User Guide
Issue 2, December 2013
This User Guide is applicable for serial numbers M42A-01051 and later
Copyright © 2013 by Studio Technologies, Inc., all rights reserved
www.studio-tech.com
50127-1213, Issue 2
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Table of Contents
Introduction ................................................................... 5
Installation ..................................................................... 8
Post-Installation ............................................................. 12
Operation ...................................................................... 13
Troubleshooting ............................................................ 16
Technical Notes ............................................................. 18
Specifications ................................................................ 22
Appendix A .................................................................... 23
Block Diagram
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Issue 2, December 2013 Model 42A User Guide
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Introduction
The Model 42A is designed to create
broadcast-standard IFB outputs from linelevel audio sources. The unit’s primary
application is to interface analog outputs
associated with digital matrix intercom systems with broadcast IFB user devices. The
Model 42A creates four independent IFB
outputs. For convenience, the four outputs
can be connected by way of eight 3-pin
male XLR connectors; four of which are
located on the back panel and four on the
front. Each IFB output provides DC power
and two analog audio signals to support
the connected IFB user devices. The Model
42A’s audio quality is excellent; little hiss,
hum, or other artifacts are present. To
ensure optimal operation, the unit provides
resources for visually and audibly monitoring the audio output signals. In addition,
the DC output voltage is monitored for fault
conditions.
Installation of the Model 42A is very simple.
Audio input connections are made using
a 25-pin D-subminiature connector. The
IFB output circuits interface using standard
3-pin XLR connectors. The compact, onerack-space package is constructed using
rugged steel and aluminum components.
The unit’s AC mains power input can range
from 100 to 240 volts, 50/60 hertz. This
“universal input” ensures correct operation
virtually anywhere in the world.
There may be persons not familiar with the
term IFB. That’s not unreasonable as it’s a
somewhat obscure acronym for interruptible foldback. On its own, the term foldback
is an alternate way of describing a cue or
monitor function. Adding “interruptible”
before it means that the cue source can be
temporarily replaced with an audio signal
originating from a producer, director, or
other production personnel. IFB outputs
are often used in the broadcast industry
for talent cueing applications, both in
studio and field settings.
Both “dry” and “wet” IFB outputs can
be deployed and their characteristics
are worth reviewing. The term “dry” IFB
typically refers to a transformer-balanced
line-level audio circuit with a nominal level
in the range of 0 to +8 dBu. This is essentially a standard analog audio circuit that
is commonly used to interconnect audio
equipment. The term “wet” IFB refers to a
circuit that combines DC power and one
or two channels of analog audio. The DC
power source is typically in the range of
Figure 1. Model 42A Interface front and back views
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Studio Technologies, Inc. Page 5
30 to 32 volts. The audio is unbalanced
with a typical nominal level of –10 dBu. The
Model 42A implements “wet” IFB outputs.
As such, in this user guide the term IFB will
always indicate this type of circuit.
IFB outputs provide an effective means of
delivering power and two channels of audio
to user devices by means of standard audio cables. These cables, ubiquitous to the
audio industry, interface using 3-pin male
and female XLR connectors. Using IFB
outputs and standard audio cables it’s a
simple matter to support user devices such
as listen-only beltpacks and announcer’s
consoles with no external power source
required. Whether the IFB source and user
devices are 100 or 1000 feet apart, reliable
operation can almost always be provided.
In many cases, the Model 42A Interface will
be used in on-air television applications.
Whether installed in a fixed location or as
part of a mobile facility, the unit is capable
of providing excellent performance. In
addition, the Model 42A can be used for
non-broadcast applications. For example,
audio recording and post-production facilities can also effectively use the Model 42A.
Combined with stereo or mono listen-only
beltpacks, also available from Studio Technologies, a variety of headphone cue systems can easily be deployed. And since the
Model 42A’s audio inputs are compatible
with standard line-level audio signals virtually any analog source can be connected.
Four Independent IFB Outputs
The Model 42A supplies four independent
IFB outputs. Each circuit consists of two
audio inputs and a 2-channel “wet” IFB
output. The audio inputs are transformer
coupled, have a nominal level of +4 dBu,
and are compatible with balanced or
unbalanced sources. In on-air television
broadcast applications the audio sources
will often be analog outputs from matrix
intercom systems. Two sources are typically designated to serve as a 2-channel
user cue signal. Generally one source is
configured in the matrix intercom system
as “interrupt” while the other is configured
as “program.” An alternate term often used
for the “interrupt” channel is “programwith-interrupt.” This may be more descriptive as the function is actually a program
source that gets interrupted with talkback
audio. The “program” channel is typically
a continuous source of program audio.
An alternate term is “program-only.”
For other applications, the Model 42A’s
audio inputs can be connected to a
2-channel or stereo audio source. This
configuration may prove useful in radio
broadcasting, audio-with-picture, or recording studio applications. The specific
application will dictate whether both cue
signals will be utilized by the end user.
In on-air sports broadcast situations a
“double-muff” (stereo) headset will provide
a user with both audio channels, one for
each ear. In interview or news-gathering
applications it’s common to use a single
“earpiece” to provide a user with only
program-with-interrupt audio.
Maintaining excellent audio performance
was a major Model 42A design goal—the
hiss, hum, and noise associated with typical IFB outputs was simply not acceptable.
The Model 42A meets those requirements
with audio that is “on-air” quality: low
distortion, high signal-to-noise ratio, and
ample headroom. On-air talent and guests,
production personnel, and technicians will
all appreciate the clean, quiet cue signals.
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The Model 42A’s IFB outputs provide
power and two channels of unbalanced
audio over a single 3-conductor output.
