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## EMS Random Voltage Generator

EMS SYNTHI MODULE III Random Voltage Generator
source: EMS infosheet / manual, posted 1 feb 2002
• Purpose and Use
• The EMS Random Voltage Generator (RVG) produces a staircase type of output, the 'steps' of which occur randomly, but whose voltage range and time variance are controllable. Let us consider the two areas of randomness - time and voltage. Time: The internal clock which decides the mean rate of events can be (a) free-running and subject only to manual control, (b) inhibited so that the next random voltage depends on a manual push button or an external pulse, (c) free-running but voltage controlled so that an external voltage influences the mean rate. If the time variance control is at zero, the mean rate is constant (though subject to change of steady rate if influenced by a control voltage). As the time variance control is turned clock­wise, the mean time random variance becomes greater, and when this control is at maximum the rate will randomly vary in a ratio of more than 100:1. The distribution of chances is rectangular, not Gaussian, so that no part of the variance range is particularly favoured. Voltage: Two voltage outputs are available and separately adjustable, but both are subject to the same time control - i.e. the steps of the staircases occur at the same times from both outputs. At zero, there is no voltage variance at all, and.as the setting is increased the variance about the mean voltage (set at the device being controlled), becomes greater, excursions being both positive and negative of the mean voltage. In addition, a trigger pulse is generated (trigger output socket) whenever a random selection is made.

• Facilities Cord
• The facilities cord provided (connections shown in the diagram) links all the outputs to a Synthi 'A' or VCS3, delivering VI to Input 1, V2 to Input 2, and Trigger to Attack. To try out the device, take two VCO's (say Oscs. 1 and 2) through the Filter and Envelope Shaper and put in pins controlling Osc.l from Input 1 and Osc.2 from Input 2. Two part chords of varying sizes will now be obtained, depending on control settings, and the envelope shaper can be set to attack each chord as desired - naturally there will be certain settings at which an envelope can never develop properly, as when Attack time is set longer than the average event time from the RVG. Try also putting one of the voltages on Filter control, which will result in timbre being pitch-dependent. There are, of course, numerous other uses than controlling pitch, and at low settings of voltage variance very natural-sounding random vibrators, wah-wah effects etc will all be found effective.

• Inhibit / Free Run
• Selecting "Inhibit" instead of "Free Run" will stop the clock, but not the random generator. Pressing "Reselect" will result in a new random voltage at Vl and V2, and each time the button is pressed a new selection appears. By using the "Select'1 input jack the occurrence of a new selection can be made dependent on an external pulse. +5v at this input will cause a single selection followed by inhibit. When this voltage is relaxed the clock will either restart (switch to 'free run') or continue inhibited (switch to 'inhibit'. The "Select" input can therefore be used to trigger single events or cause a stop-start action in the free run. It is best to use fairly sharp waveforms such as pulse or sawtooth to ensure a firm operation.

• VC of Mean Rate
• V.Cont.Mean (Voltage Control of Mean Rate) is only operative in the 'free run' mode, and a voltage here will influence the clock rate automatically. A gentle ramp, for example from Osc.3 on the Synthi, will cause a progressive speeding (or slowing) of the mean rate. There is still a variance about this rate, of course, decided by the time variance setting.

• When using the RVG with a Synthi 'A' or VCS3, these inputs should be connected by a patch cord to "Scope" or one of the Control Outputs. In larger studios it is possible to rewire the "Facilities" plug to accommodate the two inputs as well as the three outputs, and connect all five to a patch (see diagram).

Synthi Connecting Link and Facilities Plug

• PANEL CONTROLS: KNOBS
• Mean Rate alters the speed of the clock. Check mean speed set by timing with variance control at zero. Inoperative when mode switch to "inhibit"
Time Variance controls the degree of randomness of event rate, about the mean rate set. There are limits decided by the maximum and minimum possible clock rates, and the maximum time variance is achieved when mean rate is at 5 and time variance at 10, since this allows the whole clock range to be randomly selected This will not, incidentally, seem very 'fast', because there will be a content of very long time intervals
V1 and VZ Level The two independently controllable output voltages, occurring, however, at the same time. The mean about which the voltages vary can be set elsewhere by e.g. a joystick

• SWITCHES AND LAMP
• Reselect Button operates only in "inhibit' mode. For manual selection of the next random voltage, which will hold until a further selection is made
Mode Switch decides whether clock will run or not. Both 'free run' and ' inhibit' states can be influenced by external inputs
New Selection Lamp Locked to Trigger pulse. It glows whenever a new selection is made.

• JACKS
• V.Cont.Mean (SKA 20) Voltage control of mean rate. Affects rate of clock in 'free run' mode.(Input)
Select (SKA 18) operates both in 'inhibit' and 'free run' modes - see previous section (Input)

V1 and V2 (SKA 2 & 4) Sockets delivering output staircases to any desired control input (Output)
Trig (SKA 6) delivers pulses with each new selection to operate envelope shapers or gates of various kinds (Output)

Random Voltage Generator

• SUMMARY OF SPECIFICATION
• Power Supply 240 or 115 VAC 50/60Hz +10% to -23%
• Control Out Level +2.5V max (this maximum excursion of 5V can be offset to accommodate the control parameters of non-EMS products)
• Trigger Out +4V
• Mean Time Range 0.2Hz (5" per event) or slower, to 20Hz. (In some examples the clock stops when control at zero, but the slowest running position will still be 0.2Hz. or less).
• Time Variance Max. approx. 100:1 - i.e. full range of clock, provided mean rate is not set at or near one of its extremes
• Select Input +5V to operate and inhibit after one event. Returning to below +5V will restore either 'inhibit' or 'free run', whichever is set.
• V. Cont.Mean Input At mid settings of Mean Rate control, +0.5V will double the speed3 -0.5V will halve it - i.e. 0.5V/octave limited by possible clock range