Commodore 128 System Manual page 159

You can also add either the low-pass or high-pass filter to the
band-pass filter to obtain interesting effects. By mixing the band­
pass filter with the low-pass filter, you can select the band of
frequencies beneath the cutoff frequency and below. The rest are
filtered out.
By mixing the band-pass and the high-pass filters, you can select
the band of frequencies above the cutoff frequency and higher.
All the frequencies below the cutoff frequency are filtered out.
Experiment with the different combinations of filters to see all the
different types of accents you can place on your musical notes
and sound effects. The filters are designed to perfect the sounds
created by the other components of the SID chip. Once you have
created the musical notes or sound effects with the SID chip, go
back and add the FILTERing to your programs to make them as
crisp and clean as possible.
Now you have all the information you need to write your own
musical programs in Commodore 128 BASIC. Experiment with
the different waveforms, ADSR settings, TEMPOS and FILTERing.
Look in a book of sheet music and enter the notes from a musical
scale in sequence within a play string. Accent the notes in the
string with the SID control characters. You can combine your
Commodore 128 Music synthesizer with C128 mode graphics to
make your own videos or " movies" , complete with sound tracks.
Figure 7-10. Notch Reject Filter
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