[sdiy] random thought about sine shapers

[cheater cheater]

On Mon, Apr 19, 2010 at 18:21, David G. Dixon <dixon at interchange.ubc.ca> wrote:
>> That's one thing I was going to request once you got to designing the
>> shaper :-) sin/tri.. if you can get square in too, that's better, if
>> you can get PW in, that's even better. No, there's no reason you
>> shouldn't have it on the front panel :-) But make sure there still are
>> trimmers so that you can actually tune it up.
>
> I've already got all the shapers.  The sine shaper is a Thomas Henry
> special, and it's brilliant.  The shape pot (a 100-k, 18-turn Bourns trimmer
> in my units) gives near-perfect sines at about halfway (46.4k, according to
> simulations).  That's the one I'm pondering to put on the panel.
>
>> Another popular one seems to be saw/PW, but I don't know how it works.
>
> Easy: you just feed a saw to the pulse comparator rather than a triangle.

Hmm, I'm not so sure it's the exact one I'm thinking of, because the
one I am thinking off starts with 0% PW when the saw is output, and as
we increase the setting, the slope of the (rising) saw to the left of
the (negative slope) discontinuity has a 1% PW pulse there on top of
it; then as we go further the saw is slowly faded out and on the other
hand the PW gets higher until we reach 100%. I guess you could do
this: feed the saw S to the pulse comparator, call the output pulse
signal P. Have a voltage divider which divides evenly between the
pulse comparator and 'no comparator'. Then those two signals get mixed
together. I think it would/could work well this way. Note this allows
you to have a sort of 'cs 80 style' saw there.

>> > A question:  What does a typical waveform selection knob actually do?
>>  Does
>> > it simply crossfade from one static waveshape to another, or is it
>> > performing an actual waveshaping function, like a sine shape trimmer?
>>
>> do you mean potentiometer or switch?
>
> I don't know.  Are they typically rotary switches or pots?  (I have very
> little experience with actual modulars.  I've only ever laid my hands on a
> couple, and they didn't have this sort of function.)

On integrated synths they're usually switches, but sometimes there's
the tri/sin or saw/pw stuff (e.g. one oscillator would be triangle and
the other saw, and that's how you got those further waveforms, and you
had knobs for the shapers). On modulars you always have multiple
outputs. The shaper morphing is usually on additional outputs -
because you want outputs that are always reliably pw or triangle or
whatever, no matter what your 'special' outputs are doing.

>> This is where using an overdriven OTA as a sine shaper is good
>
> Yes.  A plain ol' differential NPN pair feeding a differential opamp does
> exactly the same thing, with slightly better THD specs (I think), as long as
> the correct component values are used.

I might be wrong, but I believe an overdriven OTA sine shaper might be
nicer in sound than an overdriven transistor sine shaper. Very long
since I tried them, though, but the difference in sound is certainly
there.

D.