The output power is 30 volts DC with a
maximum current of 220 milliamperes. A
major strength of the Model 42A is the IFB
output’s ability to effectively deliver DC
power over a variety of conditions. Unlike
other interface devices that use a common but less-than-ideal circuit topology, a
unique IFB output circuit was developed
by Studio Technologies to achieve the
desired performance goals. The result is a
major improvement in effectively supporting IFB user devices over a wide range of
conditions. Connected devices can draw
up to the maximum 220 milliamperes of
current with little drop in DC voltage. This
output voltage stability is the key—whether drawing 50, 100, or 200 milliamperes,
the output will remain close to 30 volts.
In practical terms this means that reliable
IFB-based cue systems can now be deployed in more stadiums, concert halls, or
motor racing facilities than was previously
possible—longer cable runs, more user
devices, excellent performance.
Audio Monitoring
To help confirm proper installation and
ensure correct operation, the Model 42A
includes a full-featured audio monitor section. Two 5-segment LED meters provide
an indication of the audio levels as they
appear directly on the IFB output connectors. This capability makes it simple to
adjust and maintain correct audio levels
so that optimal IFB performance can be
achieved. A headphone output is also provided, allowing “real world” checking of
IFB audio quality. In addition to being able
to manually select the IFB output to be
monitored by the meters and headphone
output, an “auto scan” mode is also included. This allows each of the four IFB
outputs to be monitored in a continuously
repeating sequence.
DC Monitoring and Fault
Shut Down
To prevent possible damage to the Model
42A’s output circuitry and connected user
devices, the DC output voltage present
on pin 2 of each IFB output is continually
monitored. If the voltage on an IFB output
falls below 24 volts its associated LED will
“flash” as an error indication. If this undervoltage condition continues for more than
four seconds a shut down mode will be
entered, turning off power to the IFB output. After a 10-second interval the output
will again become active.
Compatibility
The Model 42A is compatible with virtually every digital matrix intercom system,
including those from Clear-Com®, RTS®,
and Riedel®. Interfacing requires only the
connection of analog output ports from
the intercom system to the Model 42A’s
audio inputs. Optimal performance will be
gained by configuring the nominal audio
level of the intercom system’s “virtual” IFB
output ports to match the Model 42A’s
nominal +4 dBu input level. With the
Model 42A’s excellent audio and power
delivery performance it’s an ideal alternative to the interface devices offered by
the intercom system vendors. Using the
Model 42A the intercom system’s audio
quality can be maintained all the way to
the IFB users.
The Model 42A’s IFB outputs allow
virtually every type of IFB user device
to be supported. These include the
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Models 32A, 33A, and 34 listen-only beltpacks and Model 200-Series announcer
console products from Studio Technologies. The 30-series units offer a range of
features while providing excellent audio
performance. The 200-series units combine a variety of microphone control,
headphone monitoring, IFB and intercom
system interfacing, and related functions
into compact desktop units. Legacy
listen-only beltpacks from RTS, including
the 4020 and 4030, can also be directly
supported.
Alternate Applications
connected by means of a detachable
cord that is compatible with the Model
42A’s 3-pin IEC 320 C14 inlet connector.
System Components
The shipping carton contains the Model
42A Interface and associated user guide.
Units are also supplied with an AC mains
cord that has a North-American (NEMA
5-15P) standard plug on one end and an
IEC 320 C13 connector on the other end.
Units bound for other destinations may
require a different cord set; your dealer
or distributor should provide one.
In addition to broadcast intercom applications, the Model 42A can be used to create high-performance stereo headphone
cue systems. Line-level signals coming
from an audio console, a routing switcher,
or an off-air receiver can be connected
to the Model 42A’s audio inputs. The IFB
outputs can be connected to listen-only
beltpacks, several models of which are
available from Studio Technologies. For
example, the Model 35 Talent Amplifier
will allow one or two pairs of stereo
headphones to be supported. Each of
the Model 42A’s four IFB output circuits
will support up to six Model 35 Talent
Amplifiers.
Installation
In this section you will be installing and
interconnecting the Model 42A. The onerack-space unit will be mounted in
an equipment rack. Audio input connections will be made by way of a 25-pin
D-subminiature connector. IFB outputs
will be interfaced using four 3-pin XLR
connectors. AC mains power will be
Locating the Model 42A
The Model 42A’s IFB outputs provide DC
power and unbalanced audio to operate
external IFB user devices. These devices
are often IFB “beltpacks,” broadcast
announcer consoles, or other “talkback
boxes.” The Model 42A’s mounting location will dictate the length of the cable
runs needed to link the unit with the connected devices. In some cases the location choice is already established. For
example, in mobile broadcast applications the Model 42A will almost always be
located in a production truck or trailer. But
in fixed applications it may be possible to
select the Model 42A’s mounting location
so as to minimize cable length. In general,
shorter cables will lead to more reliable
and consistent system performance. It’s
also helpful if a technician can readily view
and access the monitor section that is located on the right side of the Model 42A’s
front panel. The output status LEDs and
levels meters can serve important roles
in ensuring correct IFB output operation.
Access to the headphone output jack and
level control can also be useful.
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Mounting the Model 42A
Once the desired mounting location has
been selected, the Model 42A will require
one space (1U or 1.75 vertical inches) in
a standard 19-inch (48.3 cm) equipment
rack. Secure the unit into the equipment
rack using two mounting screws per side.
possible that a stereo cue source will be
connected. In this situation the left source
would be connected to input channel 1
while the right source would be connected
to input channel 2. This might also be
the case with other professional audio
applications, such as recording and
post-production.
Audio Input Connections
The Model 42A has eight line-level audio
inputs, arranged as four 2-channel pairs.
Each pair serves one of the four IFB outputs. Each IFB output, along with its two
associated audio inputs, is completely
independent. Literally any analog audio
source with a nominal operating level
in the area of +4 dBu can be successfully connected. Transformer coupled,
the inputs are compatible with balanced
or unbalanced signals. In many applications the analog outputs from digital matrix
intercom systems will be connected to the
Model 42A’s audio inputs.
