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31
FILTER OVERSHOOT COMPENSATOR
Sources of
Overshoot
All low-pass filters exhibit a certain amount of overshoot and ringing
when presented with complex input waveforms. Generally, the sharper
the cutoff, the more pronounced the effect. Overshoots result from the
elimination of higher-order input signal components which, prior to
filtering, helped define the signal peak amplitude. Even a fully phase-
corrected filter will exhibit overshoots, and a 7-pole “elliptic” filter, as
used in the
DAVID-II
, can overshoot 3dB or more!
Other systems of overshoot control permit the primary low-pass filter
to overshoot, then isolate and re-introduce the overshoots to cancel
themselves in the signal path. The patented overshoot compensator in
the
DAVID-II
, on the other hand, so pre-conditions the limited
program signal
ahead
of the filter that there is little or no tendency for
the filter to
generate
overshoots.
Input Clipper
CR33 and CR34 form a “hard” clipper at the compensator input.
These diodes are biased to a point which represents 100%-modulation.
Since the program signal has already been limited to this same value,
the two diodes rarely clip
legitimate
program waveshapes. Clipping at
this point is limited to those fast transients which exceed limiter attack
timing, and the clipping of which is judged less obtrusive than a
temporary reduction in overall broadband level.
Phase-Lag and
Recombining
IC34B includes a phase-lag network which time-displaces the fast
leading and trailing edges of steep waveforms. This means that the
primary characteristic of a program waveform which would normally
excite
filter overshoots is instead added to the waveform
amplitude
.
CR32 and CR31, also biased to the 100%-modulation level, “strip”
these displaced-and-added components from the program signal.
IC32B compares the “stripper” input and output, recovering the
stripped-off components. As these contain much of the program
harmonic (high frequency) information, we cannot afford to simply
throw them away. By recombining these stripped-off program
components with the stripped program signal
in opposite phase
, the
spectral integrity of the program is maintained. This 180-degree
displacement of certain program overtones is not discernible to the
listener, but is quite effective in inhibiting filter overshoots.
IC19B combines the stripped-off signal components form both the right
and the left channels. This is full-wave rectified by Q8 and Q9 to flash
the front-panel
FILTER COMP
indicator whenever filter compnesator
action takes place.
LOW-PASS FILTER
The 7-pole, elliptic-function (Cauer) low-pass filter is an active version of
the classic L-C designs worked-out in Germany during the late 1940s
(probably with a slide rule!). The particular active configuration used in
