8-3010 July 1998
All filters in the ISP-100 are derived from analog prototypes. The
analog filters are approximated by the Bilinear Transform method,
prewarped to match response at the critical frequency appropriate for
the type of filter (cutoff frequency for LowPass/HighPass and Peaked
HighPass; hinge frequency for LowShelf/HighShelf; center frequency
for all others).
Each filter band may be bypassed; so as to pass the signal
unaltered.
LowPass Filters
LowPass Filters pass low frequencies and stop high frequencies.
Since it is not realistically possible to create a perfect filter that
passes low frequencies totally unaltered and stops high frequencies
completely, lowpass filter design involves compromises that allow
some rounding of the corner at the filter cutoff frequency and some
slope in the transition to the high frequency stopband. Different
compromise schemes are given different names; the ISP-100
incorporates Bessel, Butterworth, and Linkwitz-Riley lowpass filter
types.
The cutoff frequency is defined as the frequency at which the
magnitude of the filter response has fallen to -3 dB relative to the
unfiltered signal in the Bessel and Butterworth types, and to -6 dB in
the Linkwitz-Riley types. In the ISP-100 lowpass filters the cutoff
frequency is continuously variable from 20Hz to 20 kHz.
In the ISP-100 lowpass filters the passband slope is selectable from
-6 dB/octave to -12 dB/octave for the Bessel and Butterworth types.
NOTE
: Linkwitz-Riley filters are also known as Butterworth-Squared
filters because they are the product of two Butterworth filters. A -12
dB/octave Linkwitz-Riley lowpass filter may be constructed by
cascading two identical -6 dB/octave Butterworth lowpass filters; a -
24 dB/octave Linkwitz-Riley lowpass filter may be constructed by
cascading two identical -12 dB/octave Butterworth lowpass filters.
HighPass Filters
HighPass Filters pass high frequencies and stop low frequencies.
Since it is not realistically possible to create a perfect filter that
passes high frequencies totally unaltered and stops low frequencies
completely, highpass filter design involves compromises that allow
some rounding of the corner at the filter cutoff frequency and some
slope in the transition to the low frequency stopband. Different
compromise schemes are given different names; the ISP-100
incorporates Bessel, Butterworth, and Linkwitz-Riley lowpass filter
types.
The cutoff frequency is defined as the frequency at which the
magnitude of the filter response has fallen to -3 dB relative to the
unfiltered signal in the Bessel and Butterworth types, and to -6 dB in
the Linkwitz-Riley types. In the ISP-100 highpass filters the cutoff
frequency is continuously variable from 20Hz to 20 kHz.