For on-air television applications the IFB
sources are generally configured to provide a single- or dual-channel cue “feed”
to talent personnel. In such cases channel 1 of a pair is generally designated as
the “interrupt” channel while channel 2 is
the “program” channel. For other broadcast applications, such as live radio, it’s
Audio input connections are made by way
of one female 25-pin D-subminiature connector which is located on the Model 42A’s
back panel. A cable harness is required
with a 25-pin D-sub plug (male) on one
end and the desired mating connector(s)
on the other. This cable harness is not supplied by Studio Technologies. (Note that in
some locations the term “cable loom” may
be used instead of “cable harness.”) The
wiring scheme used by the D-sub complies
with the now-ubiquitous one made familiar
by TASCAM® with their DA-88® product.
A wiring harness prepared for connection
to the Model 42A’s audio inputs is identical
to a DA-88-style input harness. Please refer
to Figures 3 and 4 for connection details.
Note that unlike a DA-88-style harness, the
Model 42A’s D-sub connector’s hold-down
screws use 4-40 threads. This complies
with the original design standard for Dsubminiature connectors which used
English rather than metric thread pitch.
Figure 2. Detail of back panel showing 25-pin D-subminiature
connector used for audio inputs
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Signal Signal
Connections High (+) Low (–) Shield
IFB Output 1-Interrupt 24 12 25
IFB Output 1-Program 10 23 11
IFB Output 2-Interrupt 21 9 22
IFB Output 2-Program 7 20 8
IFB Output 3-Interrupt 18 6 19
IFB Output 3-Program 4 17 5
IFB Output 4-Interrupt 15 3 16
IFB Output 4-Program 1 14 2
Notes:
1) Connector type on Model 42A is 25-pin D-subminiature
female. Installer must provide plug (male). Connector uses
4-40 threaded inserts for locking with mating plug.
2) Wiring scheme follows TASCAM DA-88 convention.
Standard DA-88-style wiring harnesses are directly
compatible, with the exception of 4-40 screw threads being
required.
Figure 3. Connections for audio inputs using
IFB nomenclature
The Model 42A’s audio inputs have a
nominal signal level of +4 dBu. They are
transformer coupled, have an impedance
of 10 k ohms, and are compatible with
balanced or unbalanced sources. A balanced source should be wired so that signal high is connected to the + pin, signal
low to the – pin, and shield to the shield
pin. With an unbalanced source, connect
signal high to the + pin, and shield to both
the – and the shield pins. If connecting an
unbalanced source in this manner results
in hum or noise, try connecting signal high
to the + pin, and shield to the – pin; leave
the shield pin unterminated.
As previously mentioned, the Model 42A is
compatible with matrix intercom systems
from Riedel. Appendix A, located near the
end of this user guide, provides detailed
connection information.
Signal Signal
Connections High (+) Low (–) Shield
IFB Output 1-Channel 1 24 12 25
IFB Output 1-Channel 2 10 23 11
IFB Output 2-Channel 1 21 9 22
IFB Output 2-Channel 2 7 20 8
IFB Output 3-Channel 1 18 6 19
IFB Output 3-Channel 2 4 17 5
IFB Output 4-Channel 1 15 3 16
IFB Output 4-Channel 2 1 14 2
Notes:
1) Connector type on Model 42A is 25-pin D-subminiature
female. Installer must provide plug (male). Connector uses
4-40 threaded inserts for locking with mating plug.
2) Wiring scheme follows TASCAM DA-88 convention.
Standard DA-88-style wiring harnesses are directly
compatible, with the exception of 4-40 screw threads being
required.
Figure 4. Connections for audio inputs using
dual-channel nomenclature
IFB Outputs
The Model 42A’s four independent IFB
outputs each supply DC power and two
channels of unbalanced audio. The IFB
outputs are designed to connect to a variety of devices that conform to the broadcast IFB standard. In this implementation
pin 1 is used for a combination of shield,
DC power return, and audio common;
pin 2 supplies a combination of DC power
and one channel of unbalanced audio;
pin 3 supplies a second channel of unbalanced audio. The DC power supplied on
pin 2 is 30 volts with a maximum current
of 220 milliamperes. The audio superimposed on the DC power has a nominal
signal level of –10 dBu. Its audio source
is the channel 1 audio input associated
with that specific IFB output. The audio
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Figure 5. Detail of back panel showing IFB output connectors
on pin 3 also has a nominal signal level of
–10 dBu. Its audio source is the channel 2
audio input associated with that specific
IFB output.
The Model 42A’s IFB outputs are interfaced
using eight 3-pin male XLR connectors; four
which are located on the unit’s back panel
and four on the front. They are organized
in groups of two connectors per IFB output, one on the back panel and one on the
front. They are wired in parallel (“multed”)
and provide access to the identical signals.
The needs of the specific application will
dictate the manner in which the IFB output
connectors are used. For some applications, such as in fixed locations or production trucks, it may be appropriate to utilize
only the connectors on the back panel. The
permanent interface cables, one for each
IFB output, must be terminated with 3-pin
female XLR connectors. It’s recommended
for this application that the IFB outputs be
wired by way of an input/output (I/O) connector panel rather than directly to user devices. It’s also a good idea for the I/O panel
to have “mults” (multiple connectors) for
each of the IFB outputs. For troubleshooting purposes it also may be useful to have
the IFB outputs pass through points on an
audio patch bay.
When a Model 42A is used in an equipment rack that will move between broadcast events it may be desirable to not
permanently terminate any interconnecting cables to the IFB output connectors.
In this way the front- and back-panel
connectors will remain available for showspecific uses. For example, in a “booth
package” application the front-panel IFB
outputs will serve as a convenient point
to directly connect, using standard flexible microphone cables, announcer’s
consoles, listen-only beltpacks, and other
user devices.
