Post-Limiter Crossovers, Clippers, Distortion-Cancelling System - Orban 8100A/XT2 Operating Manual

6-10
Technical Information
8) POST-LIMITER CROSSOVERS, CLIPPERS, DISTORTION-CANCELLING
SYSTEM
Card #A1
The upper two bands (3.7kHz and 6.2kHz) are treated differently than the bottom three bands
(150Hz, 420Hz, and 700Hz). The 1.6kHz band is unique in that it is not followed by a
clipper.
Difference-frequency intermodulation (IM)
due to clipping is cancelled below 2.2kHz by a
feedforward
distortion-cancelling sidechain (US
patent #4,412,100). The output of each of
the two upper bands is applied to two identical
filters
(one inverting and one non-inverting)
with 6dB/octave slopes. The
6.2kHz band
filter is high-pass; the
3.7kHz band
filter is
band-pass.
A clipper
is located before the inverting filter, so that the inverting filter filters the clipped
signal, somewhat reducing out-of-band clipper-induced distortion.
The
outputs of the inverting and non-inverting filters are added in the
distortion-cancelling
summing amplifiers. If no clipping occurs, the outputs of the inverting and non-inverting
filters will cancel, and no output will be produced by the distortion-cancelling summing
amplifiers. If clipping does occur, then the output of the distortion-cancelling summing
amplifiers will represent the difference between the clipper ' s input and output as filtered by
the inverting filter (i.e., the distortion added by the clipper, as filtered through the inverting
filter).
Since the upper two bands are handled by the distortion-cancelling summing amp, the output
of this amplifier represents the sum of the filtered clipper-induced distortion produced by the
two clippers in these bands. This signal is applied to the distortion-cancelling filter (a 2.2kHz
low-pass filter with constant delay).
Meanwhile, the outputs of the inverting filters alone (containing the clipped, filtered outputs
of the upper two bands) are summed by the
band-summation amplifier.
The output
of
this
amplifier is applied to the input of the 15kHz phase-linear low-pass filter.
The outputs of the
15kHz filter
and
2.2kHz filter
are
summed.
Considering for a moment
the case where only one band is passing signal, the clipper-induced distortion component
contributed by this band to the 2.2kHz filter ' s output is equal to, and out-of-phase with,
the same distortion component in the 15kHz filter ' s output. Thus, this distortion component
is cancelled by better than 30dB within the 2.2kHz bandwidth of the distortion-cancelling
filter. Because the summation process and both of the 15kHz and 2.2kHz filters are linear,
superposition holds and the distortion component in each of the two top bands is cancelled
even when more than one band is active.
The three low-frequency bands are not treated by the distortion-cancellation sidechain. Simple
low-pass filtering
of the clipped signal is employed for distortion reduction.
The output of the
150Hz band
is applied to
a
pair of cascaded 6dB/octave
low-pass filters
before it is
summed
with the rest of the bands in the band summation amplifier. A
clipper
is located between the first and second of these filters. The output of the
420Hz band
is
applied to this clipper, and then through the second low-pass filter to the
band-summation
amplifier.
The clipper thus clips the sum of the 420Hz band and the filtered 150Hz band.