In this example we will see how to write a sine oscillator. The main component will be a “phase” generator.

Phase generator

A phase generator produces a periodic signal that goes from 0 to 1 at each period. For example the expression:

0.1 : (+,1.0:fmod) ~ _

corresponding to the following block-diagram :

oscillator

produces the periodic signal :

{0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.0}...

In this expression 0.1 is the increment between successive samples, and fmod is the floating point remainder operation used to wrap the signal between 0 and 1.

By controlling the increment we can control the frequency of the generated signal. Let say that the sampling rate is 48000 samples per second. By using an increment of 1/48000 we will produce a signal at 1 Hz, and by using an increment of f/48000 we will produce at frequency f Hz.

We can therefore define our phase generator as:

phasor(f)   = f/48000 : (+,1.0:fmod) ~ _ ;

Sine Oscillator

Once we have a phase generator it is easy to define a sine oscillator by multiplying the phase signal by 2*PI and taking the sine:

osc(f)      = phasor(f) * 6.28318530718 : sin;

We can complete our program with sliders to control the frequency and the level of the oscillator

process     = osc(hslider("freq", 440, 20, 20000, 1)) * hslider("level", 0, 0, 1, 0.1);

Final program

Instead of hardcoding the sampling rate, we can use the SR primitive defined in the math.lib library (as a foreign constant to be setup with the actual sample rate by the architecture file) in the definition of phasor. This lead us to the following program.

import("stdfaust.lib");

phasor(f)   = f/ma.SR : (+,1.0:fmod) ~ _ ;
osc(f)      = phasor(f) * 6.28318530718 : sin;
process     = osc(hslider("freq", 440, 20, 20000, 1)) * hslider("level", 0, 0, 1, 0.01);

oscillator

You can try it by pasting the above code into the online compiler and selecting the web/audio target in the exec code tab will result in the following user interface:

userinterface