Archives - 1977

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Contemporary Keyboard

January 1977
Modulation, Part III: Clangorous Sounds
Bob Moog

Clangorous sounds are periodic (pitched) sounds that have some harmonics whose frequencies are not whole-number multiples of that of the perceived pitch. Gong, drum, chime, and "ring modulator" tones are examples of clangorous sounds.
I mentioned in my September/October, 1976, column that all types of modulation produce sum and difference tones (sidebands). If the modulating frequency is in the audio range, the modulated signal contains sidebands that you hear as clangorous harmonics. The difference between frequency modulation and most other types of modulation is that frequency modulation produces many more sum and difference tones over a wider portion of the spectrum. For instance, only one sum and one difference tone are produced when one sine wave amplitude-modulates another sine wave.
However, when one sine wave frequency-modulates another, dozens of audible sidebands may be produced. Musically, frequency-modulated tones sound thicker, less pitched, more raunchy than their amplitude-modulated counterparts. This makes them particularly suitable for use as raw material in synthesizing drum, gong, and similar sounds.
The basic patch for clangorous sounds is shown below. It is the same as the patch for vibrato shown last month, except that the modulating oscillator is now oscillating in the audio frequency range, and is voltage-controlled by the same keyboard (or other pitch controller) as the tone oscillator. The modulating oscillator therefore "tracks" the tone oscillator. That is, the musical interval between the modulating and tone oscillators remains constant as the pitch control signal changes. This results in a frequency-modulated tone whose sidebands maintain their relationships to one another, and therefore whose tone color maintains a consistency as the tone's pitch is varied.
If the modulating oscillator were not voltage-controlled, the sum and difference tones would move in opposite directions as the tone oscillator's pitch was changed. This produces a characteristic counter-motion within the tone, commonly called ring-modulated sound. While it has considerable musical interest, this type of sound motion will not be discussed now.
To synthesize a gong-like sound, first listen to the modulating oscillators together. That is, mix the modulation and tone oscillator outputs without using any modulation. Set the modulating oscillator pitch a minor sixth below that of the tone oscillator. Select sine or triangular waveform for both oscillators, or filter most of the high-frequency content out of the sound. Now turn off the signal path that feeds the modulating oscillator's signal directly into the audio path. Play a note around middle C on the keyboard and turn up the modulation amount control. You will notice several things. First, new frequency components will be introduced into the sound. These are the sidebands, the sum and difference tones. As you turn the modulation amount control up, the sidebands will actually become stronger than the "natural" pitch of the tone oscillator. Second, the overall pitch of the modulated tone will go up as the amount of modulation increases, this has to do with the technical means for achieving frequency modulation that most synthesizers employ. Both this actual pitch shift and the obscuring of the natural (unmodulated) pitch by the sidebands will reduce the keyboard's pitch accuracy. When you strike middle C, don't expect to hear a gong sound with a perceived pitch of middle C!

The third thing you will notice is that fine-tuning the modulating oscillator has a strong effect on the tone color. To get a convincing gong sound, the beats between the sidebands and the natural pitch have to be very slow. Fourth, changing the waveform of the oscillator changes little more than the brightness. Ditto for the waveform of the modulating oscillators, except that some modulating waveforms will shift pitch more than others.
The bass drum is another clangorous sound that's fun to synthesize. A second tone oscillator is helpful but not necessary. The first tone oscillator should be played about two octaves below middle C, the second oscillator about a whole tone below the first, and the modulating oscillator a whole tone below the second tone. Use sawtooth on all three oscillators and feed all oscillators into the audio path if possible. Use envelopes (contours) with sharp attacks and rapid decays for both filter and loudness. If your synthesizer has a multimode filter, use the low pass mode. Adjust the filter cutoff frequency for a drum-like quality. Now turn up the modulation amount control. Notice how the modulation sidebands seem to "fill in" around the natural oscillator pitches, making the sound thicker and suggesting a rippling timpani head. If your synthesizer has provision for noise modulation, mix in some pink noise to modulate the tone oscillators. This will broaden all of the frequency components of the resultant tone, and suggest the sound of a Brobdingnagian bass drum.
Lest you think that all fast-frequency-modulated sounds are clangorous or percussive, may I suggest that you try a couple of pure-pitched sustained sounds? For a super-trumpet, set the tone oscillator for narrow rectangular or sawtooth waveform, the modulating frequency nearly in unison with the tone oscillator, and the modulating waveform sine or triangular. Turn up the modulation amount control and fine-tune the modulating oscillator for just the slightest beat. Set your filter for tasteful blare. The unison modulation produces sidebands that are very slightly out of tune with the natural tone, giving the resultant tone a rolling, choral quality.
For a good fat sound in the low register, set the tone and modulating oscillator waveforms the same as above, and the modulating frequency a fifth above the tone oscillator. Turn up the modulation amount control and filter to taste. Placing the modulating oscillator a fifth above the tone oscillator (a 3:2 frequency ratio) generates a subharmonic an octave below the natural pitch, and strengthens many of the higher harmonics. This, together with the slow beating, contributes to the tone's lush quality.
More on modulation in my next column.