User Guide
Issue 1, July 2004
This User Guide is applicable for serial numbers:
M42-00151 and later
Copyright © 2004 by Studio Technologies, Inc., all rights reserved
www.studio-tech.com
50125-0704, Issue 1
Table of Contents
Introduction .................................................................. 5
Installation .................................................................... 7
Operation ..................................................................... 11
Troubleshooting ........................................................... 11
Technical Notes ........................................................... 13
Specifications .............................................................. 16
Appendix A .................................................................. 17
Block Diagram
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Issue 1, July 2004 Model 42 User Guide
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Introduction
The Model 42 is designed to create
broadcast-standard IFB circuits 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 42 provides four independent
IFB circuits. Each IFB circuit provides DC
power and two analog audio signals to
support the connected IFB user devices.
The Model 42’s audio quality is excellent;
little hiss, hum, or other artifacts are
present. Installation 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-type connectors. The compact,
one-rack-space package is constructed
using heavy-gauge steel components.
The unit’s mains power input can range
from 100 to 230 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
interrupted foldback. (It can also be known
as interruptible foldback.) On its own,
the term foldback is an alternate way of
describing a cue or monitor function. Adding “interrupted” 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 circuits are often used in
the broadcast industry for talent cueing
applications, both in studio and field
settings. Both “dry” and “wet” IFB circuits
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 +4dBu
nominal level. This is essentially a standard 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 audio is unbalanced
with a typical nominal level of –10dBu. A
Model 42 Front Panel
Power present
LED
Model 42 Back Panel
AC mains
connection
Model 42 User Guide Issue 1, July 2004
Studio Technologies, Inc. Page 5
IFB output
circuit connections
Audio input
connections
wet IFB circuit is the type implemented
by the Model 42. As such, in this user
guide the term IFB will always represent
this type of circuit.
IFB circuits 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-type
connectors. With IFB circuits and standard
audio cables it’s a simple matter to support user devices such as listen-only belt
packs and announcer’s consoles with no
external power source required. Whether
100 or 1000 feet apart, reliable operation
can be provided.
In many cases, the Model 42 Interface will
be used in on-air television applications.
Whether installed in a fixed location or as
part of a remote facility, excellent performance can be obtained. In addition, the
Model 42 is applicable for non-broadcast
applications. For example, audio recording and post-production facilities can also
effectively use the unit. Combined with
stereo or mono listen-only belt packs,
also available from Studio Technologies,
a variety of headphone cue systems can
easily be deployed. As the Model 42’s
audio inputs are compatible with standard
line-level audio signals virtually any
analog source can be connected.
Four Independent IFB Circuits
The Model 42 supplies four independent
IFB circuits. Each circuit consists of two
audio inputs and a “wet” IFB output circuit.
The audio inputs are transformer coupled,
have a nominal level of +4dBu, 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 feed user cue signals to stereo
or monaural headsets or headphones.
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 “program-withinterrupt.” 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 42’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.
Maintaining excellent audio performance
was a major Model 42 design goal—the
hiss, hum, and noise associated with
typical IFB circuits was simply not acceptable. The Model 42 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 signal.
The Model 42’s IFB circuits provide DC
power and two channels of unbalanced
audio over a single 3-conductor output.
The DC output is nominally 30 volts with
a maximum rated current of 200 milliamperes. A major strength of the Model 42
is the IFB circuit’s ability to effectively
deliver DC power over a variety of conditions. Unlike other interface devices
that use a common but less-than-ideal
Issue 1, July 2004 Model 42 User Guide
Page 6 Studio Technologies, Inc.
circuit topology, a unique IFB 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 full
rated 200 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.
Compatibility
The Model 42 is compatible with virtually
every digital matrix intercom system,
including those from Clear-Com®, Drake,
RTS™, and Riedel Communications.
Interfacing requires only the connection
of analog output ports from the intercom
system to the Model 42’s audio inputs.
With the Model 42’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 42 the intercom system’s audio
quality can be maintained all the way to
the IFB users.
The Model 42’s IFB circuits allow virtually
every IFB user device to be supported.
These include the Model 30-series listenonly belt packs and Model 200-series
announcer console products from Studio
Technologies. The 200-series units combine a variety of microphone control,
headphone monitoring, IFB and intercom
system interfacing, and related functions
into compact desktop units. Industrystandard listen-only belt packs from RTS,
including the 4020 and 4030, can also be
directly supported.
