Studio Technologies IFB Plus 2005 - Manual (Page 29)

Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 29
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 standard
audio mode, interface 1 provides a termi-
nation impedance of approximately 2200
ohms. This allows an audio source which
uses an operational amplifier output to cor-
rectly 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 receiving audio.
Now we’ll discuss interface 1 specifics. The
tip and ring of the telephone line enters the
interface via a protection circuit. With the
Model 2, transients are definitely not wel-
come. A sealed, bifurcated contact relay
controls the connection of the telephone line
to a 600 ohm to 600 ohm coupling trans-
former. In series with the center taps of the
transformer is a loop current detector. This
circuit is quite interesting, providing both de-
sensitization and over-current protection for
the optocoupler that actually detects current
flow. The output of the optocoupler is con-
ditioned by two resistors and a capacitor to
provide “debounce” of the loop current sta-
tus signal. An LED, labeled LC, indicates the
presence of loop current. The secondary of
the transformer is connected to one section
of operational amplifier which is configured
as a variable gain buffer. A trim potentiom-
eter allows the gain to be adjusted from 4 to
20 dB. This processes the telephone line au-
dio to get it to the Model 2’s nominal internal
level of –10 dBu.
The interface mode switch controls two
functions: relay status and termination
impedance. In the telephone line mode,
the interface control switch located on the
front panel is active, allowing the operator to
control the relay. In the telephone line mode,
a 1000 ohm resistor is placed across the
secondary of the transformer. This, along
with the op-amp’s 2200 ohm input resistor,
provides the correct termination impedance
for a standard telephone line. In the stan-
dard audio mode, the 1000 ohm resistor is
not connected to the circuit, and the 2200
ohm input impedance of the op-amp’s input
resistor provides the input impedance. In
the standard audio mode, the switch is dis-
abled, the relay is held energized, and the
LED labeled STD is lit.
A simple logic circuit implements the tele-
phone interface control function. Two NAND
gates form a flip-flop, the two states being
on-hook and off-hook. In the telephone line
mode, pressing the interface control switch
to the manual off-hook position sets the
flip-flop to the off-hook state, and causing
the relay to energize via one gate of a power
driver integrated circuit. If loop current is
detected the circuit will stay in the off-hook
state. If current is not detected the circuit
will time out and return to the on-hook state.
Pressing the interface switch to the manual
hang-up position forces the circuit to the
on-hook state. In the standard audio mode
the logic circuitry is effectively disabled, the
relay is continually energized, and the LED
labeled STD is lit.
Now we’ll discuss interface 2 specifics. The
tip and ring of the telephone line enters the
interface via a protection circuit and then to
an optical coupler-based ring voltage de-
tection circuit. It provides a logic signal to
indicate when the line is “ringing.” A sealed,