Studio Technologies 374A User manual

Copyright © 2021 by Studio Technologies, Inc., all rights reserved
studio-tech.com
50658-0321, Issue 5
User Guide
Issue 5, March 2021
This User Guide is applicable for serial numbers
M374A-04001 and later with application firmware 2.1 and later
Model 374A Intercom Beltpack

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Model 374A User Guide Issue 5, March 2021
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Table of Contents
Revision History ............................................................ 4
Introduction ................................................................... 5
Getting Started .............................................................. 8
Operation ...................................................................... 14
Technical Notes ............................................................. 20
Specifications ................................................................ 24
Appendix A .................................................................... 25

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Revision History
Issue 5, March 2021:
1. Documents addition of remote control inputs.
Issue 4, February 2021:
1. Documents expansion of Call Indication function.
Issue 3, August 2019:
1. Documents revision to STcontroller (version 2.02.00 and later) which separates
microphone power and microphone gain configuration.
Issue 2, April 2019:
1. Documents new receive and send call function.
Issue 1, June 2018:
1. Initial release.

Model 374A User Guide Issue 5, March 2021
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INTERCOM BELTPACK
Introduction
The Model 374A Intercom Beltpack
begins with the features offered by tradi-
tional party-line intercom user devices and
adds a range of new capabilities along
with the advanced performance and flex-
ibility that Dante® audio-over-Ethernet
provides. With four independent talk and
listen channels the Model 374A bridges
the gap between typical single- and dual-
channel party-line devices and permanent-
ly installed multi-channel intercom panels.
Imagine the possibilities—four channels
of high-quality party-line intercom in a
compact, user-worn package.
Over a standard IP network, multiple
Model 374A units can be used in party-line
(PL) intercom applications with help from
an external Dante-enabled audio matrix
such as the Studio Technologies’ Model
5421 or Model 5422A Dante Intercom
Audio Engine units. Or, units can be used
“point-to-point” or interfaced with Dante-
compatible matrix intercom systems.
Only a single Power-over-Ethernet (PoE)
connection is required for operation. Key
user features can be easily configured us-
ing the STcontroller software application.
Configurable parameters include electret
microphone powering, microphone pre-
amplifier gain, talk button operation, call
receive indication, remote control inputs,
and headphone channel assignment. User
features include integrated sidetone, re-
mote mic kill (“talk off”), call receive and
send, and audio monitor-only modes. The
wide range of capabilities, along with the
excellent audio quality provided by the digi-
tal audio signal path, offers a unique and
powerful user experience.
Set up and configuration of the Model 374A
is simple. An etherCON® RJ45 receptacle
is used to interconnect with a standard
twisted-pair Ethernet port associated with
a local-area network (LAN). This connec-
tion provides both power and bidirectional
digital audio. A broadcast or intercom-style
stereo or monaural headset with a dynamic
or electret (DC-powered) microphone
interfaces with the unit using a 5-pin XLR
connector. A 3-conductor 3.5 mm jack al-
lows connection of two external switches
or contact closures to activate two selected
talk channels.
The STcontroller software application is
used to select the unit’s operating param-
eters. Four talk pushbutton switches can
be configured for optimal operation. Four
push-in/push-out (“pop out”) rotary con-
trols make it easy to set and maintain the
desired headphone output level. The Model
374A’s enclosure is made from an alumi-
num alloy which offers both light weight
Figure 1. Model 374A Intercom Beltpack top and bottom views

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and ruggedness. A stainless steel “belt
clip,” located on the back of the unit,
allows direct attachment to a user’s
clothing.
The audio quality of the Model 374A’s
four audio channels is excellent, with low
distortion, low noise, and high headroom.
Careful circuit design and rugged com-
ponents ensure long, reliable operation.
A wide range of applications can be sup-
ported, including sports and entertain-
ment TV and radio events, streaming
broadcasts, corporate and government AV
installations, and post-production facilities.
Dante Audio-over-Ethernet
Audio data is sent to and received from
the Model 374A using the Dante audio-
over-Ethernet media networking technol-
ogy. As a Dante-compliant device, the
Model 374A’s four audio output (Dante
transmitter) and four audio input (Dante
receiver) channels can be interconnected
(routed) with other devices using the
Dante Controller software application. The
Dante transmitter and receiver channels
are limited to supporting four Dante flows,
two in each direction. The digital audio’s
bit depth is up to 24 with a sampling rate
of 48 kHz. The Model 374A is AES67
compatible and compliant with the Dante
Domain Manager™ software application.
Two bi-color LEDs provide an indication
of the Dante connection status. The Dante
Controller’s Identify command takes on a
unique role with the Model 374A. Not only
will it cause the talk button LEDs to light
in a unique highly visible sequence, it can
also be configured to turn off any active
talk channels.
Audio Quality
The Model 374A’s completely “pro”
performance is counter to the less-than-
stellar reputation of typical party-line
(PL) intercom audio. A low-noise, wide
dynamic-range microphone preamplifier
and associated voltage-controlled-am-
plifier (VCA) dynamics controller (com-
pressor) ensures that microphone audio
quality is preserved while minimizing the
chance of signal overload. DC power
to support electret microphones can be
enabled if necessary. The output of the
microphone preamp and compressor is
routed to an analog-to-digital converter
(ADC) section that supports a sampling
rate of 48 kHz with a bit depth of up to 24.
The audio signal, now in the digital do-
main, travels through the processor and
on to the Dante interface section where it
is packetized and prepared for transport
over Ethernet.
Audio input signals arrive via four Dante
receiver channels. The supported sam-
pling rate is 48 kHz with a bit depth of
up to 24. The audio signals pass into the
Model 374A’s processor where channel
routing, headphone level control, and
sidetone creation are performed within
the digital domain. This provides flexibility,
allowing for precise control of the audio
signals and eliminates the need for the
four rotary level controls from having to
directly handle analog audio signals. The
audio signals destined for the 2-channel
headphone output are sent to a high-per-
formance digital-to-analog converter and
then on to robust driver circuitry. High
signal levels can be provided to a variety
of headsets, headphones, and earpieces.

