AudioUnit tone generator is giving me a chirp at the end of each tone generated

Question

I\'m creating a old school music emulator for the old GWBasic PLAY command. To that end I have a tone generator and a music player. Between each of the notes played

Answer 1

I think that the bit where you stop audio output between notes is the culprit:

if (self.toneUnit)
{
    AudioOutputUnitStop(self.toneUnit);
    AudioUnitUninitialize(self.toneUnit);
    AudioComponentInstanceDispose(self.toneUnit);
    self.toneUnit = nil;
}

Just leave the tone unit active and you'll have less chirping. You'll need some other way to generate silence, probably by having RenderTone continue to run but generate amplitude zero.

I was able to eliminate the slight chirps that remained by having it, on a frequency change, fade the amplitude down to nothing, update the frequenmcy, and fade back in again. This is of course what the old PC speaker couldn't do (except for a few people who rapidly switched it on again), but with a very rapid fade you can probably get the old-school effect without the chirps.

Here's my fading RenderTone function (currently using evil global variables):

double currentFrequency=0;
double currentSampleRate=0;
double currentAmplitude=0;

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 = 0.5;

    // Get the tone parameters out of the view controller
    ToneGen *toneGen = (__bridge ToneGen *)inRefCon;
    double theta = toneGen.theta;

    BOOL fadingOut = NO;
    if ((currentFrequency != toneGen.frequency) || (currentSampleRate != toneGen.sampleRate))
    {
        if (currentAmplitude > DBL_EPSILON)
        {
            fadingOut = YES;
        }
        else
        {
            currentFrequency = toneGen.frequency;
            currentSampleRate = toneGen.sampleRate;
        }
    }

    double theta_increment = 2.0 * M_PI * currentFrequency /currentSampleRate;

    // 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) * currentAmplitude;
        //NSLog(@"amplitude = %f", currentAmplitude);

        theta += theta_increment;
        if (theta > 2.0 * M_PI)
        {
            theta -= 2.0 * M_PI;
        }
        if (fadingOut)
        {
            if (currentAmplitude > 0)
            {
                currentAmplitude -= 0.001;
                if (currentAmplitude < 0)
                    currentAmplitude = 0;
            }
        }
        else
        {
            if (currentAmplitude < amplitude)
            {
                currentAmplitude += 0.001;
                if (currentAmplitude > amplitude)
                    currentAmplitude = amplitude;
            }
        }

    }

    // Store the theta back in the view controller
    toneGen.theta = theta;

    return noErr;
}