The quadrature LFO 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 has a shift of 0°, the second 90°, the third 180° and the fourth 270°. 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 ,).
It can be used also to drive four VC-panners to create rotating sound effects that fill the stereo space. The core of the circuit is based on the quadrature oscillator circuit of Ian Fritz’s ChaQuO chaos oscillator . I modified it in order to have voltage control of the frequency and also I added buffers with output LED indicators and a toggle switch that makes it possible to exchange the phase of the 90° and 270° signals in order to change the direction of the phase rotation. This simple trick is very handy when performing “barber pole” modulations. Each LED show the positive part of the corresponding signal, they are organized on a circle to visualize the phase differences between the signals.
In this video:
Music from my modular synthesizer.
Featuring the Yusynth Voltage Controlled Quadrature LFO module. The colored lights are on the module and the phosphor green “fireflies” are from my BK Precision 2120 oscilloscope. I tapped into the 90 deg and 180 deg outputs and switched them with a Q149 Signal Selector module then sent them on to the scope channel 1. Channel 2 picked up the 270 degree output. I then fiddled with ground, DC and AC readings as well as the divisions voltage readings on the scope.
Video edited and enhanced with Final Cut. Audio of three tracks mixed and mastered in Cubase 4.