AW: VC delay module ideas?

[Rene Schmitz]

At 13:01 30.10.98 +0100, you wrote:
>It's easy to build a VC clock for BBD lines. Use a 4046 as
>VCO, and a 4013 to get complementary clock pulses.
>You might even use one of the 4046's internal PD's to
>simply invert the output signal. If you want to drive long lines,
>add buffer stages (4050). If you need expo tracking, build
>an expo VCO and a 4013.

I posted a schematic to http://www.uni-bonn.de/~uzs159/bbdclock.gif
(The french text says something like "Clock generator with overlap 
suppression", the circuit is powered from 0V to -15V so in the real 
world it would be +15 where ground is indicated, and 0V where it says -15V.
Confused ??! Yep, me too ;-) 


>Now my question.
>The main reason why I want to build a tracking (i.e.
>V/Octave) delay is the use as sound source (Karplus /
>Strong - sp? - and "poor man's physical modelling" stuff.).
>The main reason why I have not yet built a tracking delay
>is the analogue filter problem. Any analogue filter included
>in the feedback loop would detune the whole thing.
>Ok, put the anti-aliasing filters out of the loop, but you
>still *want* to control the frequency response of the feedback.
>So here you have one case where digital filters are
>superior !

Hm, I always wanted something like that, though I think one can realize
it it with a BBD, a ADC->SRAM->DAC chain would be probably better, since once 
would get a wider sweep range than the 1:100 of devices like the TDA1022,
and the delay times could be longer.
Interesting to me would be "comb-filter synthesis" where the input would be 
noise and the output would be harmonically spaced narrow noise bands. 
Another use recently discussed on music-DSP is the creation of hihat sounds
with a
atan(x) function in the feedbackloop.

>Is there a way to add a few "discreet" buckets at the end of
>BBD line, and building a simple z-plane filter by weighting
>these different tabs with different resistors?
>I mean no doubt that it works in theory, but is there a simple
>practical way to do this ? How would these discreet buckets
>look like ? 

I have given this a thought once, I would have used some S/Hs 
clocked syncronous to the BBD.
I think one can simplify the anti-aliasing filter by using a 
digital filter first (this would track the frequency!) and then 
use a simpler LPF. One would have to cascade several S/Hs in series.
(Like in your analog shift register) Maybe something out of a few 4066s 
and buffer opamps can be used here. Every 2n+1-th switch is clocked by 
one phase and the 2n-th by the other. The length of the FIR is 
essentially the length of the impulse response. I've seen a DSP 
algorithm that implements a hilbert transform with a 64 stage 
FIR-filter. So that may be about the number of stages required. 
To implement a n-th order FIR-filter one has to build (2n) S/Hs! 
Since when the switches are closed every two subsequent stages 
will have the same output! But maybe one can get away with fewer stages.

(Anyone willing to build discrete BBDs?)


bye
 -René

             ,   : (uzs159 at uni-bonn.de)
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