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TELESONIEK ATELIER RELAY NO. 01

ANALOG COMPUTATION FUNCTION MODULES

In 1989, while diving heavily into analog voltage control techniques with the ARP 2500 and 2600, I was certain that an analog computer would be a great extention of the analog synthesis art, and then got the first analog computer through the Technical Highschool of Zwolle. Soon more units could be picked-up throughout the country, and I constructed a special version computer out of two original units (Hitachi 240) and spare parts.
For several years I did experiment and study on the subject, and presented results on several occasions.

Most interesting was a guest lecture (1993) at the Institute of Sonology, by setting up the analog computer system in the BEA 5 analog voltage control studio. With great support by Jo Scherpenisse for constructing interface cables, and encouragement from Stan Tempelaars, it was shown that the system could be integrated flowlessly.
Most students however, found the user interface quite hard as the technical skills needed to construct patch programmes where definitely some levels higher then normal practice in the original analog studio.
Anyhow, this set-up was great fun. Below a picture by Jo Scherpenisse of the system in Bea 5.

 

In 2012, I wrote the following article presenting my current ideas on the subject.

"Proposal for extending analog modular electronic music synthesizers with function modules from the analog computation repertoire".
(presented at the symposium ''think analog" at the Humboldt University Berlin, spring 2012)

INTRODUCTION
Most analog modular synthesizers consist of a group of basic function modules: audio- and low-frequency oscillators, various filters, amplifiers, and envelope generators (all manual or voltage controlled), a noise generator, ringmodulator, sample/hold, and some means for voltage mixing and inverting. In past and recent years, during the renewed attention to these interesting and flexible means for electronic music sound synthesis, the basic function group has been extended with several types of new functions. Typically these additions seem to focus on rhythmical functions being extentions of the basic analog step sequenser.
Modules with some logic gate funtions (AND, OR, etc.) can also been found, commonly applied to electronic switching.
However, something is still missing.

ANALOG COMPUTATION
Knowledge of the groups of functions within a typical medium sized analog computer system, and the actual experience of using both the analog computer and analog modular synthesizer combined, has brought me to this proposal for extending the basic set with several usefull analog computer function modules. The functions mostly deal with analog voltage signal processing rather than logic based functions, although the total new system should allways be seen as being an analog/hybrid system.
Both the addition of analog computer functions and its system approach will offer the analog sound synthesist an expanding control voltage universe. Some technical aspects will be defined, followed by the proposed function modules.

TECHNICAL ASPECTS
The typical analog voltage signal 'work' range within a solid state analog computer runs from –10 to +10 Volts; logic switching signals are typically 0 V for the '0' state, and about +6 V for the '1' (designated 'positive logic'). In analog modular synthesizer systems both analog and logic voltages differ widely amoung the various manufacturers, but it seems that the most common standard used is –5 V to +5 V for analog signals, and the same positive logic signals as mentioned earlier. It would be nice to have that –10V to +10V voltage work space, used for instance in the ARP 2500 and 2600 modulars. Attempts for standarization have been issued since the early seventies, but that remains difficult.

In any case these voltage definitions have to be incorporated within the electronic design of any new function module, and the same will go for the following analog computer functions proposed below. A sufficient amount of circuit components is available to fit any design related to the excisting modular system one wants to expand, and as always, better components and design leads to better function quality and stability.
ARP modular synthesizers are known for using epoxi encapsuled function modules basic to their designs. Their 4000 series modules included voltage controlled oscillators and filters, balanced modulators, noise generators etc., and in the keyboard design you could find OpAmp modules. Those encapsuled modules where also part of a product line by Teledyne / Philbrick, that offered many analog signal processing/ computation modules.
Tonus/ARP founder Allan R. Pearlman is known to have worked on that kind of design technology before starting his company.

FUNCTION MODULE PROPOSALS
1 – the variable diode function generator (VDFG)
A voltage fluctuation within the 'working range' can be altered in shape with this function. It is easy to design and produce, consisting of several opamps, diodes and resistors and a minimal series of 10 potentiometers.
If you connect a voltage ramp to the VDFG input the linear shape can be altered quite radically by means of adjusting the 10 potentiometers. The technique is named 'line segment approximation'.
Not only ramps but any kind of voltage fluctuation can be processed; LFO's, envelope generator signals, audio signals; an interesting type of wave shaping.
An (X-Y) oscilloscope for visual input versus output monitoring is recommended.

