A device that produces vibrato and chorus effects via phase modulation, by modulating the length of a very short delay line.  The canonical example is the electromechanical version found on many models of Hammond organs.  This consists of a series of allpass filter stages, implemented with simple inductor-capacitor circuits.  Each stage of the filter introduces a small amount of phase shift in the input signal.  To vary the resulting phase shift, the output circuit rapidly alternates between taps placed at various points in the line, going from shorter-length (fewer stages) to longer and then back.

In the Hammond, the output scanner consists of a mechanical rotor that is connected to the output.  The rotor has  a flat plate at one end.  As it rotates, the plate slides in between sets of stationary plates placed around the circumference of the rotor's circle.  Each set of stationary plates is connected to an output tap.  As the rotor slides in between the plates, it forms a capacitor with the stationary plates, allowing the delay line signal from the connected tap to pass to the output.  As the rotor moves from one plate set to the next, it gradually transitions to the next tap.  This provides a relatively noise-free and maintenance-free means of rapidly changing taps, thereby changing the amount of phase shift.


Demonstration of Hammond vibrato scanner rotor and capacitance plates.

In the Hammond, the generator's run motor spins the rotor continuously at about 7 Hz, and the continuously changing phase produces vibrato (variations in frequency) in the output.  A chorus effect is produced by blending the scanner output with some of the dry signal; the phase and frequency differences between the dry and scanner output signal produce the effect.  Some models have a complex multi-pole switch arrangement that allows the performer to vary the intensity of the effect by selecting taps so as to vary the distance between the shortest and longest taps of the delay line.

The vibrato scanner effect is currently emulated by some plug-ins.  Some modular synthesizer manufacturers have discussed both electromechanical and all-solid-state versions in hardware, but so far no commercial products have resulted.

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