them in an AudioBuffer and that into an AudioBufferList. After that you can write them to a file using ExtAudioFileWrite() on a ExtAudioFileRef that was created using ExtAudioFileCreateNew().

Audio File documentation: http://developer.apple.com/library/mac/#documentation/MusicAudio/Reference/ExtendedAudioFileServicesReference/Reference/reference.html%23//apple_ref/doc/uid/TP40007912

Answer 3

This is a late answer, but I struggled to get a float * buffer to write to a file in Swift.

Posting this example in case it helps someone.

enum AudioFileError: ErrorType {
    case FailedToCreate(OSStatus)
    case FailedToWrite(OSStatus)
    case FailedToClose(OSStatus)
}

func writeAudioData(audioData:NSData, toFile destination:NSURL, description:AudioStreamBasicDescription) throws {
    //get a pointer to the float buffer
    let floatBuffer = UnsafeMutablePointer<Float>(audioData.bytes)

    //get an AudioBufferList from the float buffer
    let buffer = AudioBuffer(mNumberChannels: 1, mDataByteSize: UInt32(audioData.length), mData: floatBuffer)
    var bufferList = AudioBufferList(mNumberBuffers: 1, mBuffers: buffer)


    //create the CAF file using the stream description
    var file = ExtAudioFileRef()
    var result:OSStatus = noErr

    var streamDescription = description

    withUnsafePointer(&streamDescription) { streamDescription in
        withUnsafeMutablePointer(&file) { file in
            result = ExtAudioFileCreateWithURL(destination, kAudioFileCAFType, streamDescription, nil, AudioFileFlags.EraseFile.rawValue, file)
        }
    }

    if result != noErr {
        throw AudioFileError.FailedToCreate(result)
    }

    //write the AudioBufferList to the file
    withUnsafeMutablePointer(&bufferList) { bufferList in
        result = ExtAudioFileWrite(file, UInt32(audioData.length / sizeof(Float)), bufferList)
    }

    if result != noErr {
        throw AudioFileError.FailedToWrite(result)
    }

    //close the file
    result = ExtAudioFileDispose(file)

    if result != noErr {
        throw AudioFileError.FailedToClose(result)
    }
}