Kirk Rader

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Ratcheting ​

A favorite trick of 70's and 80's electronic music in the style pioneered and epitomized by the likes of Klaus Schulze, Tangerine Dream, et al. involved a technque known as ratcheting. It depends on features of analog sequencers modeled on the one built into the venerable Moog System 55. That sequencer lives on in the form of the Behringer 960 eurorack module:

The 960's primary outputs are three control voltages, labeled A, B, and C, together with a low frequency rectangle wave representing its internal clock. The low frequency rectangle wave can be used as a gate trigger for other modules while the A, B, and C CV values can be used to control the frequency of VCO's, the amplitude of VCA's, etc. The three rows of potentiometer knobs taking up the majority of the 960's front panel are used to control the A, B, and C output CV values. The frequency range knob and frequency vernier potentiometer control the frequency of the LFO. When running, the 960 will cycle through up to eight CV values per A, B, and C row at the rate determined by the frequency of the internal LFO. The CV value at each step in the cycle is determined by the setting of the potentiometer in the corresponding column.

Consider a simple patch in which the CV output labeled A is controlling the frequency of a VCO and the LFO output is being used as the gate trigger of an envelope generator:

The result might sound like:

The tempo of the preceding "melody" is determined by the 960's LFO. The pitch of each note is determined by the value of the CV output from the A row being added to the base frequency of the VCO it is controlling.

Endless variations are possible thanks to the magic of modular synthesizers. You could add up to two more VCO's, controlled by the B and C outputs, so that the sequence consisted of a chord at each step rather than a solo note. Or one of the CV outputs could control the VCA, to vary the amplitude from note to note in the sequence along with the pitch.

Other things being equal, the 960's internal LFO emits a rectangle wave at a constant frequency, resulting in a metronome-like "rhythm." The 960 supports a mode (via a switch on the front panel) in which the C row's potentiometers control the frequency of the internal LFO on a step-by-step basis. This supports sequences where some notes are held longer than others to achieve musically interesting rhythms.

Even more complex rhythms can be achieved by extending all of the above using a technique known as ratcheting. Ratcheting adds another LFO to the patch, where the second LFO provides the gate triggers and one of the 960's CV outputs controls the fequency of the second LFO:

Note that the preceding requires a bit of fiddling with the relative frequencies and pulse widths of the 960's built-in "clock" oscillator and the second LFO that is used to generate the gate trigger for the envelope generator. It also requires the use a LFO module that provides a "trigger" or "sync" input for synchronizing the rectangle waves' phases.

When done correctly, the CV feeding the LFO (B in the preceding diagram) allows the potentiometers in that row to trigger different numbers of beats per step, at a much finer grained level than the built-in time adjustment supported by the C row. This is because the sequencer's CV output changes the frequency of the external LFO at each step and it is the external LFO that actually triggers the envelope generator.

Here is an actual patch that uses a Behringer 2600 as the sound source, with the sequencing provided by a Behrigner 960 sequencer and ratcheting via a Behinger 921 LFO (another module reproducing a component from the System 55):

which looks schematically like:

Notes:

  • None of the other modules in the 2600 nor the external eurorack case depicted in the preceding image are involved in this patch

  • The signal from the 921 LFO must be amplified a bit in order to function as a gate trigger for the 2600's envelope generators (this could be mitigated by a CV-to-trigger converter like the Behringer 961 module)

  • The melody of the chord progression is determined by the 960's A and B rows

  • The overall tempo of the chord progression is determined by the frequency of the 960's internal LFO

  • The number of times the chord plays at each step in the sequence is determined by the combination of the base frequency of the 921 oscillator and the value of the corresponding potentiometer in the 960's C row

This is what the preceding sounds like, where the base frequency of the 921 is increased and decreased at various points while the sequencer is running:

Even though the overall tempo of the chord progression remains constant, as determined by the 960's onboard "clock," each step is subdivided into differing numbers of repeated notes based on the setting of row C's potentiometers, scaled by the base frequency of the "outboard" LFO actually triggering the envelope generator.

As noted above, all of this requires some fine-tuning of the internal and external LFO's and the various C potentiometers to achieve musical sounding results. It also requires an external LFO that can be synchronized to the 960's clock using a gate (i.e. V-trigger) input. (The latter is why I used a 921 as the second LFO rather than any of the 2600's VCO's. The 2600's VCO's can be synced among themselves, but not to external waveforms.)