WOW

Why clean modulation matters

Pitch modulation changes the speed at which stored audio is read. Most requested read positions land between samples, so the plug-in has to reconstruct values that were never recorded directly. That reconstruction method has a large effect on the output.

The interpolation problem

A simple interpolator is inexpensive, but it can lose high-frequency level and create tones unrelated to the requested pitch movement. When playback speeds up, frequencies near the top of the input spectrum can be pushed beyond the output Nyquist limit. If they are not removed before resampling, they fold back into the audible band as aliasing.

Those errors are separate from the sidebands produced by pitch modulation itself. Frequency modulation naturally creates related components around the original tone; those are part of the effect. Extra interpolation spurs and folded high-frequency content are implementation artifacts.

What Wow does

Normal, HQ, and Ultra use polyphase windowed-sinc fractional-delay readers. Their low-pass cutoff follows the instantaneous playback rate: around normal speed, nearly the full input band is retained; as the read rate rises, the cutoff moves down far enough to stop content crossing the output Nyquist limit.

The filter banks are prepared away from the real-time audio thread and interpolated smoothly during processing. Normal uses 80 taps, HQ uses 96, and Ultra uses 128. The increasing tap count tightens spectral accuracy at a higher processing cost. Draft deliberately uses a simpler cubic interpolator.

Reading the comparison

The bright central curve is the modulated test tone. The additional arcs and haze around it show energy created by the resampling process. The unfiltered delay folds substantial energy through the spectrum. The middle examples reduce that energy but retain repeated spurs. Wow keeps the intended moving tone and its physically correct modulation sidebands while suppressing the unrelated structure.

The difference is easiest to hear on exposed high frequencies, extreme settings, and layered modulated signals. On darker or dense material it may be subtle, but clean resampling prevents the artifacts from accumulating.