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Call Function
A call function allows Model 374A users to
receive and send channel-specific visual
alert signals. Using 20 kHz tones, the call
signals are sent within the audio chan-
nels (“in band”) allowing interoperability
between multiple Model 374A units as well
as being compatible with legacy party-line
intercom systems. Call signals can be use-
ful to indicate to users that they are needed
“on headset” or should be actively listening
to an intercom channel. The call function
can also be used to provide realtime cues
to production personnel during the running
of live events. Call signals present on the
four audio input (Dante receiver) channels
can be detected and displayed. To opti-
mize for specific applications, a configura-
tion choice allows the way in which the
Model 374A will display Call when 20 kHz
has been detected. Pushbutton 4, on the
Model 374A’s top panel, can be configured
to serve as a call button. This allows a call
signal to be generated for talk channels 1,
2, and 3.
Configuration Flexibility
A highlight of the Model 374A is its abil-
ity to be easily configured to meet the
needs of specific users and applications.
All configuration choices are made using
the STcontroller software application that
communicates with the Model 374A by way
of an Ethernet network connection. Con-
figurable parameters include microphone
power and preamplifier gain, sidetone
audio operation, talk button operation, call
indication, remote control inputs, and head-
phone monitoring.
The microphone input can be selected for
compatibility with dynamic or electret (DC-
powered) microphones. The gain of the mi-
crophone preamplifier can be selected from
among four choices. These allow compat-
ibility with a variety of microphones that are
part of broadcast and intercom headsets.
A unique Model 374A feature is the ability
to individually configure the way in which
the four talk pushbutton switches function;
four choices are available. For standard
intercom beltpack operation either Push
to Talk or Push to Talk/Tap to Latch opera-
tion can be selected. For situations where
only monitoring of an intercom channel is
desired a Talk Disabled mode is available.
And for advanced monitoring-only situa-
tions the Talk Disabled/Phones Tap to Latch
mode can be selected such that a button
will serve in an audio on/off role.
Configuration choices allow the two remote
control inputs to be individually assigned
to activate any one of the four talk channel
functions.
Four audio channels arrive via Dante and
are destined for the 2-channel headphone
output. Each input source can be indepen-
dently routed to the left headphone chan-
nel, right headphone channel, or both the
left and right headphone channels. This
flexibility allows a variety of listening envi-
ronments to be created, including stereo,
single-channel monaural, and dual-channel
monaural.
Ethernet Data and PoE
The Model 374A connects to an Ethernet
data network using a standard 100 Mb/s
twisted-pair Ethernet interface. The physi-
cal interconnection is made by way of a
Neutrik® etherCON RJ45 receptacle. While
compatible with standard RJ45 plugs,
etherCON allows a ruggedized and locking
interconnection for harsh or high-reliability
environments. An LED displays the status
of the network connection.

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The Model 374A’s operating power is pro-
vided by way of the Ethernet interface using
the 802.3af Power-over-Ethernet (PoE) stan-
dard. This allows fast and efficient intercon-
nection with the associated data network.
To support PoE power management, the
Model 374A’s PoE interface reports to the
power sourcing equipment (PSE) that it’s
a class 1 (very low power) device.
Future Capabilities and
Firmware Updating
The Model 374A was designed such that
its capabilities and performance can be
enhanced in the future. A USB connector,
located on the unit’s main circuit board
(underneath the unit’s cover), allows the
application firmware (embedded software)
to be updated using a USB flash drive.
The Model 374A uses Audinate’s Ultimo™
integrated circuit to implement the Dante
interface. The firmware in this integrated
circuit can be updated via the Ethernet
connection, helping to ensure that its
capabilities remain up to date.
Getting Started
What’s Included
Included in the shipping carton are a Model
374A Intercom Beltpack and instructions
on how to obtain an electronic copy of this
guide. As a device that is Power-over-
Ethernet (PoE) powered, no external
power source is provided. In most applica-
tions an Ethernet switch with the required
PoE capability will be utilized.
Connections
In this section signal interconnections will
be made using the two connectors located
on the bottom of the Model 374A’s enclo-
sure. An Ethernet data connection with
Power-over-Ethernet (PoE) capability will
be made using either a standard RJ45
patch cable or an etherCON protected
RJ45 plug. A dual-channel or single-
channel (dual- or single-ear) headset
will be connected using a cable-mounted
5-pin male XLR connector. Some applica-
tions will use the 3-conductor 3.5 mm jack
that provides access to the Model 374A’s
two remote control inputs.
Ethernet Connection with PoE
A 100BASE-TX Ethernet connection that
supports Power-over-Ethernet (PoE) is
required for Model 374A operation. This
one connection will provide both the Ether-
net data interface and power for the Model
374A’s circuitry. A 10BASE-T connection
is not sufficient and a 1000BASE-T (GigE)
connection is not supported unless it can
automatically “fall back” to 100BASE-TX
operation. The Model 374A supports
Ethernet switch power management,
enumerating itself as a PoE class 1
device. Any source that is compliant with
the IEEE® 802.3af standard will function
correctly. Should the selected Ethernet
switch port support Energy-Efficient
Ethernet (EEE) it must be disabled to
help ensure reliable Dante operation.
The Ethernet connection is made by way
of a Neutrik etherCON protected RJ45
receptacle that is located on the bottom
of the Model 374A’s enclosure. This allows
connection by way of a cable-mounted
etherCON connector or a standard RJ45
plug. The Model 374A’s Ethernet interface
supports auto MDI/MDI-X so that a cross-
over cable will never be required.

