Studio Technologies ISS - Manual (Page 31)

Issue 3, June 1990 ISS User Guide
Page 32 Studio Technologies, Inc.
ISS
Two sections of op amp U7 act as com-
parators to create an indication of when the
reference ramp signal is at 5V or 10V.
These signals are used for two functions:
maintain feed to the simulator cards and
enable the simulating from right and simu-
lating from left relays.
There are four relays contained on the
Crossfade Card. Three of the relays are not
integral to the operation of the ISS but are
used to provide status indication to the
associated broadcast system. The fourth
provides the Transfer Relay Assembly with
the I/O Bypass status. Four normally open
(not shorted) relay contacts provide the
following indications: simulating from left,
simulating from right, ISS operating via the
remote control input (remote operation
enabled), and I/O Bypass request. Notice
that Remote Enable relay energizes when-
ever the IMMEDIATE crossfade speed is
selected. This is because the only time the
immediate speed is active is when the ISS
is remotely controlled.
Recognition Card
The Recognition Card contains four major
sections: Band-Pass Filters, Peak Detection,
L=R Recognition, and Left Only/Right Only
Recognition.
Band-Pass Filters: Left and right channel
audio signals enter the Recognition Card
via the three position INPUT switch. This
switch selects the input source. In the A
position the signals come from the output
of the I/O Card. In the B position the signals
come from the output of the Polarity Correc-
tion Card. In the C position signals come
from the output of the optional Tone Detec-
tion Card. The input signals then enter low-
pass Butterworth filters consisting of three
op amp sections. The signals then go
through high-pass Butterworth filters that
use two op amp sections. The resultant
signals have a 3dB band-pass of 100 to
1kHz. For accurate left versus right perfor-
mance, all 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 identical
within the pass band; 500Hz should enter
and leave at the same level. The signals
are filtered so that accurate recognition is
possible. If the high frequencies are not
removed, the normal phase shifts due to
tape head azimuth errors or short differen-
tial time delays will interfere with actual left
versus right differences. If the low frequen-
cies are not removed, the large amounts of
energy at the low frequencies will create
false recognition results. The band-passed
output signals then go to the three other
sections.
Peak Detector: The band-passed left and
right signals are summed using one op
amp section. This op amp, along with one
other, creates a half-wave rectifier circuit
that produces a DC signal that is represen-
tative of the summed amplitude of the left
and right signals. A comparator serves to
gate the DC signal so as to “speed up” the
rectification. This “speeded up” signal is a
peak DC picture of the band-passed left
and right signals. This peak DC is used by
the remaining two sections.
L=R Recognition: This circuit determines
if signals present on the left and right chan-
nels at the same time are mono, i.e., identi-
cal. A comparator is used to compare the
band-passed left channel signal with a
resistor divider scaled version of the peak
reference signal. The output of this com-
parator is 0 volts when the left channel