Most of us have experienced the ear-shattering feedback that can happen while using a PA system. Feedback makes the amplifier act like an oscillator, creating sound. In this case the feedback path is acoustic, or airborne. The principle of regenerative feedback used in an oscillator is electrical, but it works similarly.
In nature, soundmakers don’t ring forever. If you ring a bell, or set a pendulum in motion, neither will continue indefinitely. Only if you add energy will the object remain in motion. Consider how you must add this energy—not by opposing the motion, but by acting with it.
The principle holds for an electronic oscillator as well. The “tank circuit” introduced in last month’s column can produce damped oscillations, or sound that will die out. But if you add energy at the right time—i.e., in phase—the oscillations will be undamped, or continuous. This principle is called regenerative feedback.
Lee de Forest was granted the first patent for an oscillator using this regenerative feedback principle in 1915. His patent diagram (above) shows a grid-filament tank circuit with capacitor (12) and coil (10). An associated plate-filament circuit used a “tickler” coil (11) to provide regenerative feedback to the tank circuit, creating undamped oscillations (continuous sound). This oscillator could be keyed using switches which introduced one of several resistance “leak paths” (17–21) connected between the grid and the filament of the triode tube (1). The pitch produced depended on the values of the components in the circuit. In the patent specifications de Forest said that the simplest way to produce a scale was to make lighter or heavier marks with a graphite pencil used to form the resistance paths. Doodle your own keyboard, complete with unequal scale steps if you like! He also indicated that by gradually varying the capacitor in the tank circuit, a siren note of continuous pitch could be obtained. Shades of synthesizer glide! He wrote in a magazine article:
The pitch of the notes is very easily regulated by changing the capacity or the inductance in the circuits, which can be easily effected by a sliding contact or simply by turning the knob of the condenser [capacitor]. In fact, the pitch of the notes can be changed by merely putting the finger on certain parts of the circuit or even by holding the hand close to parts of the circuit. In this way very weird and beautiful effects can be easily obtained.
Several of the early electronic musical instruments were “played” by turning a knob to change pitch. De Forest’s comments also anticipate the “space-controlled” instruments of Leon Theremin, which we will see in later columns.
De Forest found that the timbre of the tone could be altered to create imitations of instruments and “…other sounds, which while pleasing to the ear were quite unlike those emitted from any musical instrument with which we are familiar.”
De Forest built a rudimentary instrument which used a single triode tube for each octave, with an arrangement of keys that allowed a single tone per octave. The output of these triode tubes was run to a set of speakers that could be placed around the room, giving a spatial dimension to the sound. (Remember, this is 1915!) De Forest was very excited about his discovery of the “music of the lamps.” He wrote:
In all my work with the audion [triode tube]—and I can imagine no device in the whole realm of physics of greater fascination than this little audion principle—I have never found any phase of its unlimited possibilities quite so interesting as this of producing musical notes. In the next twelve months I hope to be able to produce an instrument which will be far enough perfected so that I can turn it over to musicians to work out the thousand and one details of musical perfection which such men alone are capable of introducing.
In December 1915, de Forest described the Audion Piano, an instrument that used the principles he had discovered. This scheme proposed having a separate tone-generating circuit for each key on the keyboard. It’s not certain if the instrument was actually built, due to difficulties in the construction of enough efficient and stable triode tubes.
Lee de Forest’s role in the development of electronic musical instruments is very important. His development of the triode tube underlies amplification and made the oscillator possible. As we shall see in ensuing columns, we owe him much.
JULY 1978
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