Question

I want to work out a bit of code that generates the oscillator wave-type in my tone generator app. The one in this example is a sine-wave, can someone tell me how the code works, as i want to in the future make custom wave-types and square, sawtooth and triangle types.

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

{
    // Fixed amplitude is good enough for our purposes
    const double amplitude = 0.25;

    // Get the tone parameters out of the view controller
    ToneGeneratorViewController *viewController =
        (ToneGeneratorViewController *)inRefCon;
    double theta = viewController->theta;
    double theta_increment = 2.0 * M_PI * viewController->frequency / viewController->sampleRate;

    // This is a mono tone generator so we only need the first buffer
    const int channel = 0;
    Float32 *buffer = (Float32 *)ioData->mBuffers[channel].mData;

     // Generate the samples
     for (UInt32 frame = 0; frame < inNumberFrames; frame++) 
    {
        buffer[frame] = sin(theta) * amplitude;

        theta += theta_increment;
        if (theta > 2.0 * M_PI)
        {
             theta -= 2.0 * M_PI;
        } 
    }

    // Store the theta back in the view controller
    viewController->theta = theta;

    return noErr;

}

Answer 1

The actual sine wave samples are being generated and are populating the buffer in the snippet below

for (UInt32 frame = 0; frame < inNumberFrames; frame++) 
{
    buffer[frame] = sin(theta) * amplitude;

    theta += theta_increment;
    if (theta > 2.0 * M_PI)
    {
         theta -= 2.0 * M_PI;
    } 
}

In the line where buffer[frame] is being assigned, you are calling sin(theta) * amplitude, and for each iteration of the for loop, you are incrementing theta by some finite step size based on your frequency and sample rate, via

double theta_increment = 2.0 * M_PI * viewController->frequency / viewController->sampleRate;

Which is essentially dividing 2.0 * PI * frequency by your sample rate.

Incrementing the theta variable while looping through the for loop is basically advancing the time step one sample at a time until your buffer is full (i.e. frame == iNumberFrames).

If you wanted to generate something other than a sine wave, you would simply replace the following line with some other function:

buffer[frame] = sin(theta) * amplitude;

I.e. let’s say, for example, you wanted the first three terms in the infinite Fourier series that converges to a triangle wave; you might then have the following instead…

buffer[frame] = (8 / pow(M_PI,2)) * (sin(theta) - sin(3*theta)/9 + sin(5*theta)/25);