The type of interconnecting cables used
between the Model 42A’s IFB outputs and
the user devices will vary by application.
In a fixed installation it would be typical
to use 22 AWG, shielded, stranded cable
in either a single- or 2-pair configuration.
With single-pair cable, pin 1 should be
connected to shield and pins 2 and 3
connected to the cable pair. If 2-pair cable
is used, pin 1 should connect to one side
of each pair, with pin 2 going to one side
of pair one and pin 3 going to one side
of pair two. The shields can either go only
to the XLR connector shells, or to both the
connector shells and pin 1.
Shielding unbalanced audio signals can
be a tricky proposition. It is recommended that the focus be on using excellent
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Studio Technologies, Inc. Page 11
twisted-pair cable rather than worrying
about whether or not it is shielded. The foil
shields typically provided in many of types
of audio cables generally offer very limited
effectiveness. The best rule to follow is to
try to minimize exposure to large noise
sources. (Okay, so that’s hardly ever practical but at least it’s a nice dream!)
In the event that very long cable runs are
required, the resistance of the cable can
impact the DC power supplied by the
Model 42A. There’s no way to get around
the fact that some DC voltage will be
dropped in the interconnecting cable(s). A
simple “ohms law” calculation will tell you
the impact that a cable run will have. You’ll
need to know the exact cable length, the
current draw of the connected device(s),
the minimum voltage required by the connected device(s), and the resistance of the
cable’s conductors. This is generally stated as ohms per 1000 feet. Make sure that
you account for the resistance in both the
pin 1 and pin 2 legs! In general, if there is
the potential for a cable-length problem,
moving to a more substantial cable gauge,
such as 20 or 18 AWG, can be effective.
obtained. The wire colors in the AC mains
cord should conform to the internationally
recognized color code and be terminated
accordingly:
Connection Wire Color
Neutral (N) Light Blue
Line (L) Brown
Earth/Ground (E) Green/Yellow
Safety Warning: The Model 42A does
not contain an AC mains disconnect
switch. As such, the AC mains cord
plug serves as the disconnection
device. Safety considerations require
that the plug and associated inlet be
easily accessible to allow rapid disconnection of AC mains power should
it prove necessary.
As soon as AC mains power is applied
the Model 42A will begin its power-up
sequence. As a “boot-up” indication
each of the front-panel LEDs will light in a
processor-controlled sequence. After that
has completed one of the status LEDs will
remain lit. The unit is now fully operational.
AC Mains Power
The Model 42A operates directly from AC
mains power over the range of 100 to
240 V, 50/60 Hz. Being a “universal input”
device, there are no switches to set or
jumpers to install to match a location’s
nominal AC mains voltage. The Model 42A
uses a 3-pin IEC 320 C14 inlet connector
to mate with a detachable AC mains cord.
All units are supplied with an AC mains
cord that has a North-American (NEMA
5-15P) standard plug on one end and
an IEC 320 C13 connector on the other
end. Units bound for other destinations
require that the appropriate cord set be
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Post-Installation
Audio Input Source Level
Adjustment
It’s important to confirm and, if required,
adjust the level of the audio sources
that are connected to the Model 42A’s
audio inputs. The monitor section’s dual
5-segment LED level meters will help
make this task simple. Begin by using
the pushbutton switch, located on the
front panel, to select the IFB output that is
going to be calibrated. Adjust the source
levels so that the four green LEDs light
when typical audio signals are present.
The desired nominal output level of the
IFB outputs is –10 dBu. This is reflected
in the top green LED being calibrated
to, and labeled, –10. The meters’ yellow
LEDs, labeled –4, should light infrequently,
generally only when signal peaks are present. Achieving a precise level calibration is
not critical. But getting the levels within the
optimum range is very important!
It’s likely that the initial levels provided by
analog ports on a digital matrix intercom
system won’t be an exact match with the
Model 42A’s audio inputs. This shouldn’t
pose a problem as the computer control
available on contemporary intercom systems should make level adjustment very
simple. From our research we found that
the Clear-Com Eclipse™ system specifies a nominal level of 0 dBu. Since their
headroom is listed as greater than 18 dB,
increasing the nominal level of their analog output ports by 4 dB (to achieve the
desired +4 dBu) should be acceptable.
RTS in their ADAM™ and ZEUS™ systems
specify nominal output levels of +8 dBu.
With these systems correct Model 42A
performance would be provided by reducing the ports’ output levels by 4 dB. With
the Artist™ system from Riedel, the analog
ports have a nominal level of +6 dBu. A
2 dB reduction in their output level would
be beneficial. In reality, a signal with a
nominal level that deviates somewhat from
precisely +4 dBu will be acceptable.
But with the flexibility of contemporary
computer-controlled intercom systems,
there’s no reason why a precisely calibrated system can’t be easily implemented.
Audio Integrity
At this stage the Model 42A should have
been installed and the audio source levels
adjusted as required. The unit should now
be ready for many years of excellent performance. But before turning to another
task, performing a final Model 42A “reality check” is highly recommended. Using
the monitor section, along with a pair of
high-quality stereo headphones, carefully
listen to each channel associated with the
four IFB outputs. Ensure that the correct
audio sources are assigned to the correct IFB outputs. Confirm that all interrupt
channels have the correct audio levels as
they switch from normal audio to interrupt
content. Overall, the audio quality should
be excellent, with no hum, noise, hiss, or
other objectionable content. Should any
issues be detected, now is the time to
correct them. Presenting users with a
correctly implemented IFB system will
make life better for everyone involved!
Operation
Overall, the Model 42A is designed for
continuous operation with no adjustment
or maintenance required. On the audio
input side, maintaining the correct level
coming from the audio sources is very
important. As for the IFB outputs, the
cabling that connects the Model 42A to
the user devices must remain free of full or
partial short circuits. And the total current
draw of the connected user devices must
remain at 220 milliamperes or less.