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Headset Connection
The Model 374A provides a 5-pin female
XLR connector that interfaces with the
microphone and headphone connections
of an intercom or broadcast-style headset.
Refer to Figure 2 for connection details.
The microphone input connections are
compatible with most unbalanced dynamic
or electret (low-voltage DC-powered)
microphones. A balanced dynamic micro-
phone should, in most cases, also function
correctly if the signal – (low) is connected
to Model 374A’s mic in –/shield connec-
tion. No support is provided for micro-
phones that require P12 or P48 phantom
power.
To allow users of stereo (dual-earpiece or
“double muff”) headsets to hear a monau-
ral version of the two headphone output
channels does not require special wiring of
the 5-pin male XLR mating connector. The
headset’s left headphone channel should
always be wired to pin 4 and the right
headphone channel to pin 5. Configuration
choices, discussed later in this guide, can
then be used to create the desired mon-
aural output. It’s important not to connect
together (short) pins 4 and 5 of the Model
374A’s headset connector as damage to
the Model 374A’s output circuitry could
result.
Monaural (single-earpiece or “single muff”)
headsets should be wired such that its
headphone is wired only to pin 4; pin 5
should be remain unused. Configuration
choices, discussed later in this guide, can
be used to create a monaural output.
It’s possible that some Beyerdynamic
headset interconnecting cable assemblies
terminate the earpiece’s left and right con-
nections opposite from what the Model
374A and other broadcast equipment
require. These cables may terminate the
left earpiece to pin 5 of the XLR connec-
tor and the right earpiece to pin 4. If this
condition is present it will require reversing
or “flipping” the two wires in a headset’s
5-pin male XLR connector such that the left
earpiece connects to pin 4 and the right
earpiece to pin 5.
Remote Control Inputs
Provision has been made to allow exter-
nal switches or contact closures to mimic
the operation of one or two of the four talk
pushbutton switches. The exact functioning
of the two remote control inputs is deter-
mined by configuration settings, details of
which are discussed later in this guide. A
3-conductor 3.5 mm jack is located on the
Model 374A’s bottom panel and provides
access to the two remote control inputs.
The input circuitry is “active low,” which
means that they are enabled whenever
they are connected (shorted) to the com-
mon connection. Technically, each remote
control input is created with a 3.4 k ohm re-
sistor connected to +3.3 volts DC that acts
as an input “pull up.” Additional resistors
and capacitors provide ESD protection,
minimizing the chance of damage to an
input due to static discharge or other extra-
neous signals. A current flow of less than
one milliampere is required for a remote
control input to be recognized as active.
Figure 2. Headset connection pinout chart

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Prepare an interconnecting cable and
associated 3-conductor 3.5 mm plug to
reflect that the tip lead is used by Remote
Control Input 1, the ring lead by Remote
Control Input 2, and the sleeve lead is the
connection to common. Refer to Figure 3
for connection details.
circuit is limited to two. These can either be
unicast, multicast, or a combination of the
two. If the Model 374A’s transmitter chan-
nels need to be routed using more than
two flows it’s possible that an intermediary
device, such as the Studio Technologies’
Model 5422 Dante Intercom Audio Engine,
can be used to “repeat” the signals.
The four Dante receiver (Rx) channels
associated with the Model 374A’s audio in-
puts also need to be routed to the desired
Dante transmitter (Tx) channels. These four
audio signals can be sent to the Model
374A’s 2-channel headphone output.
The Model 374A supports an audio sample
rate of 48 kHz with no pull-up/pull-down
values available. The Model 374A can serve
as the Leader clock for a Dante network but
in most cases it will be configured to “sync”
to another device.
The Model 374A has a default Dante
device name of ST-M374A- followed by a
unique suffix. The suffix identifies the spe-
cific Model 374A that is being configured.
The suffix’s actual alpha and/or numeric
characters relate to the MAC address of
the unit’s Ultimo integrated circuit. The four
Dante transmitter (Tx) channels have de-
fault names of Ch1, Ch2, Ch3, and Ch4.
The four Dante receiver (Rx) channels have
default names of Ch1, Ch2, Ch3, and Ch4.
Using Dante Controller the default device
name and channel names can be revised
as appropriate for the specific application.
Model 374A Configuration
Many of the Model 374A’s operating
parameters can be configured to match
the needs of specific applications. The
STcontroller software application is used to
observe a unit’s current configuration and
perform any changes that are required. No
Figure 3. 3-conductor 3.5 mm (TRS) remote
control in pinout diagram
Dante Configuration
For audio to pass to and from the Model
374A requires that several Dante-related
parameters be configured. These configu-
ration settings will be stored in non-volatile
memory within the Model 374A’s circuitry.
Configuration will typically be done with
the Dante Controller software application
which is available for download free of
charge at audinate.com. Versions of Dante
Controller are available to support Win-
dows® and OS X® operating systems.
The Model 374A uses the Ultimo 4-input/
4-output integrated circuit to implement
the Dante architecture.
The four Dante transmitter (Tx) channels
associated with the Model 374A’s Dante
interface must be assigned to the desired
receiver (Rx) channels. This achieves rout-
ing the Model 374A’s four talk output audio
channels to the device (or devices) that will
be “listening” to them. Within Dante Con-
troller a “subscription” is the term used for
routing a transmitter flow (a group of out-
put channels) to a receiver flow (a group of
input channels). The number of transmitter
flows associated with an Ultimo integrated

