Studio Technologies IFB Plus 2007 - Manual (Page 7)

Model 2 User Guide Issue 9, January 2006
Studio Technologies, Inc. Page 7
They feature low noise, low distortion, and
high common mode signal rejection. Each
program input has a trim potentiometer
associated with it. The trim pots, accessible
from the back panel, allow the nominal
+4 dBu input level to be adjusted over a
±8 dB range.
Program audio is muted whenever an inter-
rupt takes place. A sophisticated analog
switch is used to give a noise-free mute with
absolutely no clicks or pops! If desired, a
program “dim,” rather than a full mute can
be implemented by adding two resistors to
the Model 2’s circuit board.
Compressor Circuits
Each IFB channel contains a studio-quality
compressor circuit. This serves to even out
variations in the interrupt audio signals and
smoothly control peak signal levels. They
make talent cues more intelligible and prevent
abnormally high signal levels from reaching
a user’s ears. The resulting audio quality is
very, very good.
Telephone Interfaces
The Model 2 contains two telephone inter-
faces. Both interfaces can be used to bring
audio into the Model 2 from the outside world.
These two audio signals can be indepen-
dently assigned as program sources for IFB
channels 1 and 2, as well as being used as
an input source for the voice operated (VOX)
interrupt function. Each telephone interface
has a receive level trim potentiometer that is
accessible via access holes in the front panel.
The large variations one finds in telephone
line signal levels make “on the fly” level trim-
ming a useful feature. In addition to receiving
audio, telephone interface 2 can be used to
originate an IFB feed. A switch selects if audio
will be received from the outside world, or if
audio from IFB channel 1 or 2 will be sent out
the interface.
The two telephone interfaces contain a
unique feature which allows two very differ-
ent types of telephone “lines” to be correctly
interfaced. Each interface can be indepen-
dently set to operate in either the telephone-
line mode or the standard-audio mode. A
telephone line has the profile of being a
2-wire, DC-biased (normally –48 Vdc) circuit
provided by a local telephone company. A
standard audio signal could be provided by,
for example, a fax adapter associated with a
cellular telephone.
When an interface is set to the telephone-
line mode and a DC-biased telephone line
is connected, full monitoring and control
is implemented. Each interface contains
a switch that allows the telephone line to
be seized (taken off hook) or hung up.
Loop current is monitored when the inter-
face is off hook. If a disconnect signal (a
momentary break in loop current) is detect-
ed the interface will automatically return to
the on-hook state. Each interface contains
an LED indicator that lights whenever loop
current is detected. Interface 2 also imple-
ments an auto answer function, automatical-
ly taking the telephone line to the off-hook
state when ringing voltage is detected.
In many cases a “telephone line” is actually
provided by a cellular telephone. This cellu-
lar telephone may provide an unbiased (no
DC loop current) audio output. The standard
audio mode was designed expressly to
interface with this “cell phone” arrangement.
In this mode, the interface’s loop current-
specific features are disabled, and the in-
terface appears electrically as a transformer
coupled balanced audio input.
When in the standard audio mode, an inter-
face’s front-panel switches are inactive. Also
inactive is channel 2’s auto answer function.
An LED associated with each interface