Studio Technologies ISS - Manual (Page 26)

ISS User Guide Issue 3, June 1990
Studio Technologies, Inc. Page 27
ISS
means that if one side of the line output is
grounded, the circuit is not harmed and the
other side of the line output still functions
correctly. A trim potentiometer, in series
with the non-inverting input of one of the
output op amps, is used to balance the
positive and negative output signal excur-
sions. This ensures that, as an example, a
1.00V positive excursion AC signal on the
“+” line output connection will be matched
by a 1.00V negative excursion AC signal on
the “–” line output connection.
Transfer Relay Assembly
The Transfer Relay Assembly provides the
capability to connect the audio inputs to the
audio outputs in the event of a system
malfunction or an operator initiated com-
mand. The simple circuitry consists of three
relays and a transistor control circuit. In
the normal operating mode, the transistor
is forward biased, and the relays are held
energized. One relay routes the left channel
line input signal to the ISS mainframe. The
ISS left channel line output is routed to the
output connector. The right channel is
handled the same way via another relay.
A third relay provides the I/O Bypass En-
abled Status Relay Contact, accessible to
the user via the 25-pin plug. An LED and
current limiting resistor is in parallel with
the relay coils as a status indicator. The
relays can release for two reasons: loss of
24Vdc power coming from the mainframe,
or closing of the I/O Bypass status relay
contact that comes from the mainframe.
When the relays de-energize, the left audio
input is connected directly to the left audio
output. The left channel input and output
signals to/from the mainframe are discon-
nected. The same action occurs for the
right channel. In the I/O Bypass mode, the
aforementioned relay contact shorts, pro-
viding the I/O Bypass status relay contact.
The LED is now not lit, showing that the
Transfer Relay Panel is in the transfer
mode.
Stereo Simulator Cards
The input signal enters the Type I and
Type II cards via pin 9 of the ribbon cable
bus. One section of an op amp U7 acts
as an inverting buffer. The signal then
goes through a simple Resistor/Capacitor
pre-emphasis network that precedes U4a,
which acts as a compressor in a comp-
andor circuit. U5 is an integrated circuit
compandor. The compressor attack time is
speeded by a charge pump, which reduces
transient distortion that is often associated
with compandors.
The signal now proceeds in different direc-
tions in the Type I and Type II cards. In the
Type I Card the companded signal con-
nects to the anti-aliasing low-pass filter
discussed in the next paragraph. In the
Type II Card the signal proceeds to a band
reject filter. The band reject filter is made up
of four sections of op amp U9. The 3dB
points are at 400 and 2200Hz, with an 11dB
dip at 1100Hz. This filter is set to attenuate
signals in the voice band, while leaving low
and high frequency audio signals unaf-
fected. This filter is the reason why the Type
II Card can add simulation only in the non-
voice region of the audio spectrum. The
filtered signal leaves the band-reject filter
and proceeds to the low-pass filter section.
Low-Pass Filter: Three sections of op amp
U7 form a 6-pole, 20kHz Butterworth low-
pass filter. This reduces the possibility of
audio frequencies aliasing with the clock
frequency produced by U3.