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DIP switch settings or other local actions
are used to configure the unit. This makes
it imperative that STcontroller be available
for convenient use in a personal computer
that’s connected to the related LAN.
Installing STcontroller
STcontroller is available free of charge on
the Studio Technologies website. If nec-
essary, download and install STcontroller
onto a designated personal computer.
This personal computer must be on the
same local area network (LAN) and subnet
as the Model 374A unit or units that are to
be configured.
Using STcontroller
Immediately after starting STcontroller the
application will locate and display the de-
vices that it can control. The one or more
Model 374A units that are recognized will
appear in the device list. Use the Identify
command to allow easy recognition of a
specific Model 374A unit. Double clicking
on a device name will cause the associ-
ated configuration menu to appear. Re-
view the current configuration and make
changes as required.
Parameters
STcontroller allows observation and selec-
tion of functions that include:
• Microphone Input – Electret Power
• Microphone Input – Gain
• Headphone Output Channel Routing
• Sidetone – Level
• Button Operation
• System – Mic Kill
• System – Call Indication
• System – Remote Control Inputs
Changes made using STcontroller will be
immediately reflected in the unit’s opera-
tion; no Model 374A “reboot” is required.
Each time a change is made the four talk
buttons on the top panel will momentarily
flash orange in a distinctive pattern to indi-
cate that a command from STcontroller
has been received.
Microphone Input – Electret Power
Choices are Enabled or Disabled.
If the headset has an electret microphone
that requires a source of low-voltage DC
power for operation enable the Electret
Power check box. If the associated head-
set has a dynamic (non-powered) micro-
phone do not enable the Electret Power
check box. Most broadcast headsets that
terminate on a 5-pin male XLR connector
will not require microphone power. The
on/off status is displayed by way of a red
LED, labeled MIC POWER, that is located
adjacent to the headset connector.
Note that the Model 374A cannot supply
P12 or P48 phantom power that may be
required for balanced condenser (capaci-
tor) microphones. This should not pose an
issue as this type of microphone is essen-
tially never associated with a headset that
would be used for intercom applications.
Microphone Input – Gain
Choices are 22 dB, 30 dB, 42 dB, and
48 dB.
Dynamic microphones have an output
level that is typically lower than that pro-
vided by electret microphones. As such,
the 42 or 48 dB gain settings will probably
be appropriate for them. Electret micro-
phones often have internal circuitry which
provides a relatively high output level. The
22 and 30 dB gain settings will typically be
appropriate with this type of microphone.

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When a dynamic microphone is con-
nected to the Model 374A the 42 dB gain
setting may be correct for many applica-
tions. Setting the gain for 48 dB may be
helpful in some applications, such as with
headsets that have a low microphone out-
put level. The 48 dB gain setting may also
be useful when the Model 374A is going
to be deployed at events where users are
not able to speak at normal levels, e.g.,
sporting events such as golf tournaments
where “whispering” may be necessary.
Electret microphones typically have a
higher output level due to their internal
preamplifier circuitry. As such, less pream-
plifier gain may be required. Selecting the
22 or 30 dB gain setting will probably be
appropriate in these cases.
The compressor active LED, labeled
COMP and visible adjacent to the headset
connector, can act as a guide when set-
ting the microphone preamplifier gain.
During normal talk operation the com-
pressor active LED should light intermit-
tently. If, for example, with a dynamic
microphone the LED rarely lights and the
preamp gain is set to 42 dB it might be a
good idea to change it to 48 dB. If the LED
is lit fully during normal talking in a situ-
ation where the headset has an electret
microphone and the gain is set for 30 dB,
changing it to 22 dB might be warranted.
Headphone Output Channel Routing
Choices are Left, Right, and Left and Right.
Four audio input sources (Dante receiver
channels) can be assigned to the head-
phone output channels using the Dante
Controller application. How these four
audio signals are routed to the Model
374A’s headphone output channels can
be configured using this configuration
choice. The three options provide flexibility
in how the audio sources are presented
to the user. When using a stereo (“double
muff”) headset it’s common for the audio
input sources to be routed to both the
left and right channels. But the Model
374A’s flexibility allows each source to
be independently configured for sending
to the left only, right only, or both left and
right headphone channels.
When using a stereo headset sending all
inputs to both channels is often referred to
as a dual-channel mono output. If a mon-
aural (“single muff”) headset is used the
Left configuration option allows the four
input channels to be combined to monau-
ral so that the listener can simultaneously
hear both sources.
Sidetone – Level
Choices are Off, Low, Medium Low,
Medium, Medium High, and High.
The Model 374A includes a sidetone
function that allows microphone audio
coming from the microphone preampli-
fier to be sent to the both channels of
the headphone output. This serves as a
user confirmation that they are actively
sending audio to one or more of the talk
output channels. Sidetone audio will only
be routed to the headphone output when
at least one of the talk channels is ac-
tive. The level of the sidetone audio can
be selected from among five values. The
correct value is simply the one that makes
the user most comfortable. Sidetone
audio can also be disabled by selecting
Off. Selecting Off would be useful only
in special applications such as during
troubleshooting or where user talk audio is
being returned as part of their listen audio
signals. The four rotary level controls will
not impact the sidetone level.

