Leslie had devised a marvelous apparatus capable of imposing vibrato on sound; however his rotating speaker system was mechanical and this made it very bulky, to say the least. An electronic apparatus might be more compact and easily portable, but at the time it was not at all obvious how electronic vibrato could be achieved. What was needed was some kind of medium to capture the audio signal, so it could be manipulated, that is, slowed down or sped it up to create pitch-shifting vibrato. Magnetic tape hadn’t been invented, and even primitive wire recorders weren’t readily available. Electronic vibrato seemed an impossibility with 1930s technology… to the average electronics engineer.
It was going to take a genius to crack the problem. Enter John M. Hanert. Hanert certainly was not your average engineer. He held over 60 patents to his name and was head of research at the Hammond Organ Inc in Chicago; he was the brain behind the Hammond B-3 “tone-wheel” organ and inventor of the Hammond “Novachord“, an electronic synthesiser created decades before Robert Moog even conceived of such a thing; and Hanert possessed the imagination and intellect to overcome the severe technological limitations of his day to make electronic vibrato a reality.
Hanert’s idea was to employ an electrical “transmission line” as a medium to create the vibrato effect [US Patent 2,382,413]. A transmission line is just fancy terminology engineers use to describe long cables used for radio frequency or speech transmission, for instance a telephone line. Like a guitar cable, a transmission line is not a perfect conductor of electrical signals. What Hanert did was to exploit its imperfection, more specifically, its non-ideal electrical properties, in a new and novel way.
I’m sure guitarists will be familiar with “tone-sucking“, the loss of high frequency that occurs, when using certain types of instrument cables. Tone-sucking is caused by the capacitance of the insulation surrounding the inner core of the cable. The capacitance shunts higher frequencies to ground, effectively creating a low-pass filter. The longer the cable, the greater its capacitance becomes, and the greater the loss of high frequency content.
But there’s more to it than that, a lot more. The capacitance, and inductance, of a cable also alter the phase relationship of different frequencies within a signal during transmission. This phase change, a.k.a. “phase distortion”, distorts the shape of the signal’s waveform. Phase distortion, like “clipping distortion” can seriously degrade the fidelity a signal to mangle its shape in quite bizarre, and unexpected, ways.
Phase delay distortion imposed severe limitations on the speed at which data could be transmitted over long distance telegraph lines, until inventor Michael Idvorsky Pupin figured out what was happening to cause it. Pupin went on to develop “loading coils” [US Patent 519,346] to correct and minimise phase distortion in a transmission line, and this enabled faster data transmission.
“Every electrical circuit behaves, in consequence of its self-inductance and capacity toward a periodically varying electromotive force, just as a heavy elastic body, in consequence of its inertia and elasticity, behaves toward a periodically varying disturbance.”—Michael Idvorsky Pupin
Pupin understood that, unlike resistors, which limit the flow of electrical current, capacitors and inductors are “reactive”, that is, they don’t just limit current flow, they push back against it. This is because they’re converting one form of energy to another: capacitors store energy in an electric field; and inductors store energy in a magnetic field. This energy conversion is not instantaneous, it takes time for the fields to build and collapse. I’m aware this is beginning to sound very much like a school physics lesson, but stay with me on this a little longer; it will be worth it!
Think of reactance as a kind electrical sluggishness—a common analogy is to equate electrical reactance with physical mass: the heavier an object is, the more effort required to get it moving. Imagine a steam train getting underway; it takes a lot of energy to get up to full speed. What’s actually happening is that potential energy, from burning wood or coal, is being converted in kinetic energy. Once the train attains a constant speed, it’s kinetic energy is no longer increasing and wood only needs to be burnt to overcome wind resistance and the frictional losses of the railway tracks.
Now, staying with the physical world, rather than the electrical, let’s expand on Pupin’s idea of a “periodically varying disturbance” in an “elastic body”. Think of a speaker cone moving back and forth. As it does so it causes the air to stretch and contract like an elastic band. This results in regions of high and low density; that’s what sound is: pressure variations propagating through the air. Technically, the air is the transmission medium for sound. When the air is maximally stretched or contracted (compressed) it’s stationary and only has potential energy; and when it’s halfway between these two extremes, it’s moving at greatest velocity and all its energy kinetic. In an oscillating system like this, a clock pendulum, or a guitar string there’s an ongoing exchange or conversion between potential and kinetic energy.
An electrical transmission line is analogous to air. Both are transmission mediums and both possess this sluggishness and elasticity when transmitting waves, whether as electricity or sound. And it’s this sluggishness and elasticity that results in a phase delay. And it’s this phase delay that Hanert exploited to create his vibrato. This is an early example of audio “signal processing“