CODEWITHSUNDEEP
caudiocore-audio
user3078414
user3078414

Reputation: 1937

Audio oscillator with (bi)cubic interpolation

This question is about interpolating sine wave oscillators:

Assuming that amplitude and frequency trajectories for a sine wave are defined by corresponding breakpoint functions or read from user interface, the following few lines of C-code show a common sine-wave oscillator paradigm for synthesizing inNumberFrames of mono audio samples in real time, using linear interpolation:

// ... (pre-computing initial amplitude and phase values)...
for (UInt32 frame = 0; frame < inNumberFrames; frame++) 
{                 
     buffer[frame] = sinf(phasef) * ampf[frame];                               
     phasef += osc->previousPartialPhaseIncrementf + df*frame;                         
     if (phasef > TWO_PI) phasef -= TWO_PI;
}
// ... (storing current amplitude and phase values)...

While musically satisfying in general (although it can be performance optimized using pre-computed sine wavetables and pointer arithmetics), there are occasions when linear interpolation artifacts can be heard. I'd like to know is there a free example of cubic or bicubic interpolation of amplitude and phase oscillator instantaneous values? Concerning that the render thread has real-time priority (at least in CoreAudio), it is important to keep it lightweight, also to avoid running into too many lower-priority threading issues if interpolating outside the render thread. I'd be thankful to anyone pointing me at a working example of a (bi)cubic interpolation sine-wave oscillator algorithm in C, no matter how simple (or complex) it is. Thanks in advance.

UPDATE:

Perhaps this illustration can clarify what was meant by values to be interpolated. Purple dots represent a frequency envelope breakpoint curve (connected by linear interpolation). Cyan dots represent a possibility of superimposed polynomial interpolations. First and last segments are off-scale:

enter image description here

Upvotes: 1

Views: 462

Answers (1)

ruoho ruotsi
ruoho ruotsi

Reputation: 1313

Have a look at musicdsp.org where there is a post on (almost) Ready-to-use oscillators. The end of the post contains the method that you might be interested in with the following signature (by Ollie N.)

float Oscillator::UpdateWithCubicInterpolation( float frequency )

Upvotes: 1

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