Studio Technologies ISS - Manual (Page 25)

Issue 3, June 1990 ISS User Guide
Page 26 Studio Technologies, Inc.
ISS
The Remote I/O Bypass function allows an
external signal to place the ISS in the I/O
Bypass condition. The actual signal that
controls the I/O Bypass function is derived
from remote control input circuitry located
on the Mode Select Card. Pin 20 of the
ribbon cable bus carries the signal between
the two cards. The switch on the I/O Card
selects whether the Remote I/O Bypass
function is active.
Working in conjunction with the I/O Bypass
circuitry on the I/O Card is the I/O status
relay on the Crossfade Card. A contact on
the I/O status relay controls the Transfer
Relay Assembly. The I/O status relay is
controlled by the I/O Card via pin 19 of the
ribbon cable bus.
Line Input Stage: The purpose of the line
input stage is to take a balanced audio
input signal, reduce the level by a fixed
amount, and convert it to unbalanced. For
best operating performance, the internal
operating level of the ISS was selected to
be –6dBu. This level combines a good
signal to noise figure with ample peak
signal headroom for circuits operating with
±15Vdc power sources. Since television
broadcast facilities run different average
operating levels, the line input stage has
three switch selectable input levels: 0, +4,
or +8dBu. This translates to attenuating the
input signal by 6, 10, or 14dB.
Two sections of operational amplifier and a
3-position switch serve as the gain reduc-
tion section. The input signal is capacitor
coupled to the op amps. Two 100pf capaci-
tors in conjunction with the input resistors
provide a high frequency roll-off to limit the
chance of a radio frequency interference
problem. An op amp connected in a differ-
ential mode unbalances the signal. The
output of the unbalancing op amp is ca-
pacitor coupled to the ribbon cable bus for
use by other ISS cards. Resistor locations
are available on the I/O Card for the installa-
tion of input load resistors. In most cases
this is not required, or desirable, and the
card is manufactured with no load resistors
installed.
Line Output Stage: The purpose of the line
output stage is to take an unbalanced
signal with a nominal level of –6dBu, add
gain, and convert it to a balanced, differen-
tial type. This –6dBu signal is the final
output of the other ISS cards. The amount
of gain in this circuit is set by a 3-position
switch. The audio output stage uses a
modern, sophisticated electronically bal-
anced circuit to eliminate the need for an
audio transformer.
Signal enters the line output stage using an
op amp configured as a summing junction.
In this way, signals from more than one ISS
card can be combined. The summed signal
then connects to another op amp that
boosts the level by 6, 10, or 14dB. This level
increase has been switch selected to give a
nominal output level of 0, +4, or +8dBu.
After being boosted in level, the signal
connects to two op amps, one as an invert-
ing buffer and the other as one side of a
balanced line driver circuit. The inverted
signal connects to the other side of the
balanced line driver circuit. Each side of the
line driver uses an op amp to feed a set of
current boost transistors. To preserve the
low frequency response, no output capaci-
tors are used. This is one tough circuit—it is
capable of driving high signal levels into
150 ohms! The balanced line driver circuit
utilizes cross feedback to insure stability
even when driving unbalanced loads. This