Commodore 128 Programmer's Reference Manual page 360

350 COMMODORE 128
FREQUENCY
Figure 11—11. Band-pass Filter
ADVANCED FILTERING
Each of the previous FILTERing examples used only one type of filtering at a time. You
can combine the SID chip's three methods of filtering 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 out. See Figure 11-12 for a graphic representation of a
notch reject filter.
CUTOFF
FREQUENCY
u
I I
) (
u
i f
u
ii
Figure 11-12. Notch Reject Filter
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
are filtered out.
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