User manual version 1
AV dsp amp
Designing filters
Biquad
function
Parameters
Use
Unity
-
Section is not used
LPF1
Cut-off frequency (always
-3dB)
First order lowpass
LPF2
Cut-off frequency
(asymptotically)
Q
Second order lowpass. A Q of
0.71 corresponds to
butterworth. 0.5 corresponds
to LR2. Two identical
sections with a Q of 0.71 form
an LR4 filter.
HPF1
Cut-off frequency (always
-3dB)
First order highpass
HPF2
Cut-off frequency
(asymptotically)
Q
Second order highpass. A Q of
0.71 corresponds to
butterworth. 0.5 corresponds
to LR2. Two identical
sections with a Q of 0.71 form
an LR4 filter.
Shelf1
Centre Frequency (halfway
point)
Gain
Direction
First order shelf. Useful for
baffle-step correction
Shelf2
Centre Frequency (halfway
point)
Gain
Q
Direction
Second order shelf. Useful
for correcting internal
cabinet resonances and for
the midband peak/dip combo of
most midwoofer speakers.
Asymmetric
Shelf
Pole frequency and Q
Zero frequency and Q
Equalising the bottom end of
closed-box woofers with large
magnets
Boost/Cut
Centre frequency
Q
gain
Dip/peak filter. For peaks, Q
is defined by the poles. For
dips, Q is defined by the
zeros. Thus the same filter
with opposite gains will
cancel.
The first step is equalising the magnitude responses of the drivers flat over
their entire useable frequency range.
The weapons of choice are shelving filters, and boost/cut sections. A sharp
peak followed by an equally sharp dip can be corrected using a second-order
shelving filter with a high Q.
Exercise care when deciding what to correct. When correcting for diffraction
errors, do not exceed a Q of 3 lest the cure be worse than the ailment. Errors
that are caused inside the driver, or internal cabinet resonances that emanate
through the same diaphragm, may be corrected ruthlessly provided the
–
measurement has sufficient resolution to pin them down.
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Summary of Contents for AV dsp amp
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