Studio Technologies 76DBA User Manual

Model 770

Model 770 Audio Mixer/IFB Controller

User Guide

Issue 1, October 1997

This User Guide is applicable for serial numbers SN-00151 and later.

© 1997 by Studio Technologies, Inc., all rights reserved

5520 West Touhy Avenue

Skokie, Illinois 60077 U.S.A.

Telephone [phone]

Fax [phone]

www.studio-tech.com

50301-1097, Issue 1

Model 770

Table of Contents

Foreword

Introduction .................................................................. 7

System Features.......................................................... 8

Installation .................................................................... 11

Configuration ............................................................... 21

Operation ..................................................................... 28

Technical Notes ........................................................... 38

Specifications............................................................... 41

Block Diagrams

Model 770 User Guide Issue 1, October 1997
Studio Technologies, Inc. Page 3

Model 770

This page intentionally left blank.

Issue 1, October 1997 Model 770 User Guide
Page 4 Studio Technologies, Inc.

Model 770

Foreword

It was a great pleasure to have had the chance to design the Model 770, a unique product
for a specialized application. Our goal was to take a set of requirements that were essen­tially at odds with each other and combine them into a rational product. We had to make
the Model 770 meet a highly specific application, be flexible to configure and use, be
feature-rich yet easy to operate, have good sonic performance, with everything combined
in a compact and reliable package.

Many broadcast professionals contributed their time, expertise, and good ideas to help
shape the Model 770. Without their help the unit would definitely not be as tweaked out
as it is. A hearty thanks to all of you!

Many people at Studio Technologies contributed toward making the Model 770 a reality.
Fred Roeck did the mechanical design and prepared the production documentation.
Carrie Loving designed the graphics and provided engineering CAD support. Al Lux de­signed the printed circuit boards. Mitch Budniak, Ace Consulting Engineer, designed the
power supply and did several overall reality reviews. Joe Urbanczyk designed the com­puterized test routines for the Audio Precision System One and worked on agency approv­als. Larry Leviton wrote the software that drives the itty-bitty micro-controller in the telco
interface sectionall in a few hundred bytes of C! Minas Serbekian and his crew built the
prototype units, ensuring that the final design was "manufacturable."

Please contact me with your questions, comments, and suggestions. I can be reached by
voice at [phone], fax at [phone], or via E-mail at [e-mail].

Sincerely,
Gordon Kapes

President

Model 770 User Guide Issue 1, October 1997
Studio Technologies, Inc. Page 5

Model 770

This page intentionally left blank.

Issue 1, October 1997 Model 770 User Guide
Page 6 Studio Technologies, Inc.

Model 770

Model 770

Introduction

Now for the first time mobile broadcast
applications can have both an audio mixer
and an IFB controller in one compact,
highly integrated package. Designed
expressly for electronic-news-gathering
(ENG) vehicles, the Model 770 doesnt
sacrifice features and performance to
achieve its two rack-space size. Using its
experience in on-air and IFB audio,
Studio Technologies was able to design
a product that has an extensive set of
features to meet the special needs of
fast-paced news-gathering operations,
while maintaining excellent overall audio
performance.

The Model 770 consists of three main
sections: mixer, IFB, and monitor. Each
section performs in a manner which meets
or exceeds those provided by stand
alone products. In addition, by integrat­ing audio mixer and IFB functions into one
unit, capabilities are provided that have
previously been unavailable.

What This User Guide Covers

This User Guide is designed to assist you
when installing, configuring, and using the
Model 770 Audio Mixer/IFB Controller. A
limited amount of background technical
information is also provided. A packet
containing Model 770 schematic diagrams
is not shipped with each Model 770, but is
available from the factory upon request.

Figure 1. View of Model 770 Front Panel

Figure 2. View of Model 770 Back Panel

Model 770 User Guide Issue 1, October 1997
Studio Technologies, Inc. Page 7

Model 770

System Features

Mixer Section

Mic/Line Inputs

Three transformer-coupled input channels
are provided for connection to micro­phone or line-level signals. The circuitry
is ruggedized for reliability under tough
operating conditions. Features provided
for each input channel include selectable
input sensitivity, low-cut filter, and level
control. For operator assistance, a bi-color
LED provides signal present and peak
level indication. Two push-button switches
control the routing of the input signal to
the main and auxiliary (AUX) audio buses.
For convenience the three mic/line input
sensitivity buttons, like all the Model 770s
operator controls, are located on the front
panel.

