Tracking Generators

Haible_Juergen#Tel2743 HJ2743 at
Mon Jan 22 20:05:00 CET 1996

>     However the "interpolating scanner" concept that Juergen has
>     implemented sounds like a quite useful way to generate
>     time-independent non-linear transfer functions, using crossfading
>     VCA's to "morph" between the steps. It seems to me that you would need 

>     to convert the input sweep voltage to a series of trapezoidal
>     functions that are staggered in level (is this how you do it,
>     Juergen?).

Exactly. Just the good old one-transistor sawtooth-to-triangle-converter.
Using a PNP for the right polarity, the collector current directly feeds the
OTA Iabc input. The base of each transistor is controlled by an opamp
which adds the  input voltage and a different offset voltage for each
stage. The positive Rail, to which the emitter resistors go, is stabilized
at approx. 10V. It can be fine adjusted to give exactly the desired overlap
of the different triangle windows.
If I would have used sliders instead of pots (can't make these long holes),
I really could draw the waveform with the sliders. The sanner makes a
piecewise linear interpolation between the points set by the pots.
With a sequencer or comparator method, You would get a staircase
funktion instead of a piece wise linear  function. Even with a linear Slew
Limiter You would have to control the time constant by *both*, the input
frequency, *and* the difference between two adjacent levels to get similar
result! But even then, this concept is bound too loose when You come near
very low frequencies.


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