Studio Technologies 770 - Manual (Page 15)

Model 770 User Guide Issue 1, October 1997
Studio Technologies, Inc. Page 15
Model 770
both the ring and sleeve to shield. If con-
necting to an unbalanced source in this
manner results in hum or noise, connect
tip to high (+ or hot), and ring to shield;
leave the sleeve unterminated. For flexibil-
ity it is recommended that at least one of
the IFB line input connections be routed
through a patch bay.
Telephone Interface
The Model 770’s IFB section contains a
sophisticated telephone interface which
allows connection to a telephone line or
standard audio signal. Using the telco
interface, an audio signal can be brought
in for use as a program or interrupt
source. The telco interface can also be
used to send out audio signals. We define
a telephone line as a signal having a DC
bias current associated with it. This type of
signal is sometimes referred to as “wet.”
We define a standard audio signal as one
that is isolated from any source of DC
voltage; sometimes being referred to as a
“dry” signal. While this sounds confusing,
contemporary applications sometimes
refer to both types of signals as “telco.”
To a large production vehicle that pulls up
to do a two-week golf tournament, a telco
line is “wet,” being provided by the local
telephone company on an RJ11 jack. To
a mobile ENG vehicle, a telco signal might
come from a cellular or satellite telephone
which often provides a “dry” audio signal.
A unique means of interfacing signals to
the telco interface is utilized. While most
people would consider that the Model 770
has an RJ11 jack on its back panel, it’s
actually a 6-position modular jack with all
6 positions used for electrical connections;
two are used to connect to the telephone
line or standard audio signal, two to select
telco or standard audio mode, and two to
defeat the auto-answer function. This
connection method wasn’t selected to
make life difficult, but to allow a range of
signal types to be connected to the telco
interface, while simultaneously having the
interface’s operating mode automatically
selected. This eliminates the need for
operating mode selection switches from
being located on the front or back panel.
The next paragraphs detail how this
works.
To show you “where we’re coming from,”
it may be helpful to provide some back-
ground information on what most people
refer to as an “RJ11” jack. The term RJ
comes from the connection section of Part
68 of the United States’ FCC rules, created
in the late 1970’s. A number of RJ-types
are specified, including RJ11, as well as
others such as RJ12, RJ21, and RJ48.
The different RJ-types specify in great
detail the authorized mechanical and
electrical means for telephone companies
to provide service to customers. The well-
known RJ11 actually refers to an analog
telephone line’s tip and ring connections
being terminated on pins 3 and 4 of a
6-position modular jack; pins 1, 2, 5, and
6 are not utilized and do not have to be
physically present. So the physical tele-
phone-type jack that most people refer
to as an RJ11 is really just a 6-position
device with an unspecified number of
contacts inserted. Our discussion ends
with this fact: The jack used by the Model
770’s telco interface is similar to an RJ11,
but uses all 6-positions for connections.
To access all the functions of the Model
770 telco interface requires a 6-conductor
modular jack and cord. But most “real
world” telephone jacks and interconnect-
ing cords have only two or four of the six
positions implemented. For this reason a