Audio Mixer / IFB Controller
Model 770
Audio Mixer / IFB Controller
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 electronicnews-gathering (ENG) vehicles, the Model 770 doesn’t sacrifice
features or 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 with an extensive set of features
and excellent overall audio performance, that still meets the
special needs of fast-paced news-gathering operations.
Model 770 Front Panel
Model 770
Audio Mixer / IFB Controller
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.
Mixer Section
Located in the center of the Model 770’s front panel, the
mixer section provides three mic/line inputs, two stereo
line inputs, and two monaural output buses. The features
were carefully selected to provide a balance between
technical performance and ease of use.
Mic/Line Inputs
Three transformer-coupled input channels are provided
for connection to microphone or line-level signals. Features 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 indication. Two push-button switches control the
routing of the input signal to the main and auxiliary
(AUX) audio buses. For convenience, the mic/line input
sensitivity buttons, like all of 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 to audio signals associated with
video playback. The left and right input signals, after
passing through individual level controls, are automatically mixed to monaural. This supports “real world”
applications where dialog may be on one channel and
background or “natural” sound on the other. Two buttons
control the routing of the monaural signal to the main and
AUX audio buses.
Main Audio Bus
Signals from the mic/line and stereo line input channels
combine to create the main audio bus. The main bus is
monaural, and includes a studio-quality limiter/compressor to control the dynamic range. Far from a simple
“clipper,” the limiter/compressor circuit utilizes a sophisticated laser-trimmed voltage-controlled-amplifier (VCA)
integrated circuit for quiet, low-distortion level control.
For installation flexibility, four output circuits provide
access to the main bus; three are electronically balanced,
one is transformer-balanced.
Studio Technologies, Inc.
AUX Audio Bus
Signals from the mic/line and stereo line input channels
can be directly assigned to the auxiliary (AUX) bus. The
AUX bus is monaural, and can be used for special applications such as a second bus for on-air use. Direct access to
the AUX bus is provided by means of two output circuits;
one is electronically balanced, the second is transformerbalanced.
Configurable Outputs
Three line-level outputs can serve in a wide range of
installation-specific applications. DIP switches, located
on the back panel, are used to select from five available
signal sources: main audio, AUX audio, IFB audio, IFB
program-only audio, and mic/line input channel direct.
The switches can be set to create a variety of output
configurations, such as additional main or AUX outputs,
a mix of main and AUX, additional IFB-related outputs,
or a “direct” mic/line output.
Support Functions
A push-button switch allows voice signals from the frontpanel-mounted gooseneck microphone to be routed to the
main and AUX audio buses. This “talk-back” function is
usually found only on larger audio consoles. A continuous
1kHz “0-level” sine-wave reference signal is available by
means of a connector on the back panel. The 1kHz signal
can also be assigned, by means of a front-panel pushbutton switch, to the main and AUX audio buses.
IFB Section
The IFB section provides a full set of resources to create
and maintain one channel of excellent IFB. A full set of
controls and indicators allows fast, simple operation. A
front-panel-mounted gooseneck microphone allows an
operator to make “local” interrupts. IFB audio level is
displayed by a 5-segment LED meter, while interrupt
status is displayed by an LED indicator.
Unique to the Model 770 is the ability to use each of the
IFB section’s seven audio sources as a program or an
interrupt source. A voice-operated (VOX) circuit monitors
the interrupt bus for signal activity. Whenever a signal
Model 770 Audio Mixer / IFB Controller
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, program audio can be
configured to either fully mute during interrupt activity,
or reduced in level (“dimmed”) by 10dB. While a full mute
is normally desired, the “dim” mode can serve special
applications, such as sports events.
adjustable using a trim pot located on the front panel. DIP
switches, located on the back panel, allow the selection of
main, AUX, or IFB audio as the send source. For operator
assistance, a push-button 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 initial IFB connections are being established with a remote facility.
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.
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 loopstart DC-biased (“wet”) tip-and-ring circuits. When set to
the standard audio mode, the interface is directly compatible with audio signals provided by many cellular or
satellite telephones. Becoming increasingly common for
IFB applications, these telephones often don’t provide
a “wet” interface, but do exhibit the audio levels of a
conventional telephone line.
Audio Sources
Seven audio signals can be selected to serve as program
or interrupt sources: the mixer section’s main and AUX
bus audio, 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 and
interrupt audio. Without external patching or connections,
the Model 770’s main and AUX audio buses can be directly
assigned as IFB 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
When the interface is set to the telco mode, full loop
current monitoring and control is implemented. A frontpanel 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 auto-answer 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.
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
Model 770 Back Panel
Model 770 Audio Mixer / IFB Controller
Studio Technologies, Inc.
belt pack modules from Studio Technologies. A single XLRtype 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.
