It is a hand-built device labelled as a Transient Waveform Modifier. It was probably custom built by Tristram Cary and was used in the early electronic music produced in his studio in Fressingfield, Suffolk, in the UK. It is a black metal panel with sixteen control knobs arranged in four rows of four, with a set of labels between the upper and lower pairs of rows.
A transient is a voltage pulse that is generated to trigger the envelope of a sound. Functionally, this particular device is a ‘pre-standard’ version of an envelope generator and would have done something akin to what the combination of an envelope generator and a voltage controlled amplifier (VCA) are used for in most analogue audio synthesisers.
Given a continuous tone from an oscillator, in order to use it musically you need to be able to have it rise and fall in intensity and to sound for durations from a short sharp pulse of tone to a sustained note. To do that with an analogue synthesiser one would set the rise, sustain and fall durations of the envelope generator as desired, patch it to control the gain of a VCA, and then trigger it with a pulse of short duration (a transient). The shape, or profile, of the envelope controls the amplification of the continuous waveform by contouring the sound in a form of amplitude modulation and in modern synthesisers the envelope generator usually incorporates attack, decay, sustain and release (ADSR) periods.
Before synthesisers, while all the devices that are now readily available were being first thought about, various types of signal would have been available, from the pulse generated by a press of a push-button switch to a continuous series of pulses generated by an oscillator, the sine-wave or ramp-derived tones generated by an oscillator, to the effects of putting passive components such as a capacitor to ground (which produces a time delay as it is charged with current from the pulse source) in the signal path. What was not at first available was the voltage controlled amplifier.
Electronically, it consists of four independent channels each of which utilises one row of control knobs. Each of the four rows consists of two multi-pole switches and two potentiometers. Behind the metal plate are the electronics for the four channels. These include the body of the switches and the potentiometers, an EF40 pentode audio amplifier valve, and the necessary passive electronic components to connect everything up and shape the currents so that the device works.
The left most control is a 6 position switch that sets the degree of differentiation performed on the transient pulse. Differentiation causes the signal to be sharpened up and may be thought of as the “attack” rate (slew rate) of the control voltage envelope. This control sets the rise time or ‘attack’ of the envelope.
The second control is a three position switch which determines whether the envelope waveform triggered by the transient continues to ‘grow’ (or rise) or ‘die’ and drop away (or release).
The third control is a variable potentiometer that sets the duration of the active period of the envelope, that is, it sets the hold period of the note being shaped.
The right most control is a potentiometer labelled Low Level Set. I suspect that this will be the level of the voltage held during its interim period or the voltage that it remains at between transient events.
The EF40 pentode is an audio frequency amplifier valve. It has a screen grid as well as a control grid to reduce internal stray electron noise. The control grid regulates the flow of electrons from the cathode and is effectively the signal entry point for the amplifier. However if a variable voltage is applied to the screen grid then the amplifier acts as a multiplier with the screen grid voltage modulating the gain of the valve. This makes it a voltage controlled amplifier (VCA) and as such this would have been how it was used in Cary’s Transient Waveform Modifier. The EF40 pentode is built to reduce sources of noise, with special mica supports for the grids, which reduce microphonics, and has very low hum characteristics, as all sources of stray magnetic fields generated by its heater and other grids are suppressed. Leakage currents are suppressed by keeping the electrodes separated as far as possible. It is quiet enough to render it very useful in circuits that require low noise characteristics (such as audio pre-amplifiers).
See http://www.r-type.org/pdfs/ef40.pdf for valve specs, and
Eric Barbour’s “Audio Synthesis via Vacuum Tubes” for some notes on its use as a VCA. http://www.pentodepress.com/receiving/pentode-gain.php