Appendix
196
Iridium Manual
symmetrical harmonic content while all other pulse widths
create peaks or troughs at certain frequencies. Another
special case is a pulse wave with a very narrow pulse
width – labelled as < 1% in the previous image. An infini-
tely thin pulse creates a spectrum that has all harmonics
with equal magnitudes. In a digital synthesizer, ‘infinitely’
necessarily means one sample.
The pulse wave is an artificial wave, which means that it
doesn’t occur in nature. It was built into synthesizers be-
cause it could create a lot of different timbres with a mini-
mum of technical effort. However, certain pulse widths
sound very close to the timbres of acoustic (or semi-
acoustic) instruments – i.e. guitar or bass guitar, e-piano,
or even a flute.
The most powerful feature of the pulse wave is the ability
to change its width while sounding. This is called pulse
width modulation. When the pulse width is changed the
waveform starts to sound thicker. This happens because
the effect is very similar to what you hear when you have
two oscillators running with slightly different frequencies.
They interfere and create irregular troughs in the resulting
waveforms.
The Triangle Wave
The Triangle Wave is very similar to the square wave. It is
composed of the same harmonics as the square wave but
with different magnitude ratios. The magnitude of each
harmonic is divided by the power of its number. This me-
ans that the third harmonic’s magnitude is a ninth, the fifth
harmonic is a twenty-fifth and so on. The following image
illustrates the harmonic content:
