[sdiy] QProx QT301 chip as a simple theremin controller

[Toby Paddock]

Harry,
This one doesn't self-cal. I think that was one 
of the things that put me off their chips before.
This only cals when you push the high or low 
cal button.
(must resist urge to jump around screaming "You're 
wrong Harry! Wrong wrong wrong!)  :)

I don't think the stepping is from the PWM 
output itself. It's 100kHz and the RC filter 
seems to be smoothing it OK.

I think it's 2 things.
1- (speculation) Inside is a raw 16-bit range. 
You set the high and low cal points of 8-bit 
PWM output range and it maps that on to the 
raw 16-bit range. Depending on where you set 
the cal points, there may not be enough bits. 
Especially at "far hand" end of the range. 
I don't think what I said makes any sense, 
but I don't think I can do any better.

2- There is a definite sample rate. With 
larger values of Cs, the sensitivity goes 
up, but the sample rate goes down. With 100nF 
(what they use in the evaluation board), it's 
not too bad. With 400nF it sounds like a fast 
glissandi. (Is that the right word?) This is 
seperate from the PWM frequency.

I think I'm expecting more out of this 
than it was designed for. Even with tweaking 
it probably won't be good enough for anything 
but casual use for pitch control. But it 
might be fine for controlling a filter or 
something fun like that.

Maybe an antenna under a keyboard for an LCA
(leg controlled afterthouch).
Or how about an HCF (head controlled filter).
Or an ECX (elbow controlled crossfader).

Later,
Toby Paddock

> I saw this chip before but had the impression that
> it would 'self-cal' on a periodic basis by itself...
> not acceptable in the middle of the "Lucky Man"
> solo...
> 
> Since you have pulse width info there is no reason
> to live with high ripple.
> 
> Use the pulse width output to start a linear ramp...
> S/H that ramp at the end of the PWM pulse, then reset
> the ramp. Voila... no ripple in the output.
> 
> It could still step if you can move real fast between
> samples... and you prlly still want a little lag
> filter at the output.
> 
> Let me know if I'm wrong about the self-cal.
> 
> H^) harry
> 
> 
> 
> --- Toby Paddock <tpaddock at seanet.com> wrote:
> > http://www.seanet.com/~tpaddock/c2cv.html
> > 
> > Not a real Theremin, but it's soooooooo easy.
> > 
> > Later,
> > Toby Paddock
> 
>