Synthesizer Fundamentals: Bells

By: Dane

While many timbres, like those which come from bowed strings and woodwinds, are readily recreated with synthesizers, percussive instruments are some of the most tricky to create convincing emulations of. Piano and harpsichord are most easily dealt with through sampled instruments, but the enharmonic qualities of bells and xylophone instruments are relatively easier to create. In this context, enharmonic refers to the relative balance of harmonics in a given audio source being unrelated to the fundamental frequency of the instrument. Most instruments have harmonically related overtones (such as 2nd order) which add to their perceived harmonic richness and distinguish them more readily from other instruments. Yet even if this is true of the majority of melodically based instruments, many percussion instruments defy that rule.

With these ideas in mind, let us begin creating a bell or xylophone-type instrument with a classic analog synth palette in mind. Here the oscillator functions are more important than in many other types of sounds, precisely because of the enharmonic balance of frequencies so common to acoustic tuned percussion instruments. To begin with, set the first oscillator to a high octave, using a square (or, if too bright, a triangle) waveform. This should be paired with a second oscillator, which is detuned to an enharmonic interval, preferably the major 6th or another note outside of the normal harmonic spectrum. It is important to mix the tonic oscillator relatively higher than the detuned (2nd) oscillator so as to preserve the psychoacoustic effect of the synthesizer on a listener's ears. This is also greatly improved by the ability to modulate one oscillator with another (also called frequency modulation) or the ability to ring modulate one oscillator against another (which takes both oscillators and outputs sum and difference frequencies of their respective pitches).

Most software-based analog emulations and many analog synths have this feature, and it is usually pretty easy to implement. Usually there is a switch which turns ring modulation on, and then the resulting (generally clangorous) tone can be modified further. If the ring modulation has an amount function, try using just a bit of modulation on the detuned oscillators, remembering that the oscillator with the pitch that tracks properly across the keyboard should be a lot louder than the enharmonic oscillator. In both ring and frequency modulation, the oscillators are generally referred to as carriers or modulators, depending on whether they are acting as an audible sound source or as an instrument for altering the timbre. If the tone is too clangorous or it is too difficult to discern a fundamental pitch, back off of the modulation or try simple detuning as mentioned above. Frequency modulation generally sounds somewhat different from ring modulation, but its effects for bell-type sounds are still quite useful.

While the filter is very important for most emulative sounds, especially for adding character, in xylophone sounds the harmonic content stretches well above the range of human hearing. Therefore, the traditional low-pass filter found on many analog instruments should not be affecting the sound. A high-pass filter, on the other hand, can be very useful for clearing away low enharmonic overtones which may obscure or distract from the fundamental pitch. If only a low-pass filter is available, a similar mechanism still exists. By simply increasing the resonance somewhat but leaving the cutoff frequency at its highest point, a relative drop in all frequencies beneath the cutoff still occurs, which in effect sounds like a high-end boost with an equalizer. Not the most sophisticated audio trick, but certainly useful in a pinch.

The last element of bell sounds is the envelope, which is relatively straightforward: zero attack, quick to moderate decay, no sustain, and small-to medium release. Here your preferences should take precedence, as each person's playing style, etc. will differ. Patch in a little reverb or delay and have fun!

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