The audio and DC voltage monitor functions will assist users in confirming that
correct operation is taking place. In addition, the under-voltage shutdown function
will help to protect the IFB output circuitry
should a fault condition be detected. The
“heart” of the monitor section is logic
circuitry created by a microcontroller
integrated circuit along with its associated
Model 42A User Guide Issue 2, December 2013
Studio Technologies, Inc. Page 13
firmware. This combination adds “smarts”
to otherwise fairly pedestrian functions.
Using four electromechanical relays, the
audio monitor section accesses the IFB
outputs directly on pins 2 and 3 of their
respective output connector. This ensures
that the impact of the actual wiring and
connected user devices is monitored,
rather than just observing something
internal to the Model 42A’s circuitry. Pin 2
of each of the four IFB output connectors
(one connector on the front panel and one
on the back) is also connected, by way of
interface circuitry, to analog-to-digital converter inputs on the processor. This allows
the DC output voltages to be continuously
monitored.
Associated with the audio and DC voltage
monitor functions are four status LEDs,
one pushbutton switch, two 5-segment
LED level meters, a rotary level control,
and a headphone jack. The four monitor
status LEDs are used to indicate which
IFB output is currently being audio monitored as well as presenting the status of
the DC output voltages. One of the LEDs
is always lit, indicating which IFB output
is currently being monitored by the meters
and headphone output.
Audio Monitoring
The dual 5-segment LED level meters allow a direct observation of the audio levels
present on pins 2 and 3 of the selected
IFB output’s 3-pin XLR connector pair. In
television broadcast settings, the Pin 2
(left) meter will typically display the “interrupt” signal while the Pin 3 (right) meter
will display “program.” A quick glance at
the meters will give an accurate overall
indication of a circuit’s performance.
It’s important to note that the Model 42A’s
meters are calibrated differently from the
typical “VU” scale. The level steps were
selected to effectively display the IFB
output’s nominal –10 dBu signal level.
The ballistics of the meters are also different, being a cross between VU and peak.
The bottom four LEDs are green in color
and indicate that signals are in the normal
range. The top LED, yellow in color, lights
when signals are 6 dB or greater above
–10 dBu. A correctly functioning IFB output should find normal signals lighting the
four green LEDs with the yellow LED lighting only on peaks.
Figure 6. Detail of front panel showing dual
5-segment LED level meters
The headphone output allows audible
monitoring of the selected IFB output.
The 2-channel (stereo) output is compatible with virtually any pair of stereo headphones. As the output circuitry meets
“pro audio” specifications, it’s recommended that high-quality headphones be
used. Pin 2 of the IFB output is the signal
source for the left channel of the headphone output. Pin 3 of the IFB output is
the source for the right channel. The rotary
control adjusts the output level of both the
left and right channels. Should it be necessary, there’s no reason why the headphone output couldn’t also be used as
a line-level monitor output.
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Figure 7. Detail of front panel showing headphone
section
A pushbutton switch, located on the front
panel, serves two purposes: selecting the
IFB output channel to be monitored and
enabling the auto scan feature. To select
an IFB output channel to be monitored,
press and release the IFB Circuit Select
pushbutton. Each press of the button
will advance the channel to be monitored
by one. A delay is built into the channel
selection process allowing a user to move,
for example, from channel 1 to channel 3.
By pressing the button twice in rapid
succession channel 2 will be automatically
skipped.
Unique to the Model 42A is its auto scan
feature. Pressing and holding the button
for two seconds will cause this feature to
begin operation. In this mode the monitor
source automatically “steps” through each
IFB output, pausing for eight seconds
Figure 8. Detail of front panel showing four status
LEDs and associated pushbutton switch
before moving on to the next. Ideally, this
will allow technical personnel to observe
a problem through casual viewing of the
Model 42A’s front panel.
DC Voltage Monitoring
The Model 42A’s microcontroller integrated circuit, under software control,
“watches” to ensure that the DC voltage
present on pin 2 of each IFB output connector pair is at an acceptable level. The
low-voltage threshold for the Model 42A’s
30 volt DC output is 24 volts. As discussed
in the previous section, the four LED
indicators display which IFB channel is
currently selected for audio monitoring. In
addition, the LEDs provide an indication of
the DC voltage status on the IFB outputs.
An LED will “flash” at a moderate cadence
if the voltage on pin 2 of its associated IFB
output falls below the acceptable value.
This can be caused by a temporary overcurrent or short-circuit condition, such
as when interconnecting user devices to
Model 42A IFB outputs using faulty portable cabling.
An under-voltage condition that’s present for a continuous 4-second period will
cause a fault condition to be recognized.
The status LED associated with the problem IFB output channel will indicate this
condition by flashing at a faster rate. In
addition, the output voltage on that specific IFB output channel will automatically
shut down to an essentially off condition.
A 10-second “cool-down” period will then
take place, after which the output voltage
will again become active. As soon as the
output is enabled normal output voltage
monitoring will again take place. A continuous short-circuit presented to an
IFB output will result in a continuous
4-seconds-on/10-seconds-off error cycle.
Model 42A User Guide Issue 2, December 2013
Studio Technologies, Inc. Page 15
It’s important to note that during the 10second off period no voltage monitoring
takes places. Removing the fault condition
will not result in the output voltage immediately turning on again; the 10-second
shut-down period must first elapse.
Operating Parameters
As expected with professional equipment,
whenever AC mains power is disconnected from the Model 42A, the present
operating state will be stored in nonvolatile
memory. Specifically, the IFB output channel selected for audio monitoring and the
status of the auto scan function will be
saved. This ensures that upon subsequent
power up the unit will return to its previous
operating state. If, for example, the auto
scan function was active when the Model
42A was powered down, auto scan will
begin once AC mains power is again
connected.