Stereo Line Inputs

Two stereo line-level input channels are
provided and are intended for connection
with audio signals associated with video
playback. To support real world broad­cast applications the stereo inputs auto­matically mix, using individual level con­trols, to monaural. Two buttons are part of
each input channel, controlling the routing
of the signals to the main and AUX audio
buses.

voltage-controlled amplifier (VCA) inte­grated circuit for quiet, low-distortion
operation. For installation flexibility, four
output circuits provide access to the main
bus: three are electronically balanced; the
fourth is transformer-balanced.

AUX Output

The Model 770 provides a second monau­ral audio bus. The AUX bus allows special
applications to be easily supported, such
as creating a mix-minus feed or serving
as a second output for on-air use. Signals
from the mic/line and stereo line input
channels section can be assigned to the
AUX bus. Access to the AUX bus is pro­vided by means of one electronically
balanced and one transformer-balanced
output.

Configurable Outputs

Three monaural, electronically balanced,
line-level outputs are provided and can
serve in a wide range of installation-spe­cific applications. DIP switches, located
on the back panel, are used to select
from the five available signal sources:
main output, AUX output, IFB output, IFB
program only output, and mic/line input
channel direct. The DIP switches can be
set to provide additional outputs, create
a summed output from the sources, or to
provide direct outputs from the mic/line
input channels.

Main Output

Signals from the mic/line and stereo line
input channels combine to create a mon­aural signal, called the main output bus. A
studio-quality, limiter/compressor circuit is
provided to control the dynamic range of
the main audio bus. Far from a simple
clipper, the limiter/compressor circuit
utilizes a sophisticated laser-trimmed

Issue 1, October 1997 Model 770 User Guide
Page 8 Studio Technologies, Inc.

IFB Section

The IFB section provides an extensive set
of resources to create and maintain one
channel of excellent-quality IFB audio. A
full set of controls and indicators make
operation fast and simple. A front-panel­mounted gooseneck microphone allows
an operator to make local interrupts. IFB

Model 770

audio level is displayed by a 5-segment
LED meter, while interrupt activity is dis­played by an LED indicator.

Unique to the Model 770 is the ability to
use each of the IFB sections seven pos­sible audio sources as a program or an
interrupt source. A voice-operated (VOX)
circuit monitors the interrupt bus for signal
activity. Whenever a signal meets the
requisite parameters, it is connected to
the IFB output. Using a carefully designed
fast-attack/slow-release circuit results in
very good interrupt audio performance.
Whenever interrupt is active, the program
source can be configured to either fully
mute, or be reduced in level (dimmed)
by 10dB. While a full mute is normally
desired, the dim mode can serve special
applications, such as sports events.

The IFB section doesnt take a back seat
when it comes to audio quality. Sophisti­cated ramping analog switches are used
to ensure click-free audio. Studio-quality
limiter/compressor circuits serve the inter­rupt audio signal and the gooseneck
microphone. These circuits help to mini­mize level variations, making talent cues
more intelligible and reducing the risk of
abnormally high signal levels from reach­ing the talents ears.

Audio Sources

Seven audio signals can be selected to
serve as IFB program or interrupt sources:
the mixer sections main and AUX bus,
four external line-level inputs, and an
internal telephone interface. More than
one input can be simultaneously selected
to create a mix of signals for program or
interrupt audio. Without external patching
or connections, the Model 770s main and
AUX audio buses can be directly assigned
as program or interrupt sources. Four

external line-level inputs allow a variety of
audio sources to be connected. For assis­tance during installation, input level trim
pots, located on the back panel, allow the
nominal +4 input level to be adjusted
over a ±8dB range.

Telephone Interface

The Model 770 contains a telephone
interface that, using an 8-bit micro-control­ler integrated circuit, is powerful yet simple
to use. Overall, the telephone interface is
designed to receive audio signals for use
by the IFB section, or to send out audio
from the IFB or mixer sections. A front­panel switch selects if receive audio will
serve as a program or interrupt source,
or if the telco interface will send audio.
The receive level of the telephone interface
is adjustable using a trim pot located on
the front panel. DIP switches, located on
the back panel, allow the selection of main
bus, AUX bus, or IFB audio as the send
source. For operator assistance, a push­button switch allows voice signals from
the gooseneck microphone to be sent out
the telephone interface. This talk-to-telco
function is extremely useful, especially
when an IFB connection is initially being
established with a remote facility.

