Advanced Filtering - Commodore 128 System Manual

Line 10, the ENVELOPE statement, specifies the envelope for
piano (0), which sets the attack to 0, decay to 9, sustain to 0 and
release to 0. It also selects the variable pulse waveform with a
pulse width of 1700. Line 15 sets the VOLume to 8. Line 20
chooses the TEMPO to be 10.
Line 35 FILTERS the notes that are played in lines 30 to 115. It
sets the FILTER cutoff frequency to 1200. ln addition, line 35
turns off the low-pass and band-pass filters with the two zeros
following the cutoff frequency (1200). The high-pass filter is
turned on with the 1 following the two zeros. The resonance is set
to 10 by the last parameter in the FILTER statement.
Line 30 PLAYS the notes C, D, E, F, G, A, B in that order. Line 45
PLAYS the same notes as line 30, but it specifies the SID control
characters U5 as volume level 5, V1 as voice 1 and 05 as octave
5. Remember, the SID control characters allow you to change the
synthesizer controls within a string and exercise the most control
over the synthesizer. Line 65 specifies the control characters U7
for volume level 7, V3 for voice 06 for octave 6 and X0 to turn off
the filter. Line 65 PLAYS the same notes as lines 30 and 45, but
in a different volume, voice and octave.
Line 85 has the same volume, voice and octave as line 65, and it
specifies half notes for the notes C and D, quarter notes for the
notes E and F, eighth notes for notes G and A and a sixteenth
note for the B note. Line 105 sets the volume at 7, voice 1, octave
4 and turns off the filter. It also specifies the C note as dotted half
note, E as a sharp quarter note, G and A as flat eighth notes and
B as a dotted sixteenth note.

Advanced Filtering

Each of the previous FILTERing examples used only one filter at
a time. You can combine the SID chip's three filters with each
other to achieve different filtering effects. For example, you can
enable the low-pass and high-pass filters at the same time to
form a notch reject filter. A notch reject filter allows the
frequencies below and above the cutoff to pass through the SID
chip, while the frequencies close to the cutoff frequency are
filtered. See Figure 7-10 for a graphic representation of a notch
reject filter.
7-24