In zita-rev1, the damping filter for each delay line consists
of a low-shelf filter
[452],4.20in series with a unique first-order lowpass filter
that sets
the high-frequency
to be half that of the middle-band at a
particular frequency
(specified as ``HF Damping'' in the GUI).
Since the filter
is constrained to be a lowpass,
for
, i.e., the decay time gets
shorter at higher frequencies.
Viewing the resulting damping filter
as a
three-band filter bank (§3.7.5), let
and
denote the
desired band gains at dc and ``middle frequencies'',
respectively.4.21 Then the low shelf may be set for a
desired dc-gain of
, and its input (or output) signal
multiplied by
to obtain the resulting filter
where
where
The lowpass filter
is also first order, and to provide half
the middle-band
at the beginning of the ``high'' band, the
lowpass should ``break'' to a gain of
at the ``HF Damping''
frequency
specified in the GUI. A unity-dc-gain one-pole
lowpass has the form [452]
where the pole
Squaring and normalizing yields a quadratic equation of the form
where
and the unstable solution