How do I synthesize sounds with CoreAudio on iPhone/Mac

Question

I'd like to play a synthesised sound in an iPhone. Instead of using a pre-recorded sound and using SystemSoundID to play an existing binary, I'd like to synthesise it. Partially, that's because I want to be able to play the sound continuously (e.g. when the user's finger is on the screen) instead of a one-off sound sample.

If I wanted to synthesise a Middle A+1 (A4) (440Hz), I can calculate a sine wave using sin(); what I don't know is how to arrange those bits into a packet which CoreAudio can then play. Most of the tutorials that exist on the net are concerned with simply playing existing binaries.

Can anyone help me with a simple synthesised sine sound wave at 440Hz?

Answer 1

Many of the audio technologies allow for data to be passed in instead of a sound file. AVAudioPlayer, for example, has:

-initWithData:error:
Initializes and returns an audio player for playing a designated memory buffer.

- (id)initWithData:(NSData *)data error:(NSError **)outError

However, I am not sure how you would pass in a data ptr, start the sound, and then keeping it looping by passing in other data ptrs, or repeating the same, etc.

Answer 2

What you want to do it probably to setup an AudioQueue. It allows you to fill a buffer with synthesized audio data in a callback. You would setup the AudeioQueue to run in a new thread as such:

#define BUFFER_SIZE 16384
#define BUFFER_COUNT 3
static AudioQueueRef audioQueue;
void SetupAudioQueue() {
    OSStatus err = noErr;
    // Setup the audio device.
    AudioStreamBasicDescription deviceFormat;
    deviceFormat.mSampleRate = 44100;
    deviceFormat.mFormatID = kAudioFormatLinearPCM;
    deviceFormat.mFormatFlags = kLinearPCMFormatFlagIsSignedInteger;
    deviceFormat.mBytesPerPacket = 4;
    deviceFormat.mFramesPerPacket = 1;
    deviceFormat.mBytesPerFrame = 4;
    deviceFormat.mChannelsPerFrame = 2;
    deviceFormat.mBitsPerChannel = 16;
    deviceFormat.mReserved = 0;
    // Create a new output AudioQueue for the device.
    err = AudioQueueNewOutput(&deviceFormat, AudioQueueCallback, NULL,
                              CFRunLoopGetCurrent(), kCFRunLoopCommonModes,
                              0, &audioQueue);
    // Allocate buffers for the AudioQueue, and pre-fill them.
    for (int i = 0; i < BUFFER_COUNT; ++i) {
        AudioQueueBufferRef mBuffer;
        err = AudioQueueAllocateBuffer(audioQueue, BUFFER_SIZE, mBuffer);
        if (err != noErr) break;
        AudioQueueCallback(NULL, audioQueue, mBuffer);
    }
    if (err == noErr) err = AudioQueueStart(audioQueue, NULL);
    if (err == noErr) CFRunLoopRun();
  }

You callback method AudioQueueCallback will then be called whenever the AudioQueue needs more data. Implement with something like:

void AudioQueueCallback(void* inUserData, AudioQueueRef inAQ,
                        AudioQueueBufferRef inBuffer) {
    void* pBuffer = inBuffer->mAudioData;
    UInt32 bytes = inBuffer->mAudioDataBytesCapacity;
    // Write max <bytes> bytes of audio to <pBuffer>
    outBuffer->mAudioDataByteSize = actualNumberOfBytesWritten
    err = AudioQueueEnqueueBuffer(audioQueue, inBuffer, 0, NULL);
}

Answer 3

This code works right away. It uses the AudioUnit Framework.