pwm in, pwm (half frequency) out [solved]

[befacosynth]

hi!

i want to add this to my new vco, but i can't find how, i also don't know if useful in any way, but i want to try; the idea is to take the pwm signal and divide the frequency by two but keeping the same duty cycle, any ideas?

[JovianPyx]

If you halve the PWM carrier frequency, the high and low sections of each cycle must be twice as long. This will preserve the duty cycle.

[befacosynth]

but...
how to do this electronically?
Thanks

[JovianPyx]

If this is a digital thing, it's easy to change the code to do it.

I assume you mean an analog "box" where PWM goes in and comes out at 1/2 the carrier frequency. I could do it with a PIC, but that would be digital - essentially, I would input the PWM on a digital pin and time the high and low phases. I would sample a high, low, high and a low and output a single high and a single low both at twice the time. There would be a 2 cycle lag however.

As for pure analog? Maybe someone eles?

I'm not really sure why you would really want to do this. PWM is always better when the carrier frequency is _higher_, not when it's _lower_.

[ian-s]

I think he means regular analog pwm as in early roland/arp synths. Typically a comparator is connected to a sawtooth and a modulation source, so it produces a rectangle whose width varies with the modulation voltage.
The effect is a thicker tone than a static rectangle.

Because dividers only toggle on one edge, preserving the duty cycle does not seem to be easy though. Might be possible with some extra logic. Perhaps it would be better to generate a half frequency triangle (by conditionally inverting the sawtooth with the sub square) and add another pwm comparator? You could then have two identical pulse widths or allow each rectangle to be modulated independently.

[befacosynth]

that is the idea ian! i make the first pwm from the saw and comparator, but how i can make the second saw without messing all around with a square-to-ramp converter (in a wide vco this is difficult to make well done), what you mean with 'by conditionally inverting the sawtooth with the sub square'?
some like a 'analog logic AND ' perhaps?

[ian-s]

A triangle core vco sometimes uses a couple of opamps and a switch (cmos or fet) to create a sawtooth from the triangle wave. The square wave is used to switch the inversion on and off every half cycle.

There is an example on this page which uses the technique to produce both a 1F and 2F sawtooth, the only difference being the offset on the triangle.

If you have a sawtooth core, you could use the same switched inverter with a standard square sub octave (4013?) switching the slope on alternate cycles giving a 1/2 F triangle that you can then make a second pwm output from. Might have to de-glitch the triangle though.

[befacosynth]

hi!

i didn't find in th bergfotron's page, but i do on my own way, here is the solution (attached)
as you said there is a spike on the saw, therefore is on the pwm but we can erase it withe the feedback network C7-R40
Thanks for the ideas!
BTW the combined sound with the original pwm (same duty cycle, half freq) is interesting, a plus for a new VCO

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