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Button Operation
Choices are Push to Talk, Push to Talk/Tap
to Latch, Talk Disabled, Talk Disabled/
Phones Tap to Latch, and Call (button 4
only).
This configuration section allows indepen-
dent selection of how the four buttons will
function. When the Push to Talk mode is
selected the operation is self-explanatory.
Only when a button is pressed and held
will talk audio be sent out its associated
Dante transmitter channel.
When the Push to Talk/Tap to Latch mode
is selected a “hybrid” function of sorts is
enabled. Pressing and holding the button
will enable audio to be sent out its associ-
ated Dante transmitter channel. When the
button is released audio will stop being
sent out its associated transmitter channel.
Tapping (momentarily pressing) the button
will cause the function to “latch” into the
talk active mode. Tapping the button again
will cause the function to “unlatch” and
talk will no longer be active.
Many applications are best served when
the buttons are configured in the Push
to Talk mode. This ensures that a chan-
nel can’t accidentally be left in its active
mode. But there are certainly valid situa-
tions when the Push to Talk/Tap to Latch
mode setting will prove to be very useful.
When the Talk Disabled mode is selected
microphone audio will never be sent out
the associated Dante transmitter channel.
This can be useful when the audio input
associated with this channel is going to be
monitored but talk audio is not allowed to
“join” this specific intercom channel.
When the Talk Disabled/Phones Tap to
Latch mode is selected the associated
button will not impact talk audio but will
instead control the on/off state of the au-
dio being sent to the headphone output.
In this mode talk audio will never be sent
out the associated Dante transmitter chan-
nel. This is an interesting mode, allowing
one or more Model 374A audio channels
to serve in a “monitor mixer” function. The
button will allow quick on/off control while
the rotary level control can remain in its
desired position.
When button 4 has been selected to the
Call mode it will serve as a call enable
button for talk channels 1, 2, and 3; micro-
phone audio will never be sent out Dante
transmitter channel 4. In the Call mode
button 4 will operate in a push-to-activate
manner and whenever it is active and
channels 1, 2, and/or 3 are active a 20 kHz
tone (call signal) will be added to the
active audio paths. It will not display a
call signal that’s present on audio input
channel 4.
System – Mic Kill
Choices are Off, Device ID, Global Kill
Command, and Device ID/Global Kill
Command.
The mic kill function allows an enabled
talk channel to be disabled by remote
control. Specifically, it’s possible to re-
motely change all active (on) talk channels
to their off state. This function is some-
times referred to as a “talk off” function.
There are two means available to remotely
disable all active talk channels. The Device
ID method uses the identification com-
mand generated by the Dante Controller
or STcontroller applications as a “trigger”
to disable any active talk channels. The
Global Kill Command can be generated by
applications that incorporate the Studio
Technologies’ proprietary method. This
method utilizes sending a data command

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INTERCOM BELTPACK
over the Ethernet connection. Select the
desired mic kill configuration from the four
choices. Off simply means that no remote
talk control is supported.
System – Call Indication
Choices are Off, Flash, Solid, and Flash
then Solid.
The call indication configuration allows the
manner in which each of the four talkback
button LEDs will light when a call signal is
detected on its respective Dante receiver
(input) channel. Technically, call is detect-
ed as active when a 20 kHz nominal audio
tone is present in an audio signal path.
(It would typically be a sine- or square-
wave signal that’s combined with voice
audio.) If Off is selected and a call signal
is detected in a Dante receiver (input)
channel then the associated button’s
LED will not light. If Flash is selected then
a button’s LED will flash orange if a call
signal is detected and the channel’s talk
function is not active. If the channel’s talk
function is active when a call signal is
detected then the button’s LED will flash
alternately green and orange. (Green in-
dicates that the talk function is active and
orange indicates that a call signal is de-
tected.) If the configuration is selected for
Solid then a button’s LED will light orange
when a call signal is detected; it will light
orange whether or not the talk function
associated with that button is active. If
call indication is configured for Flash then
Solid and a call signal is detected, then
a button’s LED will first flash orange (or
green and orange) and then will light solid
orange, remaining lit orange until the call
signal is no longer present.
System – Remote Control In 1
Choices are Off, Channel 1, Channel 2,
Channel 3, and Channel 4.
The action performed when Remote Control
In 1 is enabled will follow the configuration
selected from among five choices. When
Off is selected no action will be associated
with Remote Control In 1. When Channel 1
is selected activating Remote Control Input
1 will provide the same function as would
happen when pressing the talkback 1 push-
button. (The exact action will depend on
the button’s configuration.) The same
action will take place when selecting the
configuration to be channels 2, 3, or 4. But
in these cases, remote control input 1 will
control the action associated talk channels
2, 3, or 4 respectively.
System – Remote Control In 2
Choices are Off, Channel 1, Channel 2,
Channel 3, and Channel 4.
The configuration choices and related ac-
tions for Remote Control In 2 are the same
as those provided for Remote Control In 1.
Operation
At this point everything should be ready
and Model 374A operation can commence.
An Ethernet connection with Power-over-
Ethernet (PoE) capability should have been
made. A headset terminated on a 5-pin
male XLR connector should be plugged
into the headset connector. Some applica-
tions may utilize either or both of the remote
control inputs.
Using the Studio Technologies’ STcontroller
software application the unit’s configura-
tion should have been selected to meet the
needs of the specific application. The Model
374A’s Dante configuration settings should