Troubleshooting
If you’re having trouble getting the Model
42A up and running, this section may
help. If you haven’t read the previous
sections of this guide, you should do so
before proceeding.
If the Model 42A Doesn’t
Function At All
A source of AC mains power must be
connected to the Model 42A. The unit is
a “universal input” type so that applying
power in the range of 100 to 240 volts,
50/60 Hz is acceptable. Whenever AC
mains power is connected the front-panel
LEDs should go through their power-up
routine, lighting in a processor-controlled
sequence. If this does not occur confirm
that AC mains power is active (“hot”) and
that the cord is securely mated with the
inlet connector on the Model 42A’s back
panel.
In all foreseeable situations, both normal
and abnormal, the status and meter LEDs
should go through their normal power-up
routine. However, it’s possible that if all
four IFB outputs are being presented
with a short-circuit condition, the internal
36 volt power supply may enter its protection mode and shut down. In this case
no LEDs will light. If this scenario occurs,
even after confirming that AC mains power
is correctly being applied, try removing the
loads from the IFB outputs. The easiest
way to do this is to remove the 3-pin
female XLR connectors that are plugged
into the Model 42A’s back panel. If normal
operation then begins, carefully check the
IFB output wiring for fault conditions.
If the front-panel LEDs still don’t go
through their power-up routine, even
after confirming that AC mains power is
present and that the IFB outputs are not
shorted, it’s likely that the unit requires
factory service. For safety in the event of
a major internal failure, the internal 36 volt
power supply contains a fuse in series
with the incoming AC mains power. This
fuse will open (“blow”) only if a serious
failure occurs inside the unit. The fuse is
not field-replaceable. The Model 42A must
be returned to the factory or an authorized
service location for review and repair.
Maintaining Correct Input
Signal Levels
The Model 42A’s four 2-channel audio
inputs are designed for nominal signal
levels of +4 dBu. Applying signal levels
significantly lower than +4 dBu will
Issue 2, December 2013 Model 42A User Guide
Page 16 Studio Technologies, Inc.
reduce the signal-to-noise ratio (raising the
perceived noise floor) and can prevent the
connected user devices from operating optimally. Applying signal levels significantly
higher than +4 dBu will reduce the headroom and greatly increase the chance of
reaching audio “clipping.” Obviously, these
cautions are not unique to the Model 42A,
but apply to most audio equipment. The
front-panel level meters provide an easy
means of confirming that the Model 42A
is being presented with the correct audio
levels.
output load to be within the maximum 220
milliamperes will allow the output to again
operate normally. In extreme cases, such
as when the Model 42A is located in an
environment with elevated temperatures,
a few minutes may be required from the
time an overload condition is removed
to when normal operation will again take
place. Please don’t test the Model 42A’s
ability to sustain frequent overload or
short-circuit conditions! The long-term reliability of the unit can be impacted by the
stress caused by these fault conditions.
To confirm correct IFB output operation at
locations away from where the Model 42A
is installed, it’s possible to use the Model
72 Level Meter/Interface, also available
from Studio Technologies. The Model 72
is a compact, portable device that plugs
directly into IFB or intercom circuits
and provides two useful functions. Two
5-segment LED meters display the audio
levels present on pins 2 and 3 of the connected circuit. In addition, two “dry” linelevel audio outputs are provided. Complete
information on the Model 72 is available
on the Studio Technologies website.
Maintaining Correct IFB
Output Current Draw
Each of the four IFB outputs is designed
to provide up to 220 milliamperes of DC
current. By design, the IFB outputs are
protected so that an overload condition,
or even a complete short circuit, should
not cause damage. Exceeding 220 milliamperes for more than a few seconds will
typically cause the auto shut-down mode
to become active. A continuous overload
condition will cause the IFB output voltage to cycle through a 4-seconds-on/10seconds-off sequence. Restoring the IFB
The four status LEDs make it simple to
know if an excessive load, or a short circuit, is being placed on one or more of the
IFB outputs. The LEDs provide a direct indication of the IFB outputs’ DC output voltage. Each output voltage is directly related
to the amount of current being drawn, as
well as indicating when its IFB output circuit has entered its protection mode and
has essentially shut down. During normal
operation the DC level on pin 2 of the connectors associated with each IFB output
circuit will range from approximately 28 to
30 volts. An LED will begin to flash on and
off if the level falls below approximately 24
volts DC. This will occur when the current
draw is greater than approximately 220
milliamperes. As previously discussed, if
the fault lasts for more than four seconds
the LED will flash at a faster rate. In addition, the DC output will shut down for
a 10-second period.
There’s really only one piece of advice
when it comes to understanding how
to use the under-voltage status LEDs:
if they’re flashing there’s a problem that
must be corrected! The most likely cause
will be a wiring fault that creates a partial
or full short circuit between pin 1 (common) and pin 2 (power with audio) on
Model 42A User Guide Issue 2, December 2013
Studio Technologies, Inc. Page 17
one or both of the IFB output’s XLR connectors. Another cause can be due to
problems with the IFB user devices. Either
a defective user device can be drawing too much current or too many user
devices end up being connected to the
same IFB output. Troubleshooting an IFB
output circuit problem should prove quick
and easy. Begin by disconnecting the IFB
user devices. Observe the status LED and
see if the problem has gone away. If not,
review the interconnecting cables and find
the fault condition. Within ten seconds of
the problem being “cleared” the status
LED will stop flashing.
Determining the actual IFB output current
draw won’t often be required but can be
useful in a tough troubleshooting situation. This measurement can be performed
using any good-quality digital multimeter.
