Studio Technologies ISS - Manual (Page 33)

Issue 3, June 1990 ISS User Guide
Page 34 Studio Technologies, Inc.
ISS
bus. The right channel signal connects to
three points. The first is to a comparator
that produces a string of pulses that corre-
spond to the signal’s zero crossings. The
purpose of this will be discussed later. The
second point is to the input of an op amp
configured as an inverting unity gain ampli-
fier. The output of this op amp feeds an
analog switch. The third point is to another
analog switch. The outputs of the two ana-
log switches connect together and feed the
input of the right channel band-pass filter,
as well as back out on another pin on the
ribbon cable bus. The gate of one analog
switch connects to the Q output of a flip-
flop; the other to Q invert. The right channel
output polarity depends on the state of this
flip-flop. In one state the right channel input
connects directly to the output; in the other
state the right channel output is derived
from the inverting op amp. In this simple
way the left and right channels can be put
in the same relative polarity.
Band-Pass Filters: The left and right signals
enter the filter section via separate low-pass
Butterworth filters consisting of three op
amp sections. The signals then go through
separate high-pass Butterworth filters that
use two op amp sections. The resultant
signals have a 3dB band-pass of 100Hz to
1kHz. For accurate left versus right tracking,
all filter related resistors and capacitors
have 1% tolerance. The band-pass filters
have a unity gain design so that the input
and output levels should be roughly identi-
cal within the pass band; a 500Hz signal
should enter and leave at the same level.
The signals are filtered so that an accurate
determination of relative phase can be
made. If the high frequencies were not
removed, the normal phase shifts, due
to tape head azimuth errors or short
differential time delays, would interfere with
relevant left versus right differences. If the
low frequencies were not removed, the
large amounts of energy at the low frequen-
cies could create false results. The band-
passed left and right signals go on to
the Peak Detector and Polarity Detection
sections.
Peak Detector: The band-passed left and
right signals are summed using one op
amp section. This op amp, along with one
other, create a half-wave rectifier circuit that
forms a DC signal that is representative of
the summed amplitude of the left and right
signals. A comparator serves to gate the
DC signal so as to “speed up” the rectifica-
tion. This “speeded up” signal is a peak DC
picture of the band-passed left and right
signals.
Polarity Detection: The band-passed left
and right signals are fed differentially to the
inputs of an op amp. The resultant output is
the difference between the left and right
signals. This difference signal, along with
the peak signal, is fed into a comparator.
This comparator acts as a polarity normal,
polarity flipped detector. If the left and right
signals have roughly the same polarity, the
peak signal will be greater than the differ-
ence signal. If the opposite is true, the
polarity of the left and right signals are, at
least at any one instant, opposite. The
output of this comparator feeds an RC low-
pass filter, and then on to another compara-
tor. This comparator uses a fixed voltage
reference, along with hysteresis, to insure
that a true polarity reversal is occurring, and
not one that is very brief due to normal,
phase complex stereo signals. The output
of this comparator is effectively a logic
signal that changes state when a polarity