Model 374A User Guide Issue 5, March 2021
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MODEL 374A
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have been selected using the Dante Con-
troller software application. In this way the
unit’s four audio output channels (Dante
transmitter channels) and four audio input
channels (Dante receiver channels) should
have been routed, by way of Dante “sub-
scriptions,” to the receiver and transmitter
channels on associated Dante-enabled
equipment.
Initial Operation
The Model 374A will start to function
as soon as a Power-over-Ethernet (PoE)
power source is connected. However, it
may take 20 to 30 seconds for full opera-
tion to commence. Upon initial power up
the three status LEDs located on the bot-
tom panel below the etherCON RJ45 re-
ceptacle will begin to light as network and
Dante connections are established. The
green and orange LEDs that illuminate the
four pushbutton switches on the top panel
may initially light randomly and then will
light in a test sequence to indicate that the
application firmware (embedded software)
has started. Once the entire sequence has
completed and the Dante connection has
been established full operation will begin.
Ethernet and Dante Status
LEDs
Three status LEDs are located below the
etherCON RJ45 receptacle on the Model
374A’s bottom panel. The LINK ACT LED
will light green whenever an active connec-
tion to a 100 Mb/s Ethernet network has
been established. It will flash in response
to Ethernet data packet activity. The SYS
and SYNC LEDs display the operating sta-
tus of the Dante interface and associated
network. The SYS LED will light red upon
Model 374A power up to indicate that the
Dante interface is not ready. After a short
interval it will light green to indicate that it
is ready to pass data with another Dante
device. The SYNC LED will light red when
the Model 374A is not synchronized with
a Dante network. It will light solid green
when the Model 374A is synchronized
with a Dante network and an external
clock source (timing reference) is being
received. It will slowly flash green when
this specific Model 374A is serving as the
Leader clock for the network. It’s possible
that up to 30 seconds may be required for
the SYNC LED to reach its final state.
Compressor Active LED
A yellow LED indicator is located on the
bottom panel adjacent to the headset con-
nector. Labeled COMP, the LED displays
the status of the microphone audio com-
pressor function. It will light whenever the
input level from the microphone, along
with the configured preamplifier gain, is
such that the dynamic range of the talk
signal is being controlled. It’s perfectly
acceptable for this LED to light intermit-
tently whenever a user is talking at a
normal voice level into the associated
microphone. But if the COMP LED lights
solid while a user is talking at a normal
voice level this will typically indicate that
the mic preamp gain setting should be
reduced. Conversely, if the COMP LED
almost never lights when normal talking
is taking place, it’s possible that changing
the mic preamp gain to the higher setting
would be beneficial. Note that due to the
design of the circuitry the compressor ac-
tive LED will function whether or not any
of the four talk channels are active.

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Headphone Output
Four rotary potentiometers (“pots”),
located on the Model 374A’s top panel,
allow individual adjustment of the level of
the four audio input signals as they are
sent to the 2-channel headphone output.
Depending on the configuration of the unit,
each audio input can be sent to the left
channel, the right channel, or both the left
and right channels of the headphone out-
put. The pots are “push-in/push-out” type
which allow their associated knobs to be in
their “out” position to be adjusted and then
moved to their “in” position when protec-
tion from an unwanted change is desired.
Users should find the headphone output
audio quality to be excellent, with high
maximum output level and low distortion.
Audio signals do not pass directly through
the level pots. The position of the pots
is recognized by the Model 374A’s pro-
cessor which then adjusts the signal level
within the digital domain. When a pot is
in its fully counterclockwise position the
associated audio signal is fully muted. In
most cases the on/off status of the four talk
channels does not impact the headphone
output. However, one button configuration
setting (Talk Disabled/Phone Tap to Latch)
assigns the associated button to function
as a channel listen audio on/off selector.
Button Operation
Four pushbutton switches are associated
with the Model 374A’s four channels. How
they function will depend on the configura-
tion of the unit. Each button can be config-
ured independently.
Push to Talk
When a button has been configured for
the Push to Talk mode how it functions is
pretty self-explanatory. Press and hold the
button when headset microphone audio is
to be sent out the associated Dante audio
output (transmitter) channel. The button’s
green LED will light to indicate that the
output is active. The button will not be lit
when the function is not active.
Push to Talk/Tap to Latch
If a button has been configured for the
Push to Talk/Tap to Latch mode operation
is a bit different and certainly more flex-
ible. Press and hold the button to activate
the talk function. When released the talk
function will turn off. Momentarily pressing
(“tapping”) the button will cause the func-
tion to change states; off-to-on or on-to-
off. Whenever the talk function is active the
green LED will light. The button will not be
lit when the function is not active.
Talk Disabled
A button that is configured to the Talk
Disabled mode will never allow talk audio
to be sent to the associated Dante audio
output channel. But to provide “feedback”
that the button has been pressed the
button’s green LED will flash rapidly three
times. This says, in effect, “Yes, I recog-
nize that you pressed the button but I’m
not going to do anything in response!”
Talk Disabled/Phones Tap to Latch
In the Talk Disabled/Phones Tap to Latch
mode the button will never allow talk
audio to be sent to the associated Dante
audio output channel. Instead the button
will operate in a tap to enable/tap to dis-
able (alternate action or “toggle”) manner
and provide an audio channel listen on/off
function. The button’s orange LED will
light whenever audio is being sent to the
headphone output. The headphone output
level will continue to follow the setting of
the associated rotary level control.

