Studio Technologies IFB Plus 2005 - Manual (Page 31)

Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 31
Access Station Interface
The Model 2 contains interface circuitry to
allow up to four Model 22 Access Stations to
be connected. Four types of signals are sent
to and received from the access station: in-
terrupt audio, lamp voltage, control signals,
and +18 Vdc power.
Two identical circuits connect access station
interrupt audio into the Model 2. Each circuit
capacitive couples audio into an operational
amplifier configured as an inverting unity
gain buffer. The output of each op-amp
is connected to one section of SSM-2404
analog switch, part of the interrupt audio
routing circuit. This analog switch produces
a click-free joining of access station audio to
the IFB channels.
Adjustable voltage regulator integrated
circuits are used to provide power for the
pushbutton switch’s status (a.k.a. tally)
lamps located on the access stations. These
regulators were selected because of their
ability to be controlled by a logic signal,
along with their inherent over-current protec-
tion.
Three logic signals are ready to be con-
trolled by the Model 22 Access Stations:
channel 1 interrupt control, channel 2 in-
terrupt control, and monitor mute control.
These CMOS-type logic signals are active
low. Pull up resistors maintain a logic high,
as well as providing sufficient current for the
access stations to function correctly. Se-
ries resistors and shunt capacitors provide
overvoltage protection when interfacing the
nasty “outside world” signals with the Model
2’s logic gates. Static “zaps” of 8000 volts
shouldn’t damage the Model 2—but please
don’t try to prove us wrong!
Filtered and regulated +18 Vdc provides
power for the access stations. The +18 Vdc
is created by a 3-terminal, integrated circuit
regulator which provides over-current pro-
tection. This feature is especially important
as access station cable shorts may be quite
common during installation and testing.
Talent Amplifier Output
Power, IFB channel 1 audio, and IFB chan-
nel 2 audio are provided by the talent am-
plifier output. A low-noise, low-distortion
modulator circuit superimposes channel 1
audio onto DC power. The circuit uses
+26 Vdc from the power supply section
to create a +22 Vdc with channel 1 audio
output. The nominal audio level is –10 dBu,
with over 20 dB of headroom available. The
circuit has over-current protection so that
a shorted cable will not damage the output
circuitry. Another section of operational
amplifier is used to drive channel 2 audio to
the outside world. A resistor and capacitor in
series with the output protects the op-amp
from a short circuit, as well as an accidental
shorting of the power/channel 1 audio con-
nection to the channel 2 connection.
Line Outputs
Each IFB channel has a balanced line-level
output associated with it. The two circuits
are identical. Composite IFB audio at the
internal operating level of –10 dBu is boost-
ed 8 dB by one section of operational ampli-
fier. The output of the op-amp is connected
to a differential line driver integrated circuit.
This integrated circuit provides an elec-
tronically balanced, low impedance output
signal. It is capable of driving high signal
levels into 600 ohms or greater. Because of
the nature of a differential driver, an effective
6 dB boost in signal level is achieved, giv-
ing a nominal output level of +4 dBu. For
enhanced reliability the output pins of the
integrated circuit are capacitor coupled to
the output connectors. Shorting one side