Studio Technologies 2A 2015 - Manual (Page 8)

Issue 2, March 2015 Model 2A User Guide
Page 8 Studio Technologies, Inc.
Compressor Circuits
The two IFB channels contain studio-quality
compressor circuitry to control the dynamic
range of the interrupt audio. These play an
important role in how the Model 2A main-
tains high audio quality, specifically evening
out level variations presented by the talk
signals associated with various IFB users.
The compressors make talent cues more
intelligible and prevent abnormally high
signal levels from reaching user’s ears. The
resulting audio quality is very, very good.
Telephone Interfaces
The Model 2A contains two telephone
interfaces. Both interfaces can be used
to bring audio into the Model 2A from the
outside world. These two audio signals can
be independently assigned as program
sources for IFB channels 1 and 2, as well
as being used as an audio source for the
voice operated (VOX) interrupt function.
Each telephone interface has a receive-
level trim potentiometer that is accessible
via a small hole in the front panel. The large
level variations that can be presented by a
telephone line can make “on the fly” level
trimming a useful feature. In addition to re-
ceiving audio, telephone interface 2 can be
used to originate an IFB “feed” (IFB output).
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 telephone interfaces contain a unique
feature which allows two very different
types of telephone “lines” to be correctly
interfaced. Each interface can be indepen-
dently set to operate in either a telephone
line mode or a standard audio mode. A
telephone line has the profile of being a 2-
wire, DC-biased (normally –48 volts) circuit
provided by a local telephone company,
long-distance carrier, or private telephone
system. A standard audio signal could be
provided by, for example, a “dry” (no DC
voltage provided) fax adapter associated
with a cellular telephone.
When an interface is set to the telephone
line mode and a telephone line with DC
voltage is connected, full monitoring and
control is implemented. Each interface con-
tains a switch that allows the telephone line
to be answered (taken “off hook”) or hung
up (placed “on hook”). DC loop current is
monitored when the interface is active (off
hook). If a telco-provided disconnect signal
(a momentary break in loop current) is de-
tected the interface will automatically return
to its idle (on-hook) state. Telephone inter-
face 1 contains an LED status indicator that
lights whenever loop current is detected.
Telephone interface 2 has a status LED that
“flashes” when a ringing signal is detected
and lights continuously when loop current
is detected. Interface 2 also implements an
auto answer function which can automati-
cally take the telephone line to the answer
(off-hook) state when a ringing signal is
detected.
In many cases a “telephone line” in a mo-
bile broadcast application is actually pro-
vided by a cellular telephone. This cellular
telephone may provide a “dry” (no DC loop
current) audio output signal. 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
interface appears electrically as a trans-
former-coupled balanced audio interface.
When an interface has been set for the
standard audio mode its front-panel line
status switch is inactive as is its loop status
LED. Also inactive in the standard audio