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CGS505
Universal Slope Generator + Transient Generator
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The Universal Slope Generator (USG) is the ultimate patch-programmable control voltage generator in the 4U Serge system. At least two are recommended for almost every 4U Serge system, and in most cases, a number of these are desirable. People familiar with the previous series of "slewing" modules know about the importance of this kind of function in a large patchable synthesizer system.

The uses of this module are numerous, some duplicating functions found on other synthesizers, some totally unique to the 4U Serge system. Most systems require a number of control voltage generators to control the various signal processors and modifiers. Sources of trigger pulses (clocks), control voltage processors (portamento's), regular repeating voltages (LFO's) are standard synthesizer requirements. The USG is the main module providing these functions in the 4U Serge system.

The USG is a unity gain voltage follower with voltage controllable slopes. The range of control is exponential, extremely wide, and the Rise and Fall times can be. controlled independently. The range of the USG is from sub-sonic to high audio frequency. Trigger inputs and trigger outputs allows the USG to function as a transient (envelope) generator, pulse delay, or, in a "cycling mode" (to produce an LFO). Since the Rise and Fall times are voltage controllable, this adds another dimension to the above functions.

Two VC inputs are available:-

  1. [1V/OCTAVE] is calibrated to 1 volt per octave (within about 3'/.), and
  2. [VC] is fully adjustable in the negative and positive direction. This input can control either the positive slope (RISE), negative slope (FALL), or (BOTH).

The linearity and accuracy of the slewing amplifiers allows them to be used in the most exacting applications, such as processing the output of a keyboard or sequencer to produce portamento functions. Note that this portamento function has a separate rate control for rising notes and falling notes, an interesting portamento effect.

The USG can be used as a simple envelope generator, a low frequency oscillator, pulse generator, or in a variety of other applications.

As an envelope generator, the unit can be triggered in two different ways:

  1. Connect a trigger pulse to the [TRIGGER] jack. When a pulse is applied here, an envelope defined by the [RISE] and [FALL] knobs will be produced which goes from 0 to +5 volts. If a second trigger is received before the envelope has finished, it will not re-initiate the envelope. Using a pulse train into this input, the USG can be used as a frequency divider, or sub-harmonic generator. A waveshape and a pulse from the [END] jack will, be produced for each pulse applied to the [TRIGGER] jack as long as the total envelope time is shorter than the pulse period.

    If the envelope time is slightly longer than the pulse period, then the USG wilI only be triggered on alternate pulses, producing a division of two. If the envelope is slightly longer than two pulse periods, then it will only be triggered on every third pulse, producing a division by three, and so on.

  2. With a gate signal into the [IN] jack, an envelope will be produced which begins to rise at a rate set by the [RISE] knob to a level equal to the gate level. The level will remain at this level as long as the gate is present: an envelope with sustain.

    When the gate level drops back to zero at its end, the envelope will fall at the rate set by the [FALL] knob. If the gate level rises before the end of the [FALL] cycle, the output will rise again, rising toward the gate level, at a rate set by the [RATE] knob. Multiple gate signals will re-initiate the envelope, even if the envelope has not completed its cycle back to zero volts.

    A positive signal applied to the [IN] jack will always over-ride any trigger at the [TRIGGER] jack.

The USG can be used as a slew limiting processor to change discrete voltage steps into gliding voltages (portamento). Voltages from a keyboard, sequencer, or other sources can be applied to the [IN] jack, and the [RISE] and [FALL] knobs will now determine the rate of glide in the positive and negative direction, independently.

The slopes from the USG are linear (equal voltages per unit of time), but they can be altered using feedback. If the OUTPUT is patched to the [VC IN] jack, then the slope can be given an exponential or a logarithmic shape determined by the amount of feedback set by the processing knob. Since both the [RISE] and [FALL] can be switched to be controlled separately or together, the slope of either or both can be shaped using this technique. This is useful for producing slow, gradual amplitude changes with the Equal Power VCA modules.

If the [TRIGGER] jack is connected to the [END] jack, the DUSG will oscillate with a waveform and frequency set by the [RISE] and [FALL] knobs. A series of pulses will appear at the [END] jack, and the duty cycle (the time the pulse is high compared to when it is low in each cycle) is set by the [RISE] and [FALL] knobs. The [FALL] knob determine how long the pulse is low. When the USG is in the [RISE] part of the cycle or when the output is zero or less, the output of the [END] will be high. In some applications, a pulse with a very long duty cycle will cause erratic triggering in other modules. If such a symptom occurs, try increasing the [FALL] time and decreasing the [RISE] time to get the same pulse rate.

The USG may be used as a positive peak follower by setting the [RISE] time to minimum (full CW) and applying an audio signal to the [IN] jack. Adjust the [FALL] knob for a compromise between response time and the best filtering of the audio component at the DUSG output. If the [FALL] time is turned to minimum, and the [RISE] knob adjusted for optimum response time and filtering, then the unit will function as an envelope, follower-producing a negative envelope corresponding to the negative peaks ofthe input audio signal.

The Transient Generator with voltage controllable rise/fall times. It is a scaled down version of the USG and provides some of the basic functions that can be patched on the USG but occupying less panel space in your system and freeing up the CGS514 for other functions.

Common uses for the Transient Generator are:-

  1. As a simple 'AR' type envelope generation: apply a trigger to the [IN] jack to initiate an 'AR' envelope with a variable shape from 'slope' through 'triangle' to 'ramp'
  2. As a voltage-controlled low-frequency oscillator: With the [IN] jack unpatched the Transient Generator will cycle continuously, The [DC] jack gives you a DC output (0V to +5V),
  3. As a voltage-controlled clock (especially suitable for clocking the CGS359 Sequencer Programmer and the CGS86 Touch Sequencer). With the [IN] jack unpatched the CGS359 will cycle continuously. The [END] jack provides a LOGIC clock pulse (0V to +5V) with a variable duty-cycle based on the [FALL] and [RISE] settings.

When in 'oscillator' mode the frequency range extends from a high around 2.5kHz to a cycle-period in excess of 4 minutes.

PCB Dimensions: 2" x 6" with 3.2mm fixing points 0.15" from the edges
Power Consumption: +12V @ -12V @

Original article, art & design copyright by Ken Stone

Email:  elby-designs@bigpond.com
© Copyright 2000. All rights reserved.     Revised: October 5, 2021