Shepard Moon

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Shepard Moon is a module which forms the heart of 'Barber Pole' effects. These 'auditory illusions' give the impression of modulations increasing or decreasing in an endless fashion.

The most commonly encountered barber pole effect is the 'Shepard Tone', which sounds like it is constantly ascending (or descending) in pitch, without ever reaching a terminal point.

Barber Pole effects traditionally require a huge number of modules to implement due to the number of replicated signal chains, and an array of co-ordinated modulation sources. Shepard Moon takes the drudgery out of this by providing all the modulation sources AND performing the audio cross-fade mixing of all the audio signal chains. So, a six-channel Shepard Tone generator can be created with just a single Shepard Moon module and six stock CA Oscillators. Similarly, you can create barber pole filters, phasers, flangers, etc by using one Shepard Moon module and up to six instances of the processor you wish to make 'infinite'.

There are eight channels in total, each of which provides a control voltage output and an audio return input. Each channel has a fixed 'angle of rotation', which governs the phase of the control voltage output and the audio level of that channel in the final mix. The angles have been chosen to allow equal spacing between channels when using 2, 3, 4 or 6 parallel signal paths:

  • 2 signal paths: 0 degrees and 180 degrees.
  • 3 signal paths: 0 degrees, 120 degrees and 240 degrees.
  • 4 signal paths: 0 degrees, 90 degrees, 180 degrees and 270 degrees.
  • 6 signal paths: 0 degrees, 60 degrees, 120 degrees, 180 degrees, 240 degrees and 270 degrees.

Six signal paths is the 'sweet spot' between achieving a seamless effect, and avoiding a proliferation of signal paths. However, it is possible to extend Shepard Moon to eight, twelve or more signal paths by running two (or more) in parallel and using the Reset Angle knob to offset the Reset pulse by a fixed phase. For example, you can offset the reset pulse to the second Shepard Moon by one eighth of a rotation (45 degrees). This would allow you to use the 0, 90, 180 and 270 degree channels of the second Shepard Moon as 45, 135, 225 and 315 degree channels respectively.

As well as allowing you to create Barber Pole effects, you can use Shepard Moon as an 'audio sequencer'. You can feed completely separate audio signals into any of the 'angles of rotation' around the circumference of the Moon, and the module will smoothly cross-fade between them. You can use multiple Shepard Moons to create interesting polyrhythmic and polymetric patterns by running them at different rates, feeding audio into different angles of rotation, etc. The Reset Input and Reset Angle Knob allow you to synchronize multiple modules together, trigger the audio sequencer on each key press, and other creative applications.

A Preset Pack is available free-of-charge, as well as a comprehensive User Guide.

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