Commodore 128D - Page 156

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filter, it applies to all three voices. Your comput
erized tunes will play without FILTERing, but to { -j
take full advantage of your music synthesizer, i—>
use the FILTER statement to increase the sharp
ness and quality of the sound. { j
\ )
In the first paragraph of this section, The Char
acteristics of Sound, we defined a sound as a - -,
wave traveling (oscillating) through the air at a ( I
particular rate. The rate at which a sound wave
oscillates is called the wave's frequency. Recall . -
that a sound wave is made up of an overall f re- 1 )
quency and accompanying harmonics, which
are multiples of the overall frequency See Fig- - -
ure 7-2. The accompanying harmonics give the ( i
sound its timbre, the qualities of the sound
which are determined by the waveform. The -
filter within the SID chip gives you the ability to \ \
accent and eliminate the harmonics of a wave
form and change its timbre.
The SID chip filters sounds in three ways: low-
pass, band-pass and high-pass filtering. These
filtering methods are additive, meaning you can
use more than one filter at a time. This is dis
cussed in the next section. Low-pass filters out
frequencies above a certain level you specify,
called the cutoff frequency. The cutoff fre
quency is the dividing line that marks the bound
ary of which frequency level will be played and
which will not. In low-pass filtering, the SID chip
plays all frequencies below the cutoff frequency
and filters out the frequencies above it. As the
name implies, the low frequencies are allowed
to pass through the filter and the high ones are
not. The low-pass filter produces full, solid
sounds. See Figure 7-7.
148 USING C128 MODE—Sound and Music in C128 Mode