The telephone interface is unique in that it
allows two very different telephone lines
to be correctly interfaced, meeting the
needs of contemporary installations. The
two operating modes make the interface
compatible with telephone lines or stan­dard audio signals. When set to the telco
mode, the interface is compatible with
2-wire loop-start DC-biased (wet) tip­and-ring circuits. When set to the standard
audio mode, the interface is directly com­patible with audio signals provided by
many cellular or satellite telephones.

Model 770 User Guide Issue 1, October 1997
Studio Technologies, Inc. Page 9

Model 770

Becoming increasingly common for use
in IFB applications, these telephones often
dont provide a wet interface, but do
exhibit the audio levels of a conventional
telephone line.

When the interface is set to the telco
mode, full loop current monitoring and
control is implemented. A front-panel
switch allows the telephone line to be
seized (answered) or disconnected (hung
up). The interface will automatically hang
up if a telephone-company-provided
disconnect signal is received. An audible
signal and LED indicator are active when­ever ringing voltage is present. An auto­answer function is also provided.

When the interface is set to the standard
audio mode, the loop-current-specific
features are disabled, and the interface
operates as a transformer-coupled bal­anced audio input. The input level sensitiv­ity is optimized for use with telephone-type
audio signals.

The line output is transformer coupled,
with a nominal level of +4dBu. Well pro­tected from damage due to externally
generated signals, it is suitable for con­nection to devices remote from the Model

770. The intercom link output allows the
direct connection of IFB audio into broad­cast intercom systems. The output imped­ance and signal level was carefully de­signed to eliminate the need for external
pads, isolation resistors, or blocking
capacitors.

While the Model 770s monitor section
allows switch-selectable monitoring of the
IFB signal, for efficient broadcast opera­tion it is often imperative that the IFB
signal be monitored continuously. This is
why the IFB section provides a separate
line-level monitor output and associated
level control. With this output, the operator
can have a monitor loudspeaker dedi­cated exclusively to IFB.

Monitor Section

IFB Outputs

The IFB section contains four outputs:
talent amplifier, line, intercom link, and
monitor. The talent amplifier output is a
wet output that supports the connection
of belt pack modules from Studio Tech­nologies. A single XLR-type cable links
the talent amplifier output with up to four
Model 32 or Model 33 Talent Amplifiers.
The talent amplifier output provides DC
power, along with two audio channels.
The first audio channel provides IFB au­dio, while the second is configurable.
Using DIP switches, located on the back
panel, three audio sources are available:
the mixer sections main and AUX bus,
and IFB program only. This allows a broad
range of applications to be supported.

Issue 1, October 1997 Model 770 User Guide
Page 10 Studio Technologies, Inc.

The Model 770s monitor section provides
separate outputs, along with an associ­ated level control, to allow connection of
a monitor amplifier and headphones. In
addition to monitoring the main, AUX,
and IFB audio buses, provision has been
made for monitoring two external audio
sources. These monaural inputs are in­tended to be connected to off-air, micro­wave, or satellite receivers.

Multiple sources can be simultaneously
selected for monitoring. This can prove
useful by allowing, for example, an opera­tor to simultaneously monitor a local mix
and an externally provided mix-minus
signal.

A mechanical VU-type meter provides a
user-friendly indication of audio-signal

Model 770

levels. Fanatics for addressing the details,
Studio Technologies has even eliminated
the issue of burned-out meter-scale illumi­nation by implementing a solid-state, LED­based lighting scheme.

A 1kHz sine-wave signal is provided for
alignment and reference use. While the
1kHz tone can be assigned to the main
and AUX output buses, a direct line-level
output is also provided. This allows the
1kHz tone to be used as a setup or test
signal without interfering with normal
operation of the main and AUX audio
buses.

Talk Back

The Model 770 includes a talk back
function. A push-button switch allows
audio from the gooseneck microphone
to be routed to the main and AUX outputs.

Mounting, Connectors, and Mains
Power

While the Model 770 is loaded with fea­tures, it only requires two spaces in a
standard 19-inch rack. XLR-type and
¼-inch 3-conductor audio connectors are
used for audio interconnection. A standard
modular (RJ11) telephone jack is used by
the telco interface. Mains power is factory
configured for 100, 120, or 220/240V,
50/60Hz.

