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An
HF
LIMITING
control
available
to
the
user
adjusts the
threshold
of
high-
frequency
limiting
over
a range
of
approximately
3dB.
The
lowest
threshold
results
in
very little
clipping on sinusoidal
tone;
the + 3dB
threshold results in
moderate clipping of tone above approximately 2kHz. In most cases, users prefer
operating this
control
in
full " hard", which
moves
the threshold to the "+ 3dB"
point
and
results
in
minimum
high- frequency
limiting
and
maximum
high
frequency
control
by
clipping,
while
still
limiting
high
frequency
energy
which
would otherwise cause disturbing distortion if it were clipped.
Operation
of
the
high- frequency
limiter
is
metered
by
a simple
comparator
circuit which lights the appropriate front- panel high- frequency limit lamp if any
high- frequency limiting at
all
occurs. It
is primarily
useful
to
verify that
the
high- frequency limiter circuit is operating properly.
5) Peak Limiting System: ( on Cards # 0, # 1, # 8, and # 9)
The
OPTIMOD-TV
peak
limiting
system ( U.S.
Patent # 4,495,643)
is
a complex
"computed clipper" using distortion- cancelling techniques similar to those used in
other Orban second- generation processors.
Basic
peak
limiting
is
provided
by
the
exclusive
Orban "Hilbert- Transform
Clipper". This circuit ( which is explained more fully below) behaves like an SSB
RF
clipper
below
4kHz,
and
like
a conventional
hard
audio- frequency
clipper
above 4kHz. An RF clipper produces no harmonic distortion -- only IM. Thus no
harmonic distortion is produced in the
voice frequency range
below 4kHz, and
voice ( which
is
far
more
degraded
by
harmonic
than
by
IM
distortion)
is
processed as cleanly as possible.
Conversely,
above
4kHz
the
circuit
produces
more
harmonic
and
less
IM
distortion, minimizing difference- frequency IM and yielding minimum degradation
of vocal sibilance and music with substantial high frequency content.
A further crossover function ( essentially unrelated to the change in behavior at
4kHz) eliminates almost all distortion of any type below 2.2kHz at the expense
of a certain amount of overshoot. Overshoot is dealt with in the subsequent FCS
Overshoot Compensator.
It
should be noted that
it is normal for sinewaves to modulate less than 100%
when
applied
to
OPTIMOD-TV
in
its
normal
OPERATE
mode.
There
are
two
principal reasons for this:
1)
Some headroom is left between the threshold of the peak limiting system and
the
threshold of
the subsequent
overshoot
corrector
in
order
to accomodate
overshoots. With sinewaves, no overshoots are produced. Thus, the headroom is
not used, and full 100% modulation is not produced.
2)
Sinewaves have a very low peak- to- average ratio and high loudness potential
compared to program material of identical peak levels. The audio processing,
in order to maintain natural sound quality, pushes sinewaves down in level as
it
would any other similar program material with low peak- to- average ratio.
In general, any audio processor which produces 100% modulation on sinewaves
tends to sound somewhat unnatural because this psychoacoustic factor has not
been accounted for.
A-7
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