Electro-Music Engineer: CEM 3310
Application of the CEM 3310
VC Envelope Generator
As readers will have observed this new column is a development of our ICs for Electro-Music and will continue to examine ICs of specific interest for electronic music applications as well as other equipment and components for the electronic engineer working with music designs. For this article the format is changed from a simplified data sheet type of approach to one which will provide a specific practical application or some design ideas for the CEM 3310 Voltage Controlled Envelope Generator, from Curtis Electromusic Specialities, which was briefly described in the April 1981 issue of E&MM.
The features of this IC which the writer finds most intriguing is its voltage control capability, that is, the time constants of attack, decay and release as well as the sustain level are proportional to the voltage applied at the appropriate pins of the IC. This makes it easy to configure the device such that a proportion of the keyboard control voltage may be summed with one, or more, of the A, D, S and R inputs and so vary the envelope in relation to the note played. For example, if we add a proportion of a positive keyboard control voltage to the negative voltage used to vary the attack time then the result will be a faster attack time the higher the note played. Another alternative is to return a proportion of the ICs output, with or without inversion, to the control pins and obtain a virtually unlimited variety of envelope contours. Thus instead of the usual curved (exponential) shapes of the envelope contour one may obtain, say, an attack response which is concave, linear, S-shaped and so on. Since the sound contour from a synthesiser is governed by the envelope shape then the ability to depart from the conventional ADSR envelope is well worth exploring for both imitative and creative synthesis.
The main difficulty with using the feedback technique for obtaining unusual envelope shapes is the time required to establish the required conditions, especially in a polyphonic synthesiser. The next best thing is to have switch selection of various proportions of feedback to produce contours of specified shape. Doug Curtis, the president of Curtis Electromusic Specialities, has recently described such a circuit. What caught our attention, however, was another of his designs which, with a few add-ons and component changes, I have configured into a complete circuit. The design has three operating modes. The first is called NORMAL and is the conventional ADSR envelope which is familiar to readers. Second, is an AUTOMATIC mode in which a short gate pulse will cycle the envelope shaper through a complete ADR envelope. Such an envelope is illustrated in Figure 1 and it can be seen that by adjustment of the time constants it can be made to approximate the ADSR envelope.
We have found that this mode allows the envelope shaper to be interfaced with some of the programmable sound generators which usually only produce a short pulse when a new note is played. Also when gated from an LFO, or other non-keyboard trigger generators, it provides a more useful envelope than the usual AD type obtained from conventional shapers.
Furthermore, as Doug Curtis points out, the mode is useful for long attack times since the player can get on with something else while the envelope cycles through its phases. The third mode is called DAMPED and allows a closer approximation to the piano envelope. The envelope type is ADRR, as shown in Figure 2, and for a piano type sound one requires a fast attack followed by a brief decay then a long release and finally a very short release corresponding to the damper rejoining the string. In this latter mode the release of the key, which terminates the gate signal, simulates the damper action.
The complete circuit for this envelope shaper is shown in Figure 3. RV1, 3, 4 and 5 and associated resistive dividers are used to adjust the sustain level (from 0 to 100% of peak attack level) and the time constants for attack, decay and release. The time constants are also governed by C9 plus the feedback components RV6 (for matching units in a polyphonic system) and R15. With the components shown the A, D and R time values may typically be varied from two milliseconds to twenty seconds. IC3 is solely to convert the +5V output from the CEM 3310 to a +10V output and so may be bypassed, or omitted, if +5V is suitable for a particular application.
The unusual part of the circuit is built around IC1, TR1 and TR2. In the NORMAL mode a gate pulse, in the range of approximately +3 to +15V, will simply switch the output of IC1A high and this output is connected to the gate pin (pin 4) of the CEM 3310. A simultaneous trigger pulse will also be generated and applied to pin 5. It was noted in the article on this device that both a gate and a trigger pulse is necessary in order to generate an ADSR envelope but the trigger pulse is readily obtained by differentiating the gate pulse. If an independent trigger pulse is also available then this may be applied to the TRIGGER input, C5, while the gate is high and so initiate a new attack cycle for generation of multiple peak envelopes.
In the AUTOMATIC mode IC1A will again go high and produce the required gate and trigger pulses. Note, however, that IC1A is acting as a set-reset flip-flop and in this mode the length of the gate pulse at the input, S1, is of no consequence and is solely used to switch IC1A high. The envelope then progresses through the attack and decay phases and when the latter is within about 100mV of the sustain level, as determined by R10 and R11 connected to IC1B, the output of IC1ES goes high which will then reset IC1A low and cause the envelope to go into its release phase. In other words when IC1A goes low as the decay matches the sustain level then as far as the CEM 3310 is concerned the action is the same as when the normal manual gate is removed.
When S1 is switched to the DAMPED mode then the gate pulse is applied to the base of TR1, switching it on and producing a positive pulse to IC1A which, as before, initiates the envelope. Again in this mode an independent trigger may be applied, if required. With IC1A high the cycle will normally follow the same procedure as the automatic mode and it will be reset low by IC1B when the decay more or less matches the sustain level which has been manually set by RV1. When the gate pulse at S1 is removed, however, TR2 will be turned on and since its emitter is connected to the release input of the CEM 3310 it will thus short this input causing the release (denoted by R2 in Figure 2) to go to zero in several milliseconds.
The circuit also includes a manual gating facility which is disabled when the unit is operated from other sources.
It is hoped that the above focuses more attention on the rather neglected 'sound contour' and stirs some interest in utilising dynamic control of contour.