[sdiy] DCO amplitude with filtered PWM

Tom Wiltshire tom at electricdruid.net
Sat Sep 1 12:34:04 CEST 2018

> On 1 Sep 2018, at 06:55, Declare Update <declareupdate at gmail.com> wrote:
>> PWM is harmonically equivalent to detuned sawtooths.
> Are they really equivalent? I’m aware of the saw+ramp=pulse trick, but to get the common animated pwm wave out of that, you shift the phase forward then backward, which would be more like shifting the pitch up and then down, or am I totally wrong here? I do understand that the harmonic content of a saw is the same whether inverted or not, so it makes sense that their sums would be equivalent, but visualizing what they’d look like on a scope is yielding ... dissonance (hehe)

There’s a load of pictures on my website:


The “sum of phase shifted waveforms” and the “difference of phase shifted waveforms” cases aren’t identical, but they’re very similar, and they amount to the same thing.

> detuned saws, without one inverted, would not result in nearly silent “phase-y” sounds, as one might describe them. wide pwm, where the pulse becomes fairly narrow, have this high pass sort of sound going on (unsurprisingly), but does this also happen with detuned saw? my intuition says not, but maybe I’m wrong here. am I misunderstanding something? 

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.

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