[sdiy] TB303 Slide

Harry Bissell harrybissell at wowway.com
Fri Jun 8 15:32:55 CEST 2012


well the maths people can jump me if they like...

The RC glide gets to (arbitrarily close to)the final value in the same time.

For the same R, the applied voltage is greater for the larger step. so more
charging current is available early in the curve. The cap is slewing FASTER
at this point in the curve.

As the voltage difference gets smaller between the input and the capacitor voltage. everything
slows down.

You might be better off saying that in the same amount of time the error between the actual
voltage and the ~theoretic~ final voltage are equal.

That amount of time is when I SAY its 'close enough for rock and roll'

Forget whether the curves are partially congruent, they are at least similar in shape and very different
from constant slope glides...

H^) harry



----- Original Message -----
From: Tim Stinchcombe <tim102 at timstinchcombe.co.uk>
To: 'Mattias Rickardsson' <mr at analogue.org>
Cc: 'Synth DIY' <synth-diy at dropmix.xs4all.nl>
Sent: Thu, 07 Jun 2012 17:34:17 -0400 (EDT)
Subject: Re: [sdiy] TB303 Slide

> >> Does the R2R DAC in the TB 303 feeding the glide capacitor 
> directly 
> >> actually achieve constant glide time regardless of the 
> voltage span 
> >> between notes. (ie: glide time from 1V to 1.83V is the 
> same as from 
> >> 1V to 2V?)
> >
> > Yes. The output resistance of the DAC is always 100k, regardless of 
> > the voltage, and so the time constant with the 220n (C35) is 22ms - 
> > thus after about 60ms the slide is complete, no matter how large or 
> > small the voltage change was.
> 
> I guess it's "constant" glide time?
> 
> What I mean is... that it's an ordinary exponential RC 
> charging curve, having longer times for bigger steps, never 
> completing the slide? :-)
> 
> A glide from 1 V to 2 V is shorter than a glide from 0 V to 2 
> V. When gliding from 0 to 2, it goes quicker at the start and 
> by the time it passes 1 V the rest of the glide is identical 
> to a glide from 1 to 2. So the time is indeed longer... The 
> glide time is just "constant" in the sense of reaching within 
> a certain percentage (say, 90%) of the total glide distance - 
> but no such percentage can be considered a
> *complete* slide for all different distances. :-)

Agreed, each person can nit-pick what they regard as 'complete' by picking
their own favourite no. of time constants to get as close to the final
voltage as they like - however, within what I believe to be the normally
accepted convention for such a circuit, 'constant time glide' seems a
useful distinguishing description against alternatives such as 'constant
rate glide', which has markedly different characteristics.

However, I will take issue with part of this:

> A glide from 1 V to 2 V is shorter than a glide from 0 V to 2 
> V.

Mmm, well I'm not buying this argument:

> When gliding from 0 to 2, it goes quicker at the start

yes, ...but...

> and 
> by the time it passes 1 V the rest of the glide is identical 
> to a glide from 1 to 2.

...no, I think not! The 1 -> 2V curve is *always* above the 0 -> 2V curve,
with both converging toward 2V as the time gets large. Without bothering to
be rigorous about it, it looks to me that at any given time the gradient of
the 1-2 curve will always be less than that of the 0-2 curve, and so nowhere
can the 0-2 curve be 'dropped' onto the 1-2 curve!? Because the time
constant *is* the same in both cases, when the 0-2 curve passes 1V, it has
much less time to make up the remaining 1V, and so must do so at a faster
rate than the 1-2 curve! 

Tim
__________________________________________________________
Tim Stinchcombe 

Cheltenham, Glos, UK
email: tim102 at timstinchcombe.co.uk
www.timstinchcombe.co.uk

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-- 
Harry Bissell & Nora Abdullah 4eva



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