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3) Phase-Corrected Lowpass Filter/Preemphasis: ( on Card # 6)
After the outputs of the " Master" and "Bass" channels have been summed, they
are passed to a filter which performs three functions:
1)
It
lowpass-filters
the
signal
at
15kHz
and
24dB/octave
to
prevent
frequencies beyond the bandwidth of the system
from unnecessarily activating
the high frequency limiter or causing unnecessary IM distortion in the clipper;
2)
It
provides
a standard
preemphasis ( 75us
or
50us,
depending
on
region);
and,
3)
It
provides phase correction to make the delay of the lowpass filter plus
preemphasis approximately constant, thus minimizing the unavoidable increases
in
peak
level
resulting
from
the
preemphasis
and
filtering
functions.
The
lowpass
filter
is
designed
to
partially
equalize
the
frequency
response
variations in the main 15kHz lowpass filter following in the peak limiting system,
thus providing flatter overall frequency response. The preemphasis is created by
summing
a second- order bandpass filter with the flat signal. The rising side of
the filter slope
provides the
preemphasis;
the falling side provides part
of
the
lowpass filter function. The phase corrector is a fourth- order allpass filter, and is
physically placed before the lowpass filter and preemphasis.
4) High Frequency Limiter: ( on Card # 6)
In
order to perform the high- frequency limiter function, a variable- gain stage is
placed between the output of the bandpass filter creating the preemphasis ( see 3
immediately above) and the amplifier which sums the bandpass filter output with
the main signal. Thus high frequency limiting is effected by dynamically reducing
the preemphasis as required.
The
variable gain stage
is realized by a junction FET
operating as
a voltage-
controlled resistor, instead of by a VCA as in other processing functions within
OPTIMOD-TV. This simplification is possible because the high frequency limiters
in
the
left
and
right
channels
are
entirely
independent,
and
need
not
track
accurately together.
Each
channel has
its own rectifier and timing module. The timing in the high-
frequency limiter is considerably simpler than in the compressor sections because
only fast dynamic filtering occurs; there is no " compression" function.
It
should be noted that
the OPTIMOD-TV high- frequency limiter is activated by
high
frequency
energy
only.
The
high- frequency
limiter
therefore
cannot
be
activated by, for example, low frequency overshoot components from the previous
multiband compressor. This design is possible because OPTIMOD-TV's peak limiting
system
permits
considerably
greater
amounts
of
clipping
than
other
clipping
schemes without
introducing audible distortion, thus rendering the high- frequency
limiter function far less critical and permitting substantial increases in perceived
high
frequency power output. The high- frequency limiter need not " know" about
the
actual
peak
level
of
the
preemphasized
signal --
only
approximately
how
much HF energy is present.
A-6
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