Studio Technologies ISS - Manual (Page 32)

ISS User Guide Issue 3, June 1990
Studio Technologies, Inc. Page 33
ISS
meets this criteria; 15 volts when it does
not. This results in a series of short pulses
representative of the phase activity of the
signal. Another comparator is used for the
band-passed right channel. These two
output signals are sent to an exclusive-OR
(XOR) gate which produces a logic high
only when the input signals are phase
coherent. If the pulses sent to the inputs of
the XOR gate are not in phase, the XOR
gate output will stay at logic low. Input
pulses that are coherent will result in a logic
high output pulse. These XOR gate output
pulses charge a resistor-capacitor low pass
filter. The output of this filter represents the
amount of phase coherency. This output is
fed to a comparator whose trigger has been
calibrated so that when sufficient phase
coherency is detected a L=R logic high is
generated. A feedback path is used to hold
the state of the final comparator during
periods of time when the peak detector
amplitude is not sufficient for accurate
recognition. During actual broadcast opera-
tion this “hold” would come into effect when
an audio track fades out at the end of a
commercial, etc. The result is a logic high
when the two channels are determined to
be phase coherent. This logic high, called
L=R, is sent via one section of DIP switch,
and the ribbon cable bus, to the Mode
Select Card. The DIP switch allows dis-
abling of the L=R recognition feature.
Left Only/Right Only Recognition: The band-
passed left and right signals each come
into this section via a section of op amp
set for a gain of about 30dB. These op
amps create what is effectively a “signal
present” output. Very little input amplitude
will cause the output of the op amps to go
to 15 volts. Our friend the peak detector
now comes into play again. Two identical
circuits determine a left only or right only
condition. We will discuss the right only
circuit. A comparator is used to compare
the left channel “signal present” level with
the amplitude of the peak detector. The
output of this comparator goes logic low
when the amplitude of the peak detector is
greater than the amplitude of left channel
“signal present.” This logic low, in effect,
says that the left channel is not contributing
much to the peak detector amplitude. This
signal is further refined with another com-
parator section using a DC level as its
reference. A logic low on the output of this
comparator can be considered as a right
only indication. Two other comparators
produce a left only indication. These two
signals are fed to a flip-flop whose outputs
create the Left Only and Right Only outputs.
Besides going to two sections of DIP
switch, these outputs are fed back to the
“signal present” points, creating a holding
level. This holding level keeps the circuit
stable when audio is not present at a suffi-
cient level to produce an accurate detection
result. The outputs of the DIP switches are
sent to the Mode Select Card via the ribbon
cable bus.
Polarity Correction Card
The Polarity Correction Card contains five
major sections: Audio Input/Output, Band-
Pass Filters, Peak Detection, Polarity Detec-
tion, and Mode Select and Remote Control.
Audio Input/Output: Left and right channel
audio signals enter the Polarity Correction
Card via the ribbon cable bus from the
outputs of the I/O Card. The left channel
signal connects to the input of the left chan-
nel band-pass filter, as well as directly out
again via another pin in the ribbon cable