Commodore 128 System Manual page 155

Conversely, the high-pass filter allows all the frequencies above
the cutoff frequency to pass through the chip. All the ones below
it are filtered out. See Figure 7-8. The high-pass filter produces
tinny, hollow sounds.
The band-pass filter allows a range of frequencies partially above
and below the cutoff frequency to pass through the SID chip. All
other frequencies above and below the band surrounding the
cutoff frequency are filtered out. See Figure 7-9.
Figure 7-7. Low-pass Filter
Figure 7-8. High-pass Filter
Figure 7-9. Band-pass Filter
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