Studio Technologies IFB Plus 2001 - Manual (Page 34)

Issue 7, November 2001 Model 2 User Guide
Page 34 Studio Technologies, Inc.
A capacitor connected to the output of the
rectifier sets the VOX release time. The
larger the capacitor value, the longer an
absence of voice-band energy is required
for the DC output to fall below the VOX
threshold. The DC voltage from the recti-
fier is connected to one section of inte-
grated circuit comparator. A reference
voltage, created by two resistors, sets
the comparator’s switching threshold. A
resistor from the output of the comparator
to the reference input provides positive
feedback, i.e., hysteresis. This adds stabil-
ity to the circuit, preventing minor varia-
tions in the DC input voltage from causing
rapid switching. Since the output of the
comparator is an open-collector type, a
pull up resistor is required. The output of
the comparator is a “voice present” logic
signal. It goes to the logic low state when-
ever a voice signal is detected. This logic
signal is connected to one section of the
VOX output select switch. The switch
connects the VOX logic signal to the
circuitry associated with the interrupt
logic, program muting, and status LEDs.
The “voice present” logic signal is also
connected to an inverter gate. The in-
verted signal controls the VOX audio
analog switch, mentioned previously. The
analog switch goes to the on state when-
ever an input audio signal is detected. The
output of the analog switch is routed by
the VOX output select switch to the inter-
rupt summing bus for either channel 1 or
channel 2.
Telephone Interfaces
The Model 2 contains two telephone
interface circuits. Although somewhat
similar, each has unique characteristics.
Interface 1 is designed to receive audio
from the outside world. Interface 2 can
receive audio, as well as send IFB channel
1 or 2 audio. In addition, interface 2 con-
tains circuitry to automatically answer a
ringing telephone line. Each interface
contains a mode switch, allowing the
interface to be set to the telephone line or
standard audio mode. The telephone line
mode optimizes the interface for connec-
tion to a standard loop start telephone
line. This mode is sometimes referred to
as “wet,” a telephone company term for
DC current flowing through a line. The
standard audio mode optimizes the inter-
face for connection to a circuit that does
not have DC current flowing in it. This
mode is sometimes referred to as “dry,”
a telephone company term for a line with
no DC current flowing through it.
In the telephone line mode, a relay con-
trols if the telco signal is connected to the
interface. In the standard audio mode, the
relay is continually energized. Both inter-
faces in the telephone line mode provide
a telephone line termination impedance
of approximately 700 ohms. In the stan-
dard audio mode, interface 1 provides a
termination impedance of approximately
2200 ohms. This allows an audio source
which uses an operational amplifier
output to correctly interface with the
Model 2. Interface 2 in the standard audio
mode presents a source impedance of
approximately 2200 ohms when sending
channel 1 or 2 audio, and an impedance
of approximately 700 ohms when receiv-
ing audio.
Now we’ll discuss interface 1 specifics.
The tip and ring of the telephone line
enters the interface via a protection circuit
consisting of a fuse, power resistor, and
360Vdc breakdown resistor (varistor). With
the Model 2, transients are definitely not
welcome. A sealed, bifurcated contact