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 levels
were carefully designed to eliminate the need for external
“pads,” isolation resistors, or blocking capacitors.
While the Model 770’s monitor section allows monitoring
of the IFB signal, for efficient broadcast operation it is often
imperative that the IFB signal be continuously monitored.
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 speaker dedicated
exclusively to IFB.
Monitor Section
The Model 770’s monitor section gives the operator a
powerful set of resources. Separate outputs and level
controls are provided for connection to 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 external 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 monitor both a local mix and an
externally provided mix-minus signal. A mechanical
VU-type meter provides a “user-friendly” indication
of audio-signal level. To limit the chance of burned-out
meter-scale illumination, the Model 770 implements a
solid-state, LED-based lighting scheme.
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 interconnections. An “RJ11-type” modular jack is
used by the telco interface. Mains power is factory configured for 100, 120, or 220/240V, 50/60Hz.
Model 770
Mic/Line: 3-pin XLR-type, female
Outputs — Main 4, AUX 2, IFB Line, and IFB Talent
Amplifier: 3-pin XLR-type, male
All Other Audio: ¼-inch 3-conductor phone jacks
Telco Interface: 6-position modular jack (RJ11-type)
AC Mains: 3-blade IEC-type
AC Mains Requirement:
100, 120, or 220/240V, ±10%, factory configured; 50/60Hz;
40 watts maximum
Dimensions (Overall):
19.00 inches wide (48.3cm)
3.49 inches high (8.9cm)
9.50 inches deep (21.4cm)
2 standard rack spaces
13.50 pounds (6.1kg)
Audio Mixer / IFB Controller
IFB Section
General Audio Parameters:
Audio Switching and Muting: “clickless” using special “ramping” analog
Frequency Response: 20Hz-20kHz, ±0.5dB
Distortion (THD+N): 0.04%, measured at 1kHz
S/N Ratio: 71dB, referenced to +4dBu
Voice Operated (VOX) Interrupt Function:
Detection Bandpass: 400 to 1400Hz, nominal
Detect Time: less than 1mSec
Limiter/Compressors: studio quality, dual slope
Line Inputs: 4
Type: electronically balanced
Nominal Level: +4dBu, adjustable ±8dB
Talent Amplifier Output:
Application: provides power and audio signals for up to four Model 32 or
Model 33 Talent Amplifiers
IFB Line Output:
Type: balanced, transformer-coupled
Nominal Level: +4dBu
Stereo Line Inputs: 2
Type: electronically balanced
Nominal Level: +4dBu
IFB Intercom Link Out: allows direct connection to RTS/Telex® intercom
Telephone Interface:
Operating Modes: selectable for use with telephone line or standard audio
Telephone Line Requirements: 2-wire, loop start, 10mA loop current
Telephone Line Disconnect: manual, using front-panel switch; automatic
after detection of break in loop current
Telephone Line Interface Control: switch on front panel allows manual
off-hook and manual hang-up functions
Auto Answer: answers after 2 rings (can be disabled)
Receive Audio Level: –15dBu, nominal, adjustable ±8dB
Send Audio Level: –6dBu, nominal
Main Audio Bus Limiter/Compressor:
Threshold: +10dBu, nominal (6dB above nominal output level)
Slope: 5:1, nominal
IFB Monitor Output:
Type: electronically balanced
Nominal Level: –4dBu
Outputs — Main 1, 2, 3, AUX 1:
Type: electronically balanced
Nominal Level: +4dBu
Maximum Level: +27dBu into 10k ohms, +26dBu into 600 ohms
Monitor Section
Main Output 4, AUX Output 2:
Type: balanced, transformer-coupled
Nominal Level: +4dBu
Maximum Level: +21dBu into 10k ohms
Headphone Output:
Compatibility: intended for connection to headphones with impedance of
100 ohms or greater
Maximum Voltage: 8Vpp, 100 ohm load
Talk to Main and AUX:
Connects gooseneck microphone to main and AUX mixing buses
External Monitor Inputs: 2, monaural
Type: electronically balanced
Nominal Level: +4dBu
Mixer Section
General Audio Parameters:
Frequency Response: 20Hz-20kHz, ±0.2dB
Distortion (THD+N): 0.04%, measured at 1kHz
S/N Ratio: 85dB, referenced to +4dBu
Mic/Line Inputs: 3
Type: balanced, transformer-coupled
Level Range: switchable for mic or line
Low-Cut Filter: –3dB at 85Hz, 18dB/octave slope
1kHz Tone Output (Sine Wave):
Type: electronically balanced
Output Level: +4dBu, nominal
Monitor Output:
Type: electronically balanced
Nominal Level: –2dBu
Type: analog, VU scale
Illumination: 7 LEDs
Specifications subject to change without notice.
© by Studio Technologies, Inc., March 1998
50300-398, Issue 4
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