Commodore 128 Programmer's Reference Manual page 358

348 COMMODORE 128
automatically by the Commodore 128. Here are the preselected envelopes for different
types of musical instruments:
Piano
Accordion
Calliope
Drum
Flute
Guitar
Harpsichord
Organ
Trumpet
Xylophone
0
12
0
0
9
0
0
0
8
0
9
0
0
5
4
9
9
9
9
9
0
12
15
5
4
2
0
9
4
0
0
0
0
0
0
1
0
0
1
0
2
1
0
3
0
1
2
2
2
0
1536
512
2048
512
U
ENVELOPE
NUMBER INSTRUMENT ATTACK DECAY SUSTAIN RELEASE WAVEFORM WIDTH
0
1
2
3
4
5
6
7
8
9
Figure 11-8. Default Parameters for ENVELOPE Statement jj
THE SID FILTER ! j
Once you have selected the ENVELOPE, ADSR, VOLume and TEMPO, use the
FILTER to perfect your synthesized sounds. In your program, the FILTER statement ^
will precede the PLAY statement. First you should become comfortable with generating i
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 the music synthesizer, use the FILTER statement to increase j j
the sharpness and quality of the sound. ^
In the first paragraph of this section, The Characteristics of Sound, we defined a
sound as a wave traveling (oscillating) through the air at a particular rate. The rate at j j
which a sound wave oscillates is called the wave's frequency. Recall that a sound wave [J
is made up of an overall frequency and accompanying harmonics, which are multiples of
the overall frequency. See Figure 11-5. 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 j j
filtering. These filtering methods are additive, meaning you can use more than one filter J
at a time. 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. In low-pass filtering, the SID | j
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. \ j
See Figure 11-9. LI