Begin by setting the meter to measure
DC current. Then place the meter leads in
series with the pin 2 lead of the XLR connector associated with the IFB output to
be tested. The easiest way to measure
the pin 2 current is to create a simple
adapter cable using one female and one
male 3-pin XLR connector. Connect pin 1
on both connectors together. Connect pin
3 on both connectors together. Connect
separate wires to the pin 2 leads on both
connectors. Then connect the meter leads
to these two wires. The meter will indicate
the DC current being drawn while normal
operation of the connected device(s) takes
place. Be certain to connect the maximum
number of devices that might be powered by the IFB output. That is, measure
the worst-case condition and ensure that
the load is within the maximum 220 milliamperes output. If possible, leaving a
5- or 10-percent reserve margin is a good
practice.
Technical Notes
Software Version Display
A special Model 42A power-up sequence
allows the unit’s software version number
to be displayed. This is useful when working with factory personnel on application
support and troubleshooting situations.
The five LEDs associated with the level
meter for pin 2 are used to display the
major release number with a range of 1
through 5. The five LEDs associated with
the level meter for pin 3 are used to display the release sub-number which ranges
from 1 through 5. Refer to Figure 9 for a
detailed view of the LEDs and the corresponding software version numbering
scheme.
To display the Model 42A’s software
version is very simple. From the powered-down state, press and hold the
IFB channel Circuit Select button. While
continuing to press the button, apply
AC mains power to the unit. The normal
power-up sequence will take place and
then, instead of one status LED being lit,
one LED in the Pin 2 column and one LED
in the Pin 3 column will light. These two
LEDs represent the unit’s current software
version. The software version number will
Figure 9. Detail of front panel showing the status
LEDs that display software version. In this example,
the software version is 2.1.
Issue 2, December 2013 Model 42A User Guide
Page 18 Studio Technologies, Inc.
continue to be displayed until the button is
released. Once the button is released the
unit will begin normal operation.
Note that while it’s easy to determine
which software version is loaded into
the Model 42A a trip back to the factory
is required to update it. The 8-bit microcontroller that provides the unit’s logic
“horsepower” also includes internal
FLASH memory. This nonvolatile memory
is used to store the operating software
(“firmware”). Re-programming this memory requires using a specialized programming unit. While not outrageous in price,
it still costs in the range of US$500. The
programmer uses a ribbon cable and
socket to interface with a 6-pin “header”
on the Model 42A’s printed circuit board.
And, as you would guess, once connected
reprogramming takes only a matter of
seconds. But unfortunately the programmer is not something that would be found
in a typical “field shop” or repair facility.
Cable Length
There are no hard and fast rules defining
the maximum cable length possible when
connecting user devices to the Model
42A’s IFB outputs. The maximum cable
length is directly related to the amount of
resistance in the connecting cable; the
lower the resistance per foot (or meter),
the longer the cable can be. (Although
cable capacitance affects high-frequency
performance, resistance is the limiting factor in this case.) For example, a traditional
20 AWG microphone cable is Belden
8412, which has 10.9 ohms resistance
per conductor per 1000 feet. Since we’re
using two conductors to carry the signal
(pins 1 and 2) you’d get 21.8 ohms per
1000 feet of cable. By knowing the cable
resistance value, along with the minimum
voltage and maximum load current required by an IFB user device, a simple
“ohms law” calculation will tell you the
maximum cable length.
Let’s use the example of a Studio Technologies Model 210 Announcer’s Console
being connected to a Model 42A IFB
output. We’ll select Belden 8412 as the
interconnecting cable. For correct operation, the Model 210 needs at least 24 volts
DC between pins 1 and 2 of its IFB input
connector. It has a current draw of 105
milliamperes. The Model 42A’s IFB output presents an output voltage of 30 volts
across pins 1 and 2 and can supply a
maximum current of 220 milliamperes.
(As the Model 210’s current draw is well
within the Model 42A’s capability, this is
not a limiting factor.) The difference between the voltage supplied by the Model
42A (30 volts) and the voltage required by
the Model 210 (24 volts) allows a 6 volt
maximum drop over the interconnecting
cable. Using the current draw and maximum voltage drop figures, the maximum
cable resistance can easily be calculated: 6 volts divided by 0.105 amperes
equals 57 ohms. And finally, with 8412’s
21.8 ohms (total) per 1000 feet of cable,
a maximum of 2615 feet of cable can be
used and still be less than or equal to
57 ohms. Using this example as a guide,
entering the appropriate values will
allow you to determine the maximum
cable length for your application.
Cabling Issues – Crosstalk
The Model 42A’s IFB outputs conform to
a broadcast industry standard for sending
DC power and two channels of audio over
a single pair with shield audio cable. This
implementation allows standard portable
cables, such as are used for microphone
Model 42A User Guide Issue 2, December 2013
Studio Technologies, Inc. Page 19
signals, to interconnect various IFB user
devices. This method is undoubtedly convenient and practical, but is not without
limitations. The main audio quality issue
is the possibility of crosstalk between the
two audio channels. This issue arises due
to the capacitance presented by the two
wires that form the twisted pair. The greater the capacitance presented and the longer the cable run, the greater the crosstalk
will become. Is this normally a problem
during actual use? No. But it’s something
that should be noted.
one selected will depend on the specific
installation and personal technical philosophy. Two unshielded twisted pairs can be
effectively used. The first pair would carry
the DC and channel 1 audio signal and
common. The second pair would carry the
channel 2 audio signal, again along with
common. There will still be some capacitance between the conductors carrying
the two audio channels but it should be
significantly less. Two shielded pairs can
also be used as was discussed in the
Installation section of this guide.
Studio Technologies did some experimenting with various cables and the crosstalk
that was created. For example, a 1000-foot
reel of 24-gauge 2-pair unshielded telephone cable was used to link a Model 42A
IFB output with an IFB user device. One
pair carried the pin 2 (DC with channel 1
audio) and pin 3 (channel 2 audio) connections. One wire from the second pair
carried the pin 1 (DC and audio common)
connection. The inter-channel crosstalk
in the voice audio band was on the order
of –45 dB. Is this a good value for “professional” audio? Of course not. But for
the intended talent cueing applications it
should be fine. In almost all cases the
audio signals being carried are somewhat or fully phase-coherent. A bit of one
channel getting into the other won’t even
be noticed, especially since monitoring
is generally done using headsets, headphones, or earpieces.