Installation

During normal operation, no access to the
back panel of the Model 770 is required.
However, there are six trim potentiometers
that may need to be adjusted prior to the
Model 770 going into service. Refer to the
Configuration section of this guide for
details.

Locating the Unit

Physical access and mechanical noise
are the primary factors when choosing a
mounting location. By its very nature as
an audio mixer and IFB controller, an
operator will need easy access to all the
controls and the gooseneck microphone.
The gooseneck microphone is located on
the left side of the front panel so be careful
that there isnt a cooling fan blowing
directly onto it. Also, a good sight line to
the VU meter, located on the right side
of the front panel, is important.

As a device that contains high gain audio
stages, hum and noise pickup is possible
through the chassis and associated ca­bling. Locating the unit away from devices
that radiate strong AC fields is an excellent
idea.

While not likely, it is possible that the
Model 770s toroidal power transformer
may radiate energy into adjacent equip­ment. It is located at the right, bottom rear
corner of the unit, and may need to be
physically isolated from sensitive equip­ment, especially videotape recorders.

In this section you will be installing the
Model 770 Audio Mixer/IFB Controller in
an equipment rack. Input and output
connections for the mixer, IFB, and moni­tor sections will be made using the Model
770s multitude of jacks. AC mains power
will then be connected.

Model 770 User Guide Issue 1, October 1997
Studio Technologies, Inc. Page 11

Mounting

The Model 770 is intended for mounting in
a standard 19-inch equipment rack, requir­ing 3.5 inches vertically (two standard rack
spaces). It weights approximately 13.5
pounds (6.1kg), and operates on mains
power of 100, 120, or 200/240V, 50/60Hz.

Model 770

The unit is secured to the front mounting
rails of an equipment rack using two
mounting screws per side.

Connections

The Model 770s input, output, and
telco connections are made using 3-pin
XLR-type male and female connectors,
¼-inch 3-conductor phone jacks, and one
6-position modular telephone-type jack.

Caution: For reliable audio intercon­nection, the phone plugs you use must
comply with industry-standard RS-453.
Switchcraft No. 297 or Neutrik NP3C
are highly recommended.

Mixer Section

Mic/Line Inputs

The Model 770 contains three input chan­nels that are compatible with microphone
or line-level sources. Each of the three
Mic/Line inputs use a single 3-pin female
XLR-type connector. Prepare the mating
connectors (males) so that pin 2 is signal
high (+ or hot), pin 3 is low ( or cold),
and pin 1 is shield.

It is expected that the mic/line inputs will
be wired to an input/output connector
panel located on the side or rear of the
vehicle. Its also nice to have one or two
of the inputs multed to connectors inside
the vehicle, allowing the rapid connection
of microphone or line-level signals. To
minimize noise and other problems, it is
recommended that the mic/line signals
are not wired by way of points on an audio
patch bay.

Stereo Line Inputs

connection to sources located within
the vehicle, specifically the audio output
channels of videotape machines. Note that
the left and right inputs associated with the
stereo line inputs are always mixed, by
way of level controls, to monaural. The
electronically balanced input circuitry has
extremely low noise and high common­mode rejection, but no extra protection
from overvoltage or static. (Dont get us
wrongthe stereo line input circuitry is
better than whats found on most audio
mixers available todaytheyre just not as
tough as the mic/line inputs!) The stereo
line input circuitry is intended for connec­tion to balanced or unbalanced, line-level
signals with a nominal operating level of
+4dBu.

Each stereo line input channel uses two
¼-inch 3-conductor phone jacks. Prepare
the mating connectors (plugs) so that tip
is signal high (+ or hot), ring is low ( or
cold), and sleeve is shield. With an unbal­anced source connect the tip to high
(+ or hot), and both the ring and sleeve
to shield. If connecting 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 flexibility of use it is recom­mended that the stereo line input con­nections be routed through points on a
patch bay.

Main OutputsGeneral Information

The Model 770 contains four outputs
associated with the main audio bus. Main
Outputs 1, 2, and 3 are electronically
balanced, have a nominal level of +4dBu,
and are capable of driving balanced or
unbalanced loads of 600 ohms or greater.

The Model 770 contains two stereo line­level input channels. They are intended for

Issue 1, October 1997 Model 770 User Guide
Page 12 Studio Technologies, Inc.

Model 770

Note: While the electronically balanced
output circuits are capable of driving
loads of 600 ohms or greater, the out­put level will drop slightly as the load
impedance approaches 600 ohms. A

0.5dB difference in output level can
be expected as the load impedance
changes from 10k ohms to 600 ohms.
This applies to all of the electronically
balanced output circuits provided on
the Model 770.

