>From: Dan Slater <dslater@...>
>Reply-To: dslater@...
>To: Richard Quirk <r_quirk@...>
>Subject: Re: Comdyna GP-10S
>Date: Sat, 05 May 2001 22:27:38 -0700
>
>Hi Richard;
>
>Sorry for the delay in replying. I had a very busy
week
>
>Richard Quirk wrote:
>
> > Hello again,
> >
> > I posted a message to a Serge newsgroup on Yahoo
asking if anyone had used or owned any Comdyna
analogue computers. There was quite a bit of interest
so I directed them towards your website and told them
what I'd been told in letters from Ray at Comdyna.
> >
I have been asked to ask you if you wouldn't mind
explaining what the GP-10S can do (i.e. will it do the
functions as described in the analogue computers page
on your website), and how it can be used for musical
purposes - & I will post this to the newsgroup (or you
can join it yourself [
SergeModular@yahoogroups.com]).
The people on the website, like myself, don't have any
great knowledge of maths, but there are quite a few
that are very interested in buying a GP-10S if it can
lead to new musical horizons without too much
difficulty. Hopefully this makes sense.
>
Although I talked with Ray a number of times about the
GP10s I don't know its exact final configuration and
specifications. I believe that it is a scaled down
version of the GP10 with 2 rather than 4 integration
ranges (ie., 2 vs 4 capacitors) for each integrator
and it uses somewhat less expensive integration
capacitors. I believe that the GP10s has 8 coefficient
potentiometers (attenuators), 4 initial condition
potentiometers, 4 integrators, 4 summers and 8 trunk
lines. Two multipliers can be added as an extra cost
option.
>
Analog computers were originally designed to solve
differential equations. Virtually all analog computers
include 3 basic functions, integrators, summers
and coefficient potentiometers.
>
Integrator -- The integrator forms a time integral of
a voltage. It is implemented using capacitor feedback
with an operational amplifier. The integrator stages
can alternately be used as simple summing stages.
>
An electronic switch within the integrator allows the
integrator to be electronically switched between a run
state, hold state and reset state. When set to the run
state, the integrator integrates the input signal. The
hold state freezes the signal, similar to a sample and
hold. The reset mode switches the integrator into an
initial condition set mode. Associated with each
integrator is an initial condition set potentiometer.
This used to set the integrator output voltage prior
to the start of an integration. Comdyna uses a sort of
strange set of negative voltage levels to control the
integrator mode. If you want to use the run/hold/reset
capability with the GP10s you will have to produce the
mode control signal externally. A second control
signal can be used to select between the two
capacitors. This signal also would have to be provided
externally.
>
Summer -- A summing amplifier adds together several
voltages and changes the sign. The summing amplifier
is implemented as an inverting operational amplifier
circuit with multiple inputs and resistive feedback.
The GP10s has 4 of these with varying numbers of
inputs.
>
Coefficient potentiometer -- Coefficient
potentiometers multiply a voltage by a fixed value
between 0 and 1. They are just a potentiometer setup
as a voltage divider. For scientific applications,
coefficient potentiometers need to be set precisely.
Most analog computers include a switching network so
that the potentiometer multiplication factor (with
circuit loading) can be measured directly on a digital
volt meter (DVM). I think that the GP10s internally
has
this switching capability although an external meter
and control switch would be needed.
>
Trunk lines -- There are 8 lines that go to the rear
panel as a convenience. You can use these as a
convenient way of bringing inputs and outputs to the
unit. They are just wires with no active components.
>
Multiplier -- The GP10s can accept 2 multipliers. I
don't believe that these are included in the basic
unit, they would need to be ordered as an extra cost
option. The multiplier requires the use of either an
integrator or summing amplifier as it provides a
current output. It can be put in the feedback path
and then function as a divider.
>
The GP10s can be patched to do a variety of basic
functions including adding (mixing), scaling,
subtracting, multiplying (VCA / ring modulation),
switching, simple oscillating, simple filtering, etc.
But a modular analog synthesizer would generally do a
much better job here. A VC state variable filter for
example would pretty much use up all the resources of
the GP10s and not work nearly as well as an analog
modular version. For example it would have a linear,
not exponential frequency control and it would have a
smaller VC tuning range. The GP-10s is a very small
analog computer that is primarily useful at the
introductory level but would rapidly run out of
capability for more than the most basic patches just
as a tiny music synthesizer would also have
significant
limitations.
>
In general, an analog computer is not a substitute for
an analog modular synthesizer, but a tool for
exploring new concepts. The strength of the analog
computer is in the solution of linear and non-linear
differential equations. This world tends to be foreign
to the musician, just as midi, frequency shifters
and arpeggios would be foreign to the mathematician.
There are countless capabilities of the analog
computer but they are not as VCO, VCA, etc. They are
as a virtually infinite set of mathematical models.
>
Chaotic systems is an example of an area that works
well with analog computers. Chaotic systems can also
be created directly in analog modular synthesizers.
The
article on my website provides examples of both analog
computer and analog synthesizer chaos in a musical
context.
>
Polynomials can be generated by cascading integrators,
this can provide an interesting type of envelope
function, although it is a bit difficult to control
in real time. These can further be enhanced by mode
switching the integrators. Analog computers excel in
the simulation of dynamic systems. These can provide
interesting musical models, both as sound and CV
generators, although the limited computing capability
of the GP10s would limit the exploration of this
area considerably. There are many other interesting
mathematical systems but they also are generally
beyond the capability of the GP10s.
>
>Best regards;
>
>Dan Slater
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