The Sid Filter - Commodore 128 System Manual

The SID Filter

Once you have selected the ENVELOPE, ADSR, VOLume and
TEMPO, use the FILTER to perfect your synthesized sounds, ln
your program, the FILTER statement will precede the PLAY
statement. First you should become comfortable with generating
the sound and worry about FILTERing last. Since the SID chip
has only one filter, it applies to all three voices. Your
computerized tunes will play without FILTERing, but to take full
advantage of your music synthesizer, use the FILTER statement
to increase the sharpness and quality of the sound.
ln the first paragraph of this section, The Characteristics of
Sound, we defined a sound as a sound wave traveling
(oscillating) through the air at a 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 frequency and
accompanying harmonics, which are multiples of the overall
frequency. See Figure 7-2. The accompanying harmonics give
the 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
waveform 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
discussed in the next section. Low-pass filters out frequencies
above a certain level you specify, called the cutoff frequency.
The cutoff frequency is the dividing line that marks the boundary
of which frequency level will be played and which will not. ln 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.
7-20