audio - iOS Code to Convert m4a to WAV

Question

Does anyone have any code snippets that show how to convert an M4a file to WAV? I know there are libraries that convert the other way around.

Thanks.

Answer 1

Just to update for Swift 3:

func convertAudio(_ url: URL, outputURL: URL) {
    var error : OSStatus = noErr
    var destinationFile: ExtAudioFileRef? = nil
    var sourceFile : ExtAudioFileRef? = nil

    var srcFormat : AudioStreamBasicDescription = AudioStreamBasicDescription()
    var dstFormat : AudioStreamBasicDescription = AudioStreamBasicDescription()

    ExtAudioFileOpenURL(url as CFURL, &sourceFile)

    var thePropertySize: UInt32 = UInt32(MemoryLayout.stride(ofValue: srcFormat))

    ExtAudioFileGetProperty(sourceFile!,
                            kExtAudioFileProperty_FileDataFormat,
                            &thePropertySize, &srcFormat)

    dstFormat.mSampleRate = 44100  //Set sample rate
    dstFormat.mFormatID = kAudioFormatLinearPCM
    dstFormat.mChannelsPerFrame = 1
    dstFormat.mBitsPerChannel = 16
    dstFormat.mBytesPerPacket = 2 * dstFormat.mChannelsPerFrame
    dstFormat.mBytesPerFrame = 2 * dstFormat.mChannelsPerFrame
    dstFormat.mFramesPerPacket = 1
    dstFormat.mFormatFlags = kLinearPCMFormatFlagIsPacked |
    kAudioFormatFlagIsSignedInteger

    // Create destination file
    error = ExtAudioFileCreateWithURL(
        outputURL as CFURL,
        kAudioFileWAVEType,
        &dstFormat,
        nil,
        AudioFileFlags.eraseFile.rawValue,
        &destinationFile)
    print("Error 1 in convertAudio: (error.description)")

    error = ExtAudioFileSetProperty(sourceFile!,
                                    kExtAudioFileProperty_ClientDataFormat,
                                    thePropertySize,
                                    &dstFormat)
    print("Error 2 in convertAudio: (error.description)")

    error = ExtAudioFileSetProperty(destinationFile!,
                                    kExtAudioFileProperty_ClientDataFormat,
                                    thePropertySize,
                                    &dstFormat)
    print("Error 3 in convertAudio: (error.description)")

    let bufferByteSize : UInt32 = 32768
    var srcBuffer = [UInt8](repeating: 0, count: 32768)
    var sourceFrameOffset : ULONG = 0

    while(true){
        var fillBufList = AudioBufferList(
            mNumberBuffers: 1,
            mBuffers: AudioBuffer(
                mNumberChannels: 2,
                mDataByteSize: UInt32(srcBuffer.count),
                mData: &srcBuffer
            )
        )
        var numFrames : UInt32 = 0

        if(dstFormat.mBytesPerFrame > 0){
            numFrames = bufferByteSize / dstFormat.mBytesPerFrame
        }

        error = ExtAudioFileRead(sourceFile!, &numFrames, &fillBufList)
        print("Error 4 in convertAudio: (error.description)")

        if(numFrames == 0){
            error = noErr;
            break;
        }

        sourceFrameOffset += numFrames
        error = ExtAudioFileWrite(destinationFile!, numFrames, &fillBufList)
        print("Error 5 in convertAudio: (error.description)")
    }

    error = ExtAudioFileDispose(destinationFile!)
    print("Error 6 in convertAudio: (error.description)")
    error = ExtAudioFileDispose(sourceFile!)
    print("Error 7 in convertAudio: (error.description)")
}