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Call (Button 4 Only)
When button 4 is configured for the Call
mode it will allow call signals to be gener-
ated whenever talk channels 1, 2, and/or 3
are active. The button functions in a push-
to-call manner and will light orange when-
ever it is pressed and held. No microphone
audio will be routed to audio output 4. But-
ton 4 will never light in response to a call
signal being present on audio input 4. The
headphone output level will continue to
follow the setting of the rotary level control
associated with audio input 4.
Remote Control Inputs
The Model 374A allows two switches or
contact closures to be connected, by way
of a 3-conductor 3.5 mm jack, to two re-
mote control inputs. Both remote control
inputs can be configured to perform the
same action as talkback button 1, 2, 3, or
4. The remote control inputs can also be
configured to be off, and thus not perform
any function.
Whether or not the two remote control
inputs are utilized will depend on the se-
lected application. Even if connections
are made to either or both of the remote
control inputs how they specifically func-
tion will depend on several configuration
settings. The configuration selections are
performed using STcontroller with the de-
fault setting for both remote control inputs
being off. The remote control inputs can
be individually configured to duplicate the
action of any one of the four talk pushbut-
ton switches. When a remote control input
is enabled it will cause an action identical
to that caused by pressing or tapping the
related push-button switch. For example,
if remote control input 1 is assigned to the
pushbutton function associated with talk 1
and that pushbutton is configured for
latching mode, remote control input 1 will
also function in a latching manner for talk
channel 1.
Sidetone Function
The Model 374A includes a sidetone func-
tion that sends microphone audio to both
the left and right headphone output chan-
nels whenever one or more of the talk
buttons are active. The quality of the side-
tone audio should be excellent and will pro-
vide Model 374A users with a confidence
signal that they are actively talking to other
intercom users. Sidetone audio will always
be sent to both headphone channels and
is not a configurable choice. This is be-
cause the function is trying to simulate
what a user would hear if they didn’t have
a headset covering their ears. It is not in-
tended to indicate to the user which Dante
audio output (transmitter) channel or chan-
nels are actively sending audio.
The exact sidetone level is configured from
among five values within the STcontroller
application. There is also a selection choice
for turning off sidetone. There is no means
of adjusting the sidetone level using any
physical buttons or controls on the Model
374A unit. Also, the four headphone level
controls on the top panel of the unit do not
impact the sidetone level.
In most cases the exact sidetone level
setting is not critical and typical users will
not be concerned about revising it to a
specific value. But setting the sidetone level
to something reasonable is important. Set-
ting the level too low will encourage users
to speak too loudly; setting it too high and
users will be tempted to speak hesitantly.
And while the user isn’t provided with a
sidetone level control or other adjustment
means, should the need arise STcontroller
allows rapid adjustment of the sidetone

Issue 5, March 2021 Model 374A User Guide
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level. In most cases the default sidetone
level, Medium, should provide an appropri-
ate level.
Call Function
General
The Model 374A includes a call function that
allows users, and users of compatible equip-
ment, to signal each other. A call receive
function is provided as part of each of the
Model 374A’s four channels. Any combina-
tion of the four can have their call function
active at any one time. Technically, a call
signal is achieved by sending a nominal
20 kHz audio tone on the desired audio out-
put channel. This signal, normally a sine- or
square-wave, is summed (mixed) with nor-
mal talk audio. The Model 374A’s four audio
input channels are always monitoring for the
presence of 20 kHz. A call signal is recog-
nized when a continuous 20 kHz tone is
present on the audio input for that channel.
Note that any Model 374A channel that has
been configured to Talk Disabled/Phones
Tap to Latch mode will not respond to the
presence of a call signal. Also, if channel 4
has been configured to the Call mode that
channel will not respond to a call signal on
audio input 4.
The Model 374A will not confuse a normal
talk audio signal with a call signal. Digital
filters within the unit’s processor integrated
circuit limits each channel’s high-frequency
response, helping to ensure that false call
detection won’t take place. Digital filtering
is also performed on the Dante receiver
(input) audio signals before they are sent
to the headphone outputs. This prevents
users with extended high-frequency sensitiv-
ity, such as presenters and guests associ-
ated with dog shows, from ever receiving
undesirable audio content.
By using 20 kHz tones for call signaling the
Model 374A is compatible with legacy in-
tercom equipment, including the venerable
RTS® BP-325 beltpack. When intercon-
necting Model 374A and BP-325 units using
an appropriate Dante-enabled interface,
such as the Studio Technologies’ Model
45DR Intercom Interface, call signaling
is fully compatible. Compatibility with the
Clear-Com® method of call functionality
is also possible by using the Model 45DC
Intercom Interface. (It converts the DC call
signal associated with pin 3 of a Clear-Com
party-line circuit to a 20 kHz tone.)
Devices such as the Studio Technologies’
Model 44D Audio Interface will also send
and receive 20 kHz signals that are com-
patible with the Model 374A’s call function.
To generate a call signal a Model 44D will
respond to the activation of a GPI (general-
purpose input) signal by summing (com-
bining) a 20 kHz tone with the normal audio
signal that is being transported “in band”
via a Dante audio path. On the call receive
side, audio signals that enter the Model
44D will be monitored for the presence of
20 kHz. If a call signal is detected the GPO
(general-purpose output) associated with
that audio channel will activate.
Receive
Whenever a Model 374A’s Dante receive
(input) channel receives a valid 20 kHz
nominal call signal the action of the associ-
ated button’s LED will follow the unit’s
call indication configuration. This one
configuration will impact the action of all
button LEDs. If the call indication function
has been configured for Disabled a but-
ton’s LED will not respond to the presence
of a call signal. If the call indication configu-
ration has been selected for Flash then
a button’s LED will flash orange when a
call signal has been detected on its

