(VCO) centered or edge mod style - re; PWM???

[Rex Coil 7]

What's up with this?

The pulse waveform can be modulated in two ways. You can have either 'centred' or 'edge' modulation styles. The former will modulate both edges of the pulse output. The latter modulates only one edge. This can have a profound effect on audio rate PWM.

Profound how, precisely?

I have been unable to locate video or audio representations of this. Crucially, an A/B performance will be required to demonstrate how this affects things musically.

Thanks!

[Synthbuilder]

Profound how, precisely?

I haven't any audio samples of this unfortunately but I may try and do some. The 2U VCO is an old project now and the webpage's main content hasn't been updated in some time.

The essential difference is that centred does not produce a phase shift when the pulse width is modulated, while edge does. Phase shifts are perceived as pitch changes if the modulation frequency is high enough.

PWM is typically created from the sawtooth waveform and is therefore equivalent to the edge setting. With this you perceive both a change in frequency and waveshape when the PW is modulated fast enough.

The centred setting is created from the triangle waveform. One can create this with a comparator module and the triangle output from a VCO. When PW is modulated you hear only the shape changing with no apparent frequency modulation.

Tony

[Synthbuilder]

I've uploaded a sound set to the VCO project webpage:

http://www.oakleysound.com/vco.htm

A direct link to the sample is here:

http://www.oakleysound.com/vco_pwm.mp3

There are eight samples in this set. The first pair of samples have the pulse width modulated by an sine wave LFO at a slow speed and differ only by the position of the PWM type switch. The first sound in the first pair uses the 'edge' setting, while the second sound uses the 'centred' setting. Unlike the 'edge' setting, the 'centred' setting does not introduce a varying phase shift which means there is no perception of frequency modulation. As we can hear the perceived pitch shift in 'edge' mode is not that obvious at these lower modulation frequencies.

The second pair of sounds is the same sound but with a faster modulating frequency. Again, 'edge' is first and then 'centred' and this time the difference is more pronounced. The third pair of sounds is the same patch but at an even higher modulation frequency. The difference is now very obvious. The fourth sample is the previous patch but the switch is moved between 'edge' and 'centred' repeatedly. The fifth sample uses a triangle audio rate modulation from a slightly detuned VCO. Again, the switch is moved between the two settings to highlight to the difference in the sounds.

Tony

[Rex Coil 7]

Now, that's how you sell a VCO, folks! I posted the question, and inside of roughly ten hours ~or so~ (give or take), an explanation was given and sound samples were produced and provided. Nicely done, good sir.

So I believe I understand what "edge" and "centered" refer now. Visually speaking. As I imagine viewing the effect on a scope, here's what I see:

** Edge - meaning the pulse itself is stretched and compressed (for lack of better descriptive terms) starting from one edge of the squared off waveform.

** Centered - meaning the pulse itself is stretched and compressed starting from the center of the squared off waveform.

Do I have this right? Forgive my down home descriptions, I'm no engineer, not by a long shot.

Thanks again, Tony, for your prompt reply and thorough explanations.

EDIT: Also, the audio really demonstrates the difference and importance between the two approaches, musically speaking.

[Synthbuilder]

** Edge - meaning the pulse itself is stretched and compressed (for lack of better descriptive terms) starting from one edge of the squared off waveform.

** Centered - meaning the pulse itself is stretched and compressed starting from the center of the squared off waveform.

That's exactly it.

The way I describe it when I'm actually there in person is to hold my hands up in front of me. Centred is both hands towards the middle and PWM is both hands moving towards and away from each other like a slow clap. Edge is holding the left hand stationary on the left side and PWM would be the right hand moving horizontally away from and back to the left hand.

Tony