Main Output 4 is transformer balanced,
with a nominal level of +4dBu. It is ca­pable of driving full level into a balanced
or unbalanced load of 2k ohms or greater.

Main Outputs 1, 2, and 3

Main Outputs 1, 2, and 3 are intended for
connection only to devices located within
the vehicle. It is recommended that either
Main Output 1, 2, or 3 be used as the on­air source. Each output utilizes a ¼-inch
3-conductor phone jack for interconnec­tion. Prepare the mating connectors
(plugs) so that tip is signal high (+ or hot),
ring is low ( or cold), and sleeve is shield.
Whether these outputs are wired via a
patch bay will depend on the specific
installation.

While balanced operation is preferred,
unbalanced operation is not a problem
for the output circuitry. To connect to an
unbalanced load connect the tip to high
(+ or hot), and both the ring and sleeve
to shield. For optimal unbalanced opera­tion, it is important to connect both ring
and sleeve together directly on the plug
that mates with the Model 770, and not at
the other end of the cable.

Main Output 4

Main Output 4 has been optimized for
connection to devices located external
to the vehicle. This ruggedized, trans­former-coupled output has a nominal level
of +4dBu. A 3-pin male XLR-type connec­tor is used for interconnection. Prepare the
mating connector (female) so that pin 2
is high (+ or hot), pin 3 is low ( or cold),
and pin 1 is shield. It is recommended that
Main Output 4 be wired, via a patch bay,
to the vehicles input/output connector
panel.

Main Output 4 was specifically designed
to provide a fully isolated output with good
sonic performance. It was NOT designed
to drive full level into 600 ohm loads! While
a 600 ohm load can be connected without
damage, a 2dB level drop should be
expected. Used with 2k ohm or greater
loads, Main Output 4 will provide its full
specified performance.

AUX OutputsGeneral Information

The Model 770 contains two outputs
associated with the AUX audio bus. AUX
Output 1 is electronically balanced, has a
nominal level of +4dBu, and is capable of
driving balanced or unbalanced loads of
600 ohms or greater. AUX Output 2 is
transformer balanced, with a nominal level
of +4dBu. It is capable of driving full level
into a balanced or unbalanced load of 2k
ohms or greater.

AUX Output 1

AUX Output 1 is intended for connection
to devices located only within the vehicle.
The output utilizes a ¼-inch 3-conductor
phone jack for interconnection. Prepare
the mating connector (plug) so that tip is
signal high (+ or hot), ring is low ( or
cold), and sleeve is shield. This output

Model 770 User Guide Issue 1, October 1997
Studio Technologies, Inc. Page 13

Model 770

should be wired via a patch bay to allow
ready access.

While balanced operation is preferred,
unbalanced operation is not a problem
for the output circuit. To connect to an
unbalanced load connect the tip to high
(+ or hot), and both the ring and sleeve to
shield. For optimal unbalanced operation,
it is important to connect both the ring and
sleeve together directly on the plug that
mates with the Model 770, and not at the
other end of the cable.

AUX Output 2

AUX Output 2 has been optimized for
connection to devices located external
to the vehicle. This ruggedized, trans­former-coupled output has a nominal level
of +4dBu. A 3-pin male XLR-type connec­tor is used for interconnection. Prepare the
mating connector (female) so that pin 2 is
high (+ or hot), pin 3 is low ( or cold),
and pin 1 is shield. It is recommended that
AUX Output 2 be wired, via a patch bay, to
the vehicles input/output connector panel.

AUX Output 2 was specifically designed
to provide a fully isolated output with good
sonic performance. It was NOT designed
to drive full level into 600 ohm loads! While
a 600 ohm load can be connected without
damage, a 2dB level drop should be
expected. Used with 2k ohm or greater
loads, AUX Output 2 will provide its full
specified performance.

Configurable Outputs

Each of the three configurable outputs
is electronically balanced, has a nominal
level of +4dBu, and is capable of driving
balanced or unbalanced loads of 600
ohms or greater. The outputs are intended
for connection to devices located only

within the vehicle. DIP switches are associ­ated with each configurable output. Details
on using these switches can be found in
the Configuration section of this guide.

