[sdiy] DCO amplitude with filtered PWM

[Richie Burnett]

> Detuned saws without one inverted absolutely produces nearly silent phasey sounds as the fundamental of one approaches 180 degrees and all the bass drops out.

Actually the amplitudes of all the odd harmonics tend to zero, leaving just components at 2f, 4f, 6f, 8f, etc... which happens to be the harmonic series for a sawtooth one octave up in pitch.

It is not the same effect as you get when mixing a sawtooth and a ramp together with varying phase shift.

The latter produces a pulse train with varying duty ratio. (The positive and negative gradients of the two waveforms cancel each other out leaving just alertnating up and down steps in the resulting waveform.) With no phase shift between the ramp and sawtooth waveforms you get complete cancellation of every harmonic resulting in silence. For very small phase shifts you get very narrow pulses with a characteristic thin fizzy sound.

You can also think of this process as comb filtering of a sawtooth with an adjustable delay. It amounts to the same effect. You start with the full sawtooth spectrum then various harmonics get attenuated by various amounts by the notches of the comb. 

-Richie,
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