Studio Technologies ISS - Manual (Page 34)

ISS User Guide Issue 3, June 1990
Studio Technologies, Inc. Page 35
ISS
reversal is detected. This logic signal clocks
a flip-flop, whose output is configured to
change state on a clock transition. The flip-
flop output feeds the D input of another flip-
flop. This second flip-flop outputs the final
polarity correction signals that control the
polarity correction audio circuitry. The
output of the second flip-flop has two con-
straints placed on it. The first requirement
is that the D signal is sent to the output only
during a zero crossing of the right channel
audio signal. As previously discussed, the
right channel audio is sent to a comparator
that produces an invert clock signal. This
invert clock signal is a string of pulses that
correspond to the zero crossings of the
right channel audio. The second require-
ment is that the flip-flop is enabled, or
more accurately, has not been disabled
by the front panel switch or remote control
request.
To review: The Polarity Detection circuit
simply looks to see if the left and right
signals are in the same relative polarity;
there is no absolute reference. The circuit
performs the same action when going
between what, to a broadcaster, is two
different events. The first event is the transi-
tion from polarity correct to polarity re-
versed audio. The circuitry detects this
condition and flips the right channel’s
polarity. The circuitry now detects the left
and right signals as polarity correct. The
second event is the broadcast audio source
going from polarity incorrect back to polar-
ity correct. The circuitry again sees a prob-
lem and flips the right channel again. The
right channel audio now passes through
the analog switches without a polarity flip.
Mode Select and Remote Control: A switch
controls the operating mode of the Polarity
Correction Card. One remote control input,
and two relay outputs give remote control
access to several Polarity Correction
Card functions. Two LEDs provide status
indication.
The Remote Disable input allows operation
of the card to be disabled. An optocoupler
integrated circuit provides isolation between
the source of the remote control signal and
the ISS circuitry. A resistor in series with the
optocoupler photodiode provides current
limiting to protect the remote control signal
source and the photodiode. A diode is
connected anode to cathode/cathode to
anode across the photodiode to prevent
a polarity reversal on the remote control
signal from damaging the optocoupler. The
optocoupler output signal is buffered using
an inverting Schmidt trigger gate. The
output of the buffer is now truly a “clean”
logic signal. This logic signal directly fol-
lows the remote control input. A switch
selects whether the Remote Disable input
will respond to a continuous or pulse signal.
In the continuous mode, the output of the
Schmidt inverting buffer is sent directly to
the next section of circuitry. In the pulse
mode, the output of a D flip-flop, whose
input is from the Schmidt inverting buffer,
is connected to the next section of circuitry.
A single pole, three position switch controls
three sections of analog switch which set
the mode of the card. In the DISABLE
position the flip-flop that controls the right
channel audio analog switches is disabled.
In the OPERATE position the flip-flop is
active but a Remote Disable request is
ignored. In the OPERATE + REMOTE
position the flip-flop is active, and a Remote
Disable request will disable the flip-flop.
Two relay contacts provide status indication
to the outside world. The Remote Disable