Each of the configurable outputs is acces­sible via a ¼-inch 3-conductor phone jack.
Prepare the mating connectors (plugs) so
that tip is signal high (+ or hot), ring is low
( or cold), and sleeve is shield. To con­nect to an unbalanced load connect the
tip to high (+ or hot), and both the ring
and sleeve to shield.

It is highly recommended that the config­urable outputs be routed via a patch bay.
Or, if there are no immediate needs for
these outputs, be certain to terminate
them on a patch bay for future use.

IFB Section

IFB Line Inputs

The Model 770s IFB section allows four
line-level audio sources to be connected.
These sources can serve as program or
interrupt audio signals, their function being
selectable by the operator using front­panel controls. The direct-coupled, elec­tronically balanced input circuitry is
intended for connection to sources lo­cated within the vehicle. The nominal input
level is +4dBu, but a trim potentiometer
associated with each input gives a ±8dB
input sensitivity range. The Configuration
section of this guide has details on using
the trim pots.

Each of the four IFB line inputs use a
¼-inch 3-conductor phone jack for inter­connection. Prepare the mating connec­tors (plugs) so that tip is signal high (+ or
hot), ring is low ( or cold), and sleeve is
shield. With an unbalanced source con­nect the plugs tip to high (+ or hot), and

Issue 1, October 1997 Model 770 User Guide
Page 14 Studio Technologies, Inc.

Model 770

both the ring and sleeve to shield. If con­necting 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 flexibil­ity 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 back­ground 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 well­known 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 tele­phone-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 interconnect­ing cords have only two or four of the six
positions implemented. For this reason a

Model 770 User Guide Issue 1, October 1997
Studio Technologies, Inc. Page 15

Model 770

6-conductor jack, along with a 6-conduc­tor cord, is included with each Model 770.
(However, the reality is that in most Model
770 applications, access to four of the six
contacts in the interfaces jack is sufficient.
Using a 4-conductor jack and cable will
provide access to the telco/standard input
and the wet/standard audio mode select
function. The last two contacts in the telco
interfaces 6-conductor jack allows the
auto-answer function to be disabled, a
condition that is less frequently required.)

Now down to the nitty gritty of making
connections! A 6-position modular jack is
most often organized as three pairs. Well
use this terminology in our explanation.
Pair 1, pins 3 and 4, of the 6-position
modular jack are used to connect to the
telco or standard audio signal. Pair 2, pins
2 and 5, of the jack are used to control
the input mode; shorting pins 2 and 5
together places the telco interface in the
standard audio mode. Pair 3, pins 1 and
6, of the jack are used to control the auto­answer mode; shorting pins 1 and 6
together causes the interface not to auto­matically answer when a ringing tele­phone-line signal is present. A typical
6-position modular jack will have the
following terminal colors:

Pin No. Wire Color Alternate Color

1 White W-GN
2 Black W-OR
3 Red BL-W
4 Green W-BL
5 Yellow OR-W
6 Blue GN-W

The question that you may be rightfully
thinking is: Why doesnt the Model 770
simply have switches to select between
telco and standard audio mode, and to
defeat the auto answer function? This

would be adequate if only one source was
going to be connected to the telco inter­face, but thats not the reality for most
broadcast settings. A number of telco
signals may be available in a single ve­hicle, each requiring the ability to be
connected to the Model 770s telco inter­face at a moments notice. By terminating
these various signals onto 6-position
modular jacks, or a modular-jack patch
panel, they can be quickly patched into
the Model 770s telco interface. By con­necting jumper wires across pins 2 & 5
and 1 & 6 of the source jacks as required,
the telco interface will automatically config­ure to the correct operating mode as that
source is connected.

If you are going to use a modular-jack
patch panel, be careful to select one
that has all six contacts available in
each jackmany will only provide
access to two or four.

When selecting your telco sources it
may be useful to review the character­istics of the Model 770s telco interface.
When set to the telephone-line mode, a
DC-biased signal (often 48V), with
high-voltage ringing, is expected to be
connected. The telco interface is trans­former-isolated, and very well protected
from damage due to static and transients.
It has a nominal audio impedance of 600
ohms to match the requirements of a
normal telephone line. When set to the
standard audio mode, no DC ringing
voltage is expected. Transformer-coupling
is still present, as is the 600 ohm imped­ance. The receive and transmit audio
levels are still matched to the require­ments of telephone devices, not the much
hotter +4dBu professional audio level!

Issue 1, October 1997 Model 770 User Guide
Page 16 Studio Technologies, Inc.