Studio Technologies IFB Plus 2005 - Manual (Page 7)

Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 7
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 inde-
pendently 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 potentiome-
ter 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 trimming a useful feature. In addi-
tion 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 chan-
nel 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 dis-
plays when the standard audio mode is se-
lected. Even during operation in this mode,
the loop current LEDs remain active, serving
as a useful diagnostic tool. They will display
if your “standard audio signal” is actually a
telephone line!
Voice Operated (VOX) Interrupt
The Model 2 contains circuitry to allow an
audio signal to serve as both an interrupt
audio source and a control signal. This
eliminates the need for a separate push-
to-talk button or contact closure. The VOX
feature allows an audio signal from a remote
source, such as a two-way radio or tele-
phone line, to serve as the interrupt source.
The VOX function was optimized for detec-
tion of audio signals in the voice band. As
voice detection is quite tricky to perform,
great care was taken when designing this
function.