Studio Technologies IFB Plus 2005 - Manual (Page 30)

Issue 8, December 2005 Model 2 User Guide
Page 30 Studio Technologies, Inc.
bifurcated contact relay controls the con-
nection of the telephone line to a 600 ohm
to 600 ohm coupling transformer. In series
with the center taps of the transformer is a
loop current detector. An LED, labeled LC,
indicates the presence of loop current. The
secondary of the transformer is connected
to the input of an operational amplifier which
is configured as a variable gain buffer. A trim
potentiometer allows the gain to be adjusted
from 4 to 20 dB.
Four sections of analog switch set the audio
flow through the interface. Control signals
for the analog switches are derived from
the front-panel audio routing switch, along
with some simple logic gates. One section
of analog switch controls the output of the
receive operational amplifier. It is active only
when the interface is in the receive mode.
This analog switch prevents channel 1 or 2
audio from “feeding back” to other Model
2 circuits when the interface is in the send
mode. The other three sections of analog
switch control the flow of send audio, as well
as adjusting the interface termination imped-
ance. The secondary of the transformer, via
a 1000 ohm resistor, connects to one of the
three remaining sections of analog switch.
In the standard audio mode this switch is al-
ways in the on state, keeping the 1000 ohm
resistor effectively connected across the
transformer’s secondary. This resistor, along
with the 2200 ohm resistor on the input of
the receive op-amp, provides a 700 ohm ter-
mination impedance. In the standard audio
mode this analog switch is active when the
routing switch is set to send channel 1 or
channel 2 audio. The two remaining analog
switches control if channel 1 or channel 2
audio is connected to the send driver opera-
tional amplifier.
The interface mode switch, located on the
back panel, controls two functions: relay sta-
tus and termination impedance. In the tele-
phone line mode the interface control switch
is active, allowing the operator to control the
relay. In the standard audio mode the switch
is disabled, 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, causing the relay
to energize via one gate of a power driver
integrated circuit. The output of the ring
voltage detector can also set the flip-flop.
If ringing voltage is detected for a sufficient
time period the resistor/capacitor circuit on
the input of the flip-flop is brought to the
logic low state, and the flip-flop is set. 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. Notice that there
is what appears to be a “bug” in this circuit
when in the telephone line mode. If the inter-
face control switch is moved to the manual
off-hook position, and then moved immedi-
ately to the hang-up position, the phone line
will not “hang up.” This is because approxi-
mately three seconds are required for the
ringing detection capacitor to recharge after
being discharged by the manual off-hook
switch action. If a pause of three seconds
occurs, or the interface control switch is
held in the hang-up position for about three
seconds, the telephone line will hang up.
Normal operation will not be affected by this
condition.