Question

I am experiencing precision loss when im using doubles and i cant seem to find where the precision is lost.
I am writing a software synthesizer and for some reason the input frequency for an oscillator(sound-wave generator) gets heavily aliased somewhere along the way.
The oscillator is supposed to generate a triangle waveform and the correct frequency is passed to the method but when the result is played in the speakers i can clearly hear that the sound snaps to different frequencies.

I am using NAudio (http://naudio.codeplex.com/) and this method is run once for every sample i want to generate.

Here is the code:

double counter = 0;
int samplecounter = 0;

public double GetNextTriangle(double frequency)
{
    double samplesperwave = (double)Parent.WaveFormat.SampleRate / frequency;
    double length = (double)samplecounter / samplesperwave;
    if (length.CompareTo(0.25) == -1)
    {
        counter = length * 4.0;
    }
    else if (length.CompareTo(0.75) == -1)
    {
        counter = 1.0 - (length - 0.25) * 4.0;
    }
    else
    {
        counter = -1.0 + (length - 0.75) * 4.0;
    }
    samplecounter++;
    samplecounter = samplecounter > samplesperwave ? 0 : samplecounter;
    return counter;
}

Thanks in advance! //Mats

Answer 1

The floating point operations here are well conditionned. But you may see an alisaing problem related to sampling if 1/frequency is not a multiple of 1/sampleRate.

Try this matlab code if you can

sampleRate=5000;
frequency=700;
sampleperwave=sampleRate/frequency;
samplecounter=0:floor(sampleperwave);
samplecounter=repmat(samplecounter,1,5);
length=samplecounter/sampleperwave;
wave=-1+4*(length-0.75);
wave(length<0.75)=1-4*(length(length<0.75)-0.25);
wave(length<0.25)=4*length(length<0.25);
figure; stem(wave); hold on; plot(wave,'r')

You could try to declare samplecounter as double and increment it with a modulo

samplecounter++;
samplecounter = samplecounter % samplesperwave;

Or back in matlab

samplecounter=0:length(samplecounter)-1;
samplecounter=rem(samplecounter,sampleperwave);
len=samplecounter/sampleperwave;
wave=-1+4*(len-0.75);
wave(len<0.75)=1-4*(len(len<0.75)-0.25);
wave(len<0.25)=4*len(len<0.25);
figure; stem(wave); hold on; plot(wave,'r')