The ED151 voltage controlled quadrature LFO and is a special module that was not found on many modular synthesizers back in 1970s. Mostly, only companies like Polyfusion, Buchla or Serge would propose such a weird LFO.

Basically, it is a sinewave LFO with four outputs which are running at the same rate but are phase shifted by increment of 90°. That is, the first output (red) has a phase shift of 0°, the second output (yellow) has a phase shift of 90°, the third output (orange) has a phase shift of 180° and the fourth output (green) has a phase shift of 270°.

The oscillator has a basic range from longer than 15 seconds per cycle to over 700Hz and can be used to create many effects. Swirling a sound at an audio rate produces interesting spatial and modultive effects.

Additional features include linear VCA's for each of the outputs, so that spatial swirls can be made to decreasein size, effectively spiralling inward. A [HOLD' input 'freezes' the outputs whenever pulsed high. Thus a swirl can be stopped at a given location,with the sound just 'hanging there' until operation is resumed. Switches on the panel allowing either or both of the oscillator outputs to be disabled, a feature which has been found to be useful for live performance.

What is it useful for? Mostly it is for modulating simultaneously up to four sources at the same rate but out of phase. This makes it possible to achieve so called "Barber pole" effects or "Shepard tones". For example, imagine that you connect each output of the Quad-LFO to the V/Oct input of four different VCOs, then you'll hear continuous though steady (paradoxical, ain't it!) rising and falling intermangled pitches (see Shepard Tones and Tritone Paradox).

It can be used also to drive four VC-panners to create rotating sound effects that fill the stereo or quadraphonic space.

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