Stochastic Voltage Generator

  • 1979 SVG Stochastic Voltage Generator Buchla Marbles
  • 1979 SVG Stochastic Voltage Generator Buchla Marbles

Clock-synced random CV generator with
looping, CV recording, and quantization

Based on Marbles by Mutable Instruments, the SVG expands significantly on the randomization concepts that Don Buchla first introduced with the Model 266 "Source of Uncertainty" module in the 1970s. The SVG provides clock-synchronized random voltage generation for your Buchla-format modular system. Sequences of up to 64 steps can be locked and looped. External CVs can be sampled and quantized. Three random pulse outputs and four random CV outputs are provided. Additional features not present on Marbles, including extended loop lengths and reset capabilities, are now available.

Tech Specs

  • Three random pulse and four random CV outputs
  • Sync to internal or external clock source
  • External clock divider/multiplier options
  • Loop, record, and quantize internal/external CV sources
  • Up to six user-programmable CV quantization scales
  • New "reset" feature provides precise loop points
  • Maximum loop length upgraded to 64 steps

Quick Reference

Review the Marbles manual for information about the core functions of the SVG. There are several differences between Marbles and the SVG which are outlined below.

LED switches/modes. Marbles uses the front panel graphics colors to indicate various settings. On the SVG, the same settings are indicated by the color of the LED switch which is used to change these modes. Compared to the Marbles manual, the SVG labels switch [E] as T-OUT, switch [B] as TIME, switch [J] as VOLT, and switch [N] as X-OUT.

T section alternate modes. Tapping the T-OUT switch will cycle through three primary modes which dictate the correlated relationships of the T1/T3 outputs. The LED colors indicate which mode is active. Green is a "Bernoulli Gate" similar to the Branches module from Mutable Instruments where either T1 or T3 will be randomly selected in a mutually-exclusive way. Orange assigns random subdivisions or multiples of the master clock to T1/T3. Red mimics common kick/snare drum patterns. Each of the three primary modes also has a hidden alternate mode. These are not officially documented in the Marbles manual (receiving only an oblique mention in a forum post). To access an alternate mode, press the T-OUT switch for 3 seconds. The LED will blink slowly if the primary mode is active, or rapidly if the alternate mode is active. Tap the switch to toggle between primary/alternate modes for the current LED color, then wait 3 seconds to exit. The alternate modes are as follows. Green Alt eliminates the correlation between T1 and T3, providing two independent Bernoulli gates. Orange Alt provides a randomization of T1/T3 based on inverted ratios, where one output is a divisor of the clock rate (i.e. 1/4) and the other is a multiplier of the same rate (i.e. 4/1). Red Alt provides a Bernoulli gate which is similar to the primary green mode, in which the T1/T3 gate are correlated, but adds a "null" state in which neither the T1 or T3 outputs will generate a pulse.

Reset function. Marbles does not have a Reset jack so without complicated firmware hacks there is no way to reset a looped sequence to a specific starting point. The SVG adds a dedicated Reset jack for this purpose and also provides a method for manually setting Reset points. When a sequence is first locked (short press on either the T or X switches), no Reset point exists. When an initial Reset pulse is received at the Reset jack, this pulse will set the Reset point for the loop. Subsequent pulses will then reset the loop to that point. If you want to manually perform Reset-related actions without using the Reset jack, first press the X-OUT switch, then press either T or X. Pressing T will perform a manual reset. If no Reset point exists, this first tap will set the Reset point, after which point the loop may be manually reset to that point. Pressing X will set the current step of the loop as the new Reset point. Note that there's no visual indication of Reset-related events... use your ears while listening to a locked T/X loop. The new Reset point applies to both the Reset jack and manual reset events using the X-OUT + T shortcut. The new Reset point is erased when both T and X are no longer locked/looped.

Longer loop lengths. The maximum loop length on Marbles is limited to 16 steps. The SVG provides for lengths of up to 64 steps. Press the TIME switch for 3 seconds to access longer loop lengths. The color of the blinking LED indicates the maximum: green is 16 steps, orange is 32 steps, red is 64 steps. With the 16 step maximum engaged, available loop lengths are 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, and 16 steps. With a 32 step maximum: 1, 4, 6, 8, 10, 12, 14, 16, 20, 24, 28, and 32 steps. With a 64 step maximum: 1, 8, 12, 16, 20, 24, 28, 32, 40, 48, 56, and 64 steps. Note that these loop lengths are mostly ratios of common musical time signatures based on quarter notes such as 3/4, 4/4, 5/4, 7/4, etc. After 3 seconds, this menu will exit automatically and the new loop length setting will be saved. Adjust the Recall Loop Length slider to change the loop length. Note that moving the slider fully to the left (value of 1 step) will freeze the T/X outputs at their current states (this can also be automated by modulating the RECALL CV input).

Loop Superlock. This feature, described on the Mutable forum but not documented in the Marbles manual, adds the ability to prevent T/X loops from being changed by the RECALL knob/CV. For example this allows you to adjust the degree of randomness for the T outputs while leaving the X outputs unaffected (or vice versa). On the SVG you can "superlock" T and/or X by pressing the T and/or X switches for 3 seconds. The normal slow blink of these switches will become a faster blink to indicate that the T and/or X loop will not be affected by the Recall knob. To disengage the superlock and allow the RECALL knob/CV to affect the loop, press the T and/or X switches for 3 seconds again.

Virtual attenuators. The SVG (like Marbles) does not have physical CV attenuators. However the SVG (unlike Marbles) does have an alternate method of attenuating external CVs. Simply press the T switch (top left of the module) and then turn any of the seven parameter knobs. Turn the knob all the way to the left to fully attenuate (0% modulation) or all the way to the right to fully maximize (100% modulation). In between values are possible of course, for example turning a knob to its halfway point will attenuate the CV to about 50% of its maximum level. When you release the hold on the T switch, the module will remember any adjustments that were made and store them to memory. If you want to attenuate CVs as well as invert them for negative modulation, press the X switch (top right of the module) and then turn the parameter knobs. The center point of the knob will be the 0% point, turning fully to the right will maximize the CV, and turning fully to the left will both maximize and invert the CV. Adjustments are saved across power cycles so if a CV input does not seem to be working correctly, press the T switch and turn the associated knob fully to the right to remove any saved attenuation values.

External input. Marbles allows for sampling of external CVs, but does not have a dedicated input for this. Instead the SPREAD CV input must be used, preventing the modulation of SPREAD while sampling external CVs. To avoid this limitation, the SVG has a dedicated external CV input, allowing the SPREAD parameter to be modulated while external CVs are being sampled. Press the switch next to the EXT IN jack on the SVG to enable external CV sampling. The SPREAD CV jack will continue to work as it should.