Model 374A User Guide Issue 5, March 2021
Studio Technologies, Inc. Page 19
MODEL 374A
INTERCOM BELTPACK
associated audio input and the talk function
is not active. If a call tone is detected when
the talk function is active then the associ-
ated button’s LED will flash, alternating
between green and orange. If the call indi-
cation has been configured for Solid then a
button’s LED will light orange whenever a
call signal is detected. If Flash then Solid is
selected for the call indication configuration
a button’s LED will first flash orange (or or-
ange and green if the talk function is active)
and then light solid orange.
Send
The Model 374A allows a call signal to be
sent on channels 1, 2, and 3. This requires
that button 4 be configured to the Call
mode. To send a call signal is simple: just
enable any combinations of channels 1, 2,
and/or 3 and simultaneously press and hold
button 4. When call sending is active the
orange LED associated with button 4 will
light. At the same time the channel or chan-
nels that are active in their talk mode (from
among channels 1, 2, and 3) will add 20
kHz to their audio output channels and have
their associated LED flash, alternating be-
tween green and orange. (Green indicates
that the talk function is active and orange
indicates that the call function is active.)
Release button 4 and the sending of call
signals will cease. As expected, pressing
button 4 when no talk channels are active
will result in no call tones being generated.
How to Identify a Specific
Model 374A
The Dante Controller software application
offers an Identify command that can be
used to help locate a specific Model 374A.
When Identify is selected it will send a
command to a single Model 374A unit. On
that specific unit the four pushbutton LEDs
will light in a unique pattern. In addition,
the SYS and SYNC status LEDs, located
directly below the etherCON RJ45 recep-
tacle on the bottom panel, will slowly flash
green. After a few seconds the LED identifi-
cation pattern will cease and normal Model
374A button LED and Dante status LED
operation will resume.
If configured, the Identify command can
also cause the mic kill function to activate.
For details please refer to the next para-
graph of this guide.
Mic Kill Function
The Model 374A includes a mic kill function,
allowing talk buttons that have been placed
in their enabled (on) state to be remotely
changed to their disabled (off) state. This
allows one or more talk channels on a spe-
cific Model 374A that have been enabled
to be remotely disabled. The reason for this
function is simple. It’s common in intercom
applications for users to enable a talk chan-
nel and then go “off headset,” forgetting that
they’ve left the talk channel enabled. While
they take a break or go to lunch, all other
users listening to that channel may hear
unwanted audio! Having the ability to dis-
able this talk channel can be very useful.
A configurable choice in the STcontroller
software application is used to select which
of the mic kill methods, Device ID or Global
Kill Command, will be active. Either or both
can be enabled as desired. It’s also possible
to disable the mic kill function.
If the Device ID method of activating
mic kill is enabled, whenever the specific
Model 374A is identified using the Identify
command within Dante Controller or
STcontroller all enabled talk channels will
be forced to their disabled state. If the
Global Kill Command method is enabled

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INTERCOM BELTPACK
the Model 374A will respond to any such
request and force all talk channels to their
disabled state.
Technical Notes
IP Address Assignment
By default the Model 374A’s Ethernet in-
terface will attempt to automatically obtain
an IP address and associated settings
using the DHCP (Dynamic Host Configu-
ration Protocol). If a DHCP server is not
detected an IP address will automatically
be assigned using the link-local protocol.
This protocol is known in the Microsoft®
world as Automatic Private IP Addressing
(APIPA). It is also sometimes referred to
as auto-IP. Link-local will randomly assign
a unique IP address in the IPv4 range of
169.254.0.1 to 169.254.255.254. In this
way multiple Dante-enabled devices can
be connected together and automatically
function, whether or not a DHCP server
is active on the LAN. Even two Dante-
enabled devices that are directly intercon-
nected using an RJ45 patch cable will,
in most cases, correctly acquire IP ad-
dresses and be able to communicate
with each other.
An exception does arise when trying to
directly interconnect two Dante-enabled
devices that use Ultimo integrated circuits
to implement Dante. The Model 374A
uses the Ultimo “chip” and, as such, a
direct one-to-one interconnection to an-
other Model 374A (or other Ultimo-based
product) is not supported. An Ethernet
switch linking the two units is required
to successfully interconnect two Ultimo-
based devices. The technical reason that
a switch is required relates to the need for
a slight latency (delay) in the data flow.
And an Ethernet switch will provide this.
While this is certainly an anomaly, since
PoE power is required for Model 374A
operation it’s highly unlikely that an ap-
plication would use two Model 374A units
without a PoE-enabled Ethernet switch
being present.
Using the Dante Controller software ap-
plication the Model 374A’s IP address and
related network parameters can be manu-
ally set for a fixed (static) configuration.
While this is a more-involved process than
simply letting DHCP or link-local “do their
thing,” if fixed addressing is necessary
then this capability is available. But in this
case it’s highly recommended that each
unit be physically marked, e.g., directly
using a permanent marker or “console
tape,” with its specific static IP address. If
knowledge of a Model 374A’s IP address
has been misplaced there is no reset but-
ton or other method to easily restore the
unit to a default IP setting.
In the unfortunate event that a device’s IP
address is “lost,” the Address Resolution
Protocol (ARP) networking command can
be used to “probe” devices on a network
for this information. For example, in Win-
dows OS the arp –a command can be
used to display a list of LAN information
that includes MAC addresses and cor-
responding IP addresses. The simplest
means of identifying an unknown IP ad-
dress is to create a “mini” LAN with a
small PoE-enabled Ethernet switch con-
necting a personal computer to the Model
374A. Then by using the appropriate ARP
command the required “clues” can be
obtained.