Commodore 128D - Advanced Filtering

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 through 115. It
sets the FILTER cutoff frequency to 1200. In 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, V2 as voice 2 and 05 as octave 5.
Remember, the SID control characters allow you to change the syn
thesizer controls within a string and exercise the most control over
the synthesizer. Line 65 specifies the control characters U7 for vol
ume 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 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 8, voice 1, octave 4 and turns off the
filter. It also specifies the C note as a dotted half note, E as a sharp
quarter note, G and A as flat eighth notes and B as a dotted sharp
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.
152 USING C128 MODE—Sound and Music in 0128 Mode