2 – the linear ramp voltage generator (LRVG)
Havings access to a (on/off switchable) linear ascending or decending voltage has proved to be very usefull in voltage control technique. This can be accomplished by means of an analog integrator with logic control of its 3 states (reset(or initial condition)-compute-hold), a reference voltage source at both the integrator input and the initial condition (IC) input (switchable by logic between – and +), and 2 potentiometers.
The function is usefull at very low frequencies (especially combined with the VDFG) producing slow varying control voltages for oscillators, filters and amplifiers etc.

3 – the 2nd order differential equation model (2DE)
This is probably the most usefull and multi-functional circuit, consisting of 2 integrators, an inverter, several potentiometers and multiple in- and outputs. It will work from very low frequency into audio frequency range, and can be set to sine oscillation (damped and undamped) having quadrature outputs with the addition of a second inverter, and it will function as a state-variable filter, or sometimes called multi-mode filter, having 12dB/oct high- and lowpass, and 6dB/oct bandpass outputs simultaniously available. Putting high- and lowpass in parallel gives ofcourse the bandreject or bandstop/notch filter response.
Two of the most famous electronic musical instrument filters were based on this circuit, the Oberheim SEM filter, and the ARP 1047 multi-mode filter/resonator.

By taking one step back to the original analog computing function however, and adding the logic mode control feature (reset-compute-hold), this circuit becomes much more powerfull.
Voltage control of this function module is accomplished by exchanging the potentiometers by multipliers (or OTA's). Another advantage of this circuit when used in oscillator mode is that it produces a real sine wave, as compared to the sine wave approximations usually found in analog VCO modules (triangle wave 'sine curved' by means of diodes, resulting in several harmonics).

4 – precision potentiometer module (PP)
Just a set of ten-turn good quality potentiometers (buffered) with good readable dials to be able to set control voltages acurately. Acurate voltage and parameter adjustments are often problematic on modular synthesizers (costs/budget choices) but are of great importance to the advanced synthesist. In addition a digital voltmeter comes in handy.

5 - analog/hybrid computer functions already found on some modulars:
a- a sample/hold module that has a track/hold function (in analog computers a sample/hold is composed of two track/holds in series, alternately logic controlled)
b- an analog voltage comparator with additional inverted logic output
c- an oscillator with single shot function
d- R-S flip-flops, for proper logic switching of proposed module number 2 and 3, so not only for frequency- and clock-division.
e- electronic switches, bi-directional not nessesary, A/B switching recommended
f- manual switches: on/off, A/B, and momentary types

6 – analog/hybrid computer system approach
In analog sound synthesis technique the voltage flow through the electronic system is the working material; its speed, direction and form define the resulting sound movements.
Awareness of this parallel parametric process can be enhanced by adding measuring and indicating instruments to the total system. Typical test & measurement instruments are multi-channel oscilloscopes, DC analog and digital voltage meters, lamp displays indicating logic states , frequency counters, XY- and Y-t recording and so on.
In analog computation one has that total feel of awareness for all the voltage fluctuations through the patched program, and it is that particular approach which is very usefull in analog sound synthesis.

CONCLUSION
Analog and analog/hybrid computation functions offer an application repertoire widely explored during the age of high analog signal processing art during the 1950 until 1980 era. The huge amount of dedicated practical and theoretical literature contains programs (function module interconnections, patches) that can expand the analog modular synthesizer repertoire, and enlarge the art of voltage control technique.
It is dearly hoped that this basic proposal will start something that should have been common practice already.

RESOURCES
Wells, T. The Technique of Electronic Music, 1981, schirmer books, new york
Paynter, H.M. Palimpsest of the Electronic Analog Art, 1965, philbrick researches, massachusetts
Kulk, H.J. : Use of the Analog/Hybrid Computer in Electronic Music Studios, 1994


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