Inovonics 235, Инструкция по эксплуатации и техническому обслуживанию

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The
PEAK PROCESSING
selector controls analog switch IC14 to set
peak limiter release time . The faster the release, (a lower value of
resistance switched through IC14), the greater the incidence of program
clipping and its attendant increase in perceived loudness.
At the two least-aggressive
PEAK PROCESSING settings ( 1 and 2 ), a
small DC bias from R43 and R44 is introduced into the limiter release
equation. This assigns a fixed loss to the limiter circuit and reduces the
total amount of peak level control to approximately 5dB.
Limiter release timing is programmed for the most useful working
range of limiter/clipper action. The signal level at the output of IC26B
which represents 100% symmetrical modulation is +5dBu.
FILTER OVERSHOOT COMPENSATION (Schematic Sheet 2, Page 30)
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 helped to
define the signal peak amplitude before they were filtered out. Even a
fully phase-corrected filter will exhibit overshoots, and a 9-pole
“elliptic” filter, like the one used in the Model 235 Processor, can
overshoot 3dB or more... perhaps much 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 235, on the other hand, pre-conditions the limited program signal
ahead of the filter so there is little or no tendency for the filter to
generate overshoots in the first place.
Phase-Lag and
Recombining
As previously related, Q7 and Q8 constitute a “hard” clipper at the
input of the compensation circuit and are biased to a point which
represents 100%-modulation.
IC25B buffers the output of the Q7/Q8 clipper, and incorporates a
gentle second-order rolloff above 15kHz. IC24B is an all-pass, phase-
lag stage 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 . A second clipper, Q22 and
Q23, also biased to the 100%-modulation level, “strips” these displaced-
and-added components from the program signal. IC22B compares the
“stripper” input and output to recover the stripped-off components.
These contain much of the program harmonic (high frequency)
information, so we cannot afford simply to 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