Is it possible to reduce the crosstalk that
is created? Absolutely, as long as a nonstandard cable connection is made. This
becomes a trade-off between an improved
crosstalk figure and ease of installation
and use. Using two full pairs can significantly reduce crosstalk. Several connection schemes are possible; the exact
Superior Power Delivery and
Audio Quality
As previously discussed, one of the Model
42A’s strengths is its ability to very effectively deliver energy to the connected IFB
user devices. This allows more devices
to be supported over longer cable runs.
How does the Model 42A accomplish this?
Simply by having circuitry that is superior
to that used in most of the “industrystandard” equipment. In most IFB interface devices, an adjustable voltage
regulator integrated circuit is used as
a combination of audio modulator and
current limiter. While this is a simple and
inexpensive solution, it’s not without
significant limitations. The major problem
with this method is the type of voltagecurrent “knee” that is created. As the load
current increases past about 50 percent
of the rated maximum the output voltage
begins to decrease. This means that the
usable power delivered to the connected
device(s) will start to drop well before the
rated output is reached. This limitation will
become significant in applications that use
long cable runs. As the IFB output voltage begins to drop problems with user
device performance can occur. Contrast
Issue 2, December 2013 Model 42A User Guide
Page 20 Studio Technologies, Inc.
0
5
10
15
20
25
30
35
0 50 100 150 200 250
Current (mA)
Voltage (V)
4010
Model 41
this situation with the performance provided by the Model 42A. The DC voltage
supplied by its IFB outputs won’t “poop
out” when loaded over its 0 to 220 milliamperes range. This will allow IFB beltpack
and announcer’s console devices to function correctly in many more applications.
Figure 10 shows the IFB output voltagecurrent curves for the RTS 4000-series and
the Model 42A Interface. The performance
differences are quite interesting.
It’s interesting to note the reason why
typical IFB output audio quality is less
than pristine. It’s not hard to notice the
background “hiss” that is always present
on pin 2 (DC with channel 1 audio) of the
interface connector. Technically, it’s white
noise that comes from the adjustable
voltage regulator being used as an “AM”
modulator and current limiter. The noise
is an artifact of the design topology and
simply can’t be overcome. How does
Studio Technologies know this? Because
our first “breadboard” designs used this
method and achieved the same poor
results! Only after the problem came
to light did work on an improved circuit
begin. The results were worth the effort.
Model 42A
Figure 10. IFB output voltage-current curves for RTS 4000-Series and Model 42A Interface
Model 42A User Guide Issue 2, December 2013
Studio Technologies, Inc. Page 21
Specifications
General Audio:
Frequency Response – Pin 2 Output
(DC with Channel 1 Audio):
+0/–3 dB, 20 Hz-20 kHz; +0/–1 dB, 100 Hz-20 kHz
Frequency Response – Pin 3 Output
(Channel 2 Audio):
+0/–3 dB, 20 Hz-20 kHz; +0/–1 dB, 100 Hz-20 kHz
Distortion (THD+N) – Pin 2 Output:
(DC with Channel 1 Audio):
0.05%, measured at 1 kHz, +4 dBu
S/N Ratio – Pin 2 Output:
(DC with Channel 1 Audio):
86 dB, ref +4 dBu out, A-weighted
Audio Inputs: 8, organized as four 2-channel
inputs
Type: transformer balanced, compatible with
balanced or unbalanced sources
Impedance: 10 k ohms, nominal
Nominal Level: +4 dBu
IFB Outputs: 4
Type: DC power with two channels of unbalanced
audio (“wet” IFB)
Connections:
Pin 1: common
Pin 2: 30 volts DC (nominal) modulated with
channel 1 audio (–10 dBu nominal)
Pin 3: channel 2 audio (–10 dBu nominal)
Maximum Audio Output Level:
Pin 2: +9 dBu with +23 dBu on audio input
Pin 3: +14 dBu with +28 dBu on audio input
DC Current Output: 220 mA maximum
Monitor Section – Headphone Output:
Type: 2-channel (stereo), drives headphones by
way of 100 ohm series resistors
Compatibility: intended for connection to headphones with impedance of 100 ohms or greater
Maximum Voltage: 8 Vpp, 100 ohm load
Monitor Section – Output Voltage Detection:
Measures DC voltage level directly on pin 2 of IFB
output connector pairs. Status LEDs flash when
level is less than approximately 24 volts DC.
Connectors:
Audio Inputs: 25-pin female D-subminiature,
4-40 threads on locking hardware (two places)
IFB Outputs:
Headphone Output: ¼-inch 3-conductor phone
jack
AC Mains: 3-blade, IEC 320 C14 (mates with IEC
320 C13 connector)
AC Mains Requirement: 100-240 volts (±10%),
50/60 Hz, 40 watts maximum
Dimensions (Overall):
19.00 inches wide (48.3 cm)
1.72 inches high (4.4 cm)
6.4 inches deep (16.3 cm)
Mounting: one space (1U) in a standard 19-inch
rack enclosure
Weight: 3.7 pounds (1.7 kg)
Specifications and information contained in this
User Guide subject to change without notice.
8 (two per IFB output), 3-pin male XLR
Issue 2, December 2013 Model 42A User Guide
Page 22 Studio Technologies, Inc.
Appendix A
Interfacing Riedel® Artist™ Matrix Intercom Systems with the Model 42A Interface
Information courtesy of
Riedel Communications Inc.
Model 42A User Guide Issue 2, December 2013
Studio Technologies, Inc. Page 23
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