A TECHNICAL VIEWPOINT

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Tremolo and vibrato sound much the same. And to tell the truth, while some theoreticians will spend considerable time discriminating between the two, when referred to an electronic "signal" as seen on an oscilloscope before it reaches the loudspeaker and in terms of propagated waves (the kind we listen to and call sound) there is little absolute difference between the forms.

This can be shown by a frequency analysis of the two forms. Academically, tremolo is a fluctuation of amplitude applied to a sound of constant frequency. In traditional instruments it may be achieved by opening and closing a shutter that attenuates the sound passing through it. The old home harmonium of a few generations ago did this, with the name "vox humana" (not to be confused with the true stop of that name on professional organs). Vibrato is a variation of pitch, a fluctuation of frequency, applied to a sound of constant amplitude. In traditional instruments, the best known example is the way a guitarist achieves the effect by trembling his hand so his finger rolls the string at the point where it touches the fret board.

In an electronic organ, as well as introducing vibrato by changing the oscillator frequency, it can be achieved by moving the loudspeaker assembly or by using a baffle that has the same effect by changing the distance the sound wave has to travel to escape into the room. This kind of speaker is commonly called a "Leslie" after the first one of its kind to achieve the effect.

Suppose a frequency is modulated in one or the other of these ways, electrically or electronically, at a rate of 6 Hz, which makes a good vibrato or tremolo rate. Say the frequency is a concert pitch A of 440 Hz. The pure tremolo, or flactuating amplitude, (such as found in "Fender" amps) will add frequencies of 434 and 446 Hz, in equal proportions and phased so that together they alternately add to and subtract from the steady 440 Hz waveform, without shifting the phase of the resultant wave. So the frequency content of a concert A with tremolo is a dominant 440 Hz with smaller (and equal) amounts of 434 and 446 Hz.

Now assume the frequency is shifted at the same rate, in what is called vibrato. Frequency or pitch shift can also be interpreted as phase shift. When frequency is raised, phase advances; when frequency is lowered, phase delays. They are not the same but are related. They can be reguarded only in interchangeable terms so long as frequency is repetitively changing as happens in vibrato.

If the same frequency, 440 Hz, is alternately advanced and retarded in phase at a rate so that the complete variation is repeated six times a second, the frequency will wobble above and below 440 Hz while maintaining that average value. How far above or below 440 Hz the frequency swings will depend on how far the phase shifts from its median value in each one-twenty-fourth of a second (a guadrant of the 6-Hz variation).

Regardless of how far the frequency shifts, the fact that its rate or repetition frequency is a 6 Hz means that the phase movement repeats at that rate. Consequently, the most dominant effect in an analysis of the frequency content will be the same two frequencies produced by the tremolo but in different phase relationship.

To keep the amplitude constant, if more than a few degrees of phase change occur other sideband frequencies such as 428 and 452 Hz will have to be added. But the dominant ones will be still be 434 and 446 Hz. And in a practical situation, a warbling tone will seldom fluctuate only its amplitude or only its frequency without affecting the other.

Even when the tone generated is produced by pure frequency or pure amplitude variation, the acoustic wave it produces has to travel round a room with real dimensions before we hear it. Thus the wave delivered to the loudspeaker may be pure tremolo, for example, but before it has left the diaphragm very far, it will have components of vibrato in it as well because of the phase shifts that occur as the various component-frequency waves travel across the room, or vice versa.

Admittedly, vibrato can give a richer sound than tremolo, for a reason that should be fairly obvious from the foregoing discussion. In generating tremolo the only extra frequencies added are a single pair of sidebands. A deep vibrato can add quite a number more frequencies. But a shallow vibrato produces essentially the same frequencies as tremolo.

Where the tone being modulated has a relatively high frequency, a vibrato will cause the phase to "go round" a few times at least, compared to a wave of the same steady frequency. This means that tremolo cannot possibly have as many extra frequencies, although the fluctuation might otherwise be as deep. But where the tone being modulated is low in frequency, as in the bass notes, the phase shift corresponding to vibrato will be relatively small because there will not be enough cycles above or below the normal frequency to allow phase to shift very far. This means that for low notes tremolo and vibrato become virtually indistinguishable.

You might be able to tell the difference by looking at the waveform on an oscilloscope screen before it goes to the loudspeaker, but if you pick up the same sound with a microphone and put it back on the oscilloscope screen you would have virtually no means of knowing how it started.

From the musical viewpoint, a violin can be played vibrato, and it is not impossible to play a guitar that way. However, in order to get a pure rhythmic vibrato sound from a guitar - without getting hand cramps or using a whammy bar, it must be done electronically.

Courtesy of N.Crowhurst, Electronic Musical Instruments © 1977