Alternate Applications
In addition to broadcast intercom applications, the Model 42 can be used to create
high-performance stereo headphone cue
systems. Stereo line-level signals coming
from audio consoles, routing switchers, or
off-air receivers can be connected to the
Model 42’s audio inputs. The IFB circuits
can be connected to listen-only belt packs,
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 four Model 42
IFB circuits will support up to six Model 35
Talent Amplifiers.
Installation
In this section you will be installing and
interconnecting the Model 42. 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 circuits will be
interfaced using four 3-pin XLR-type
connectors. AC mains power is connected
by means of a detachable cord set that is
compatible with the Model 42’s 3-pin IEC
320 C14-type inlet connector.
System Components
The shipping carton contains the Model 42
Interface and associated user guide. Units
destined for North America and Japan
also include an AC mains cord. Your
Model 42 User Guide Issue 1, July 2004
Studio Technologies, Inc. Page 7
dealer or distributor should provide an
AC mains cord for other destinations.
Locating the Model 42
The Model 42’s IFB circuits provide DC
power and unbalanced audio to operate
external IFB user devices. These devices
are often IFB “belt packs,” broadcast
announcer consoles, or other “talkback
boxes.” The Model 42’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
field broadcast applications the Model 42
will almost always be located in a production truck or trailer. But in fixed applications it may be possible to select the
Model 42’s mounting location so as to
minimize cable length. In general, shorter
cables will lead to more reliable and
consistent system performance.
Mounting the Model 42
Once the desired mounting location has
been selected, the Model 42 will require
one space (1.75 vertical inches) in a standard 19-inch (48.3cm) equipment rack.
Secure the unit into the equipment rack
using two mounting screws per side.
Audio Input Connections
The Model 42 has eight line-level audio
inputs, arranged as four 2-channel pairs.
Each pair serves one of the four IFB circuits. Each IFB circuit, along with its two
associated audio inputs, is completely
independent. 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 one of a pair is generally
designated as the “interrupt” channel
while channel two is the “program” channel. For other broadcast applications, such
as live radio, it’s possible that a stereo cue
source will be connected. In this situation
the left source would be connected to input
channel one while the right source would
be connected to input channel two. This
might also be the case with other professional audio applications, such as recording and post-production.
Audio input connections are made by way
of one female 25-pin D-subminiature connector which is located on the Model 42’s
back panel. A cable harness is required
with a 25-pin D-sub plug (male) on one
end and the desired mating connectors
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 42’s
audio inputs is identical to a DA-88-style
input harness. Please refer to Figures 1 and
2 for connection details. Note that unlike
a DA-88-style harness, the Model 42’s
D-sub connector’s hold-down screws use
4-40 threads. This complies with the original design standard for D-subminiature
connectors which used English rather
than metric thread pitch.
The Model 42’s audio input circuits have
a nominal signal level of +4dBu. They
are transformer coupled, have a nominal
impedance of 10K ohms, and compatible
with balanced or unbalanced sources.
Balanced sources should be wired so
that signal high is connected to the + pins,
signal low to the – pins, and shield to the
shield pins. With unbalanced sources,
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Page 8 Studio Technologies, Inc.
Connections High (+) Low (–) Shield
Signal Signal
IFB Circuit 1-Interrupt 24 12 25
IFB Circuit 1-Program 10 23 11
IFB Circuit 2-Interrupt 21 9 22
IFB Circuit 2-Program 7 20 8
IFB Circuit 3-Interrupt 18 6 19
IFB Circuit 3-Program 4 17 5
IFB Circuit 4-Interrupt 15 3 16
IFB Circuit 4-Program 1 14 2
Notes: 1) Connector type on Model 42 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-type wiring harnesses are directly
compatible, with the exception of 4-40 screw threads
being required.
Figure 1. Connections for Audio Inputs using IFB
Nomenclature
Signal Signal
Connections High (+) Low (–) Shield
IFB Circuit 1-Channel 1 24 12 25
IFB Circuit 1-Channel 2 10 23 11
IFB Circuit 2-Channel 1 21 9 22
IFB Circuit 2-Channel 2 7 20 8
IFB Circuit 3-Channel 1 18 6 19
IFB Circuit 3-Channel 2 4 17 5
IFB Circuit 4-Channel 1 15 3 16
IFB Circuit 4-Channel 2 1 14 2
connect signal high to the + pins, 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 + pins,
and shield to the – pins; leave the shield
pins unterminated.
As previously mentioned, the Model 42 is
compatible with matrix intercom systems
from Riedel Communications. Appendix A,
located near the end of this user guide,
provides detailed connection information.
Audio Input Source Level
Adjustment
Literally any audio source with a nominal
operating level of +4dBu can be successfully connected to the Model 42’s audio
inputs. 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 42. It’s important to confirm and,
if required, adjust the nominal output level
provided by the specific system. In reality,
a nominal level that deviates somewhat
from exactly +4dBu is acceptable. But
with the power of contemporary computercontrolled intercom systems, there’s no
reason why a precisely calibrated system
can’t be easily implemented.
From our research we found that the Clear-
Notes: 1) Connector type on Model 42 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-type wiring harnesses are directly
compatible, with the exception of 4-40 screw threads
being required.
Com Eclipse™ system specifies a nominal
level of 0dBu. Since their headroom is
listed as greater than 18dB, increasing
the nominal level of their analog output
ports by 4dB (to achieve the desired
+4dBu) should be acceptable. RTS
in their ADAM™ and ZEUS™ systems
Figure 2. Connections for Audio Inputs using DualChannel Nomenclature
Model 42 User Guide Issue 1, July 2004
Studio Technologies, Inc. Page 9
specify nominal output levels of +8dBu.
With these systems maximum Model 42
performance would be provided by reducing their nominal output levels by 4dB.
With the Artist™ system from Riedel, the
analog ports have a nominal level of
+6dBu. A 2dB reduction in their output
level would be beneficial.
IFB Circuits
As mentioned previously, the Model 42
has four independent IFB output circuits
each supplying DC power and two channels of unbalanced audio. The IFB circuits
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 nominal with a maximum current
of nominally 200 milliamperes. The audio
superimposed on the DC power has a
nominal signal level of –10dBu. Its audio
source is the channel one audio input
associated with that specific IFB circuit.
The audio on pin 3 also has a nominal
signal left of –10dBu. Its audio source
is the channel two audio input channel
associated with that specific IFB circuit.
The Model 42’s IFB circuits are interfaced
using four 3-pin male XLR-type connectors
which are located on the unit’s back panel.
The associated interface cables, one for
each IFB circuit, must be terminated with
3-pin female XLR-type connectors. In most
cases the IFB circuits should be wired by
way of an input/output connector panel
rather than directly to the external devices.
It’s also recommended that the panel have
“mults” (multiple connectors) for each
of the IFB circuits. For troubleshooting
purposes it also may be useful to have
the IFB circuits pass through points on
an audio patch bay.
The type of interconnecting cable used
between the Model 42’s IFB circuits and
the user devices will vary by application.
In a fixed installation it would be typical
to use 22AWG, shielded, stranded cable
in either a single- or 2-pair configuration.
With single-pair cable, pin 1 should be
connected to shield. Pins 2 and 3 would
connect 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 twisted-pair cable, rather
than worrying about whether or not it is
shielded. The typical foil shields used
in much of the contemporary audio cable
generally offers 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 42. There’s no way to get around
the fact that some DC voltage will be
dropped by the interconnecting cable. A
simple ohms law calculation will tell you
the impact a specific cable run will have.
You’ll need to know the current draw of the
Issue 1, July 2004 Model 42 User Guide
Page 10 Studio Technologies, Inc.
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, 18,
or 16 can be effective.
Safety Warning: The Model 42 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.
AC Mains Power
The Model 42 operates directly from AC
mains power of 100 to 230V, 50/60Hz.
Being a “universal input” device, there are
no switches to set or jumpers to install to
match a location’s nominal mains voltage.
For locations that have a mains power
source of 240 volts, contact Studio Technologies for confirmation that a direct
connection can be made.
The Model 42 uses a 3-pin IEC 320 C14type inlet connector to mate with a detachable mains cord set. For units shipped to
North America and Japan a cord is supplied that has a North-American (NEMA
15L) standard plug on one end and an
IEC 320 C13-type connector on the other.
Units bound for other destinations require
that the appropriate cord set be obtained.
The wire colors in the mains cord must
conform to the internationally recognized
color code and should be terminated
accordingly:
Connection Wire Color
Neutral (N) Light Blue
Line (L) Brown
Protective Earth (E) Green/Yellow
As soon as AC mains power is applied,
the Model 42’s power present LED will
light. The unit is now fully functional.
Operation
There are no switches, potentiometers,
or user controls associated with the
Model 42. It’s designed for continuous
operation with no adjustment or maintenance required. Proper operation will
take place as long as attention is paid
to the level presented on the audio inputs
and the loading placed on the IFB circuits.
Troubleshooting
If you’re having trouble getting the Model
42 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 42 Doesn’t Work
At All
A source of AC mains power must be
connected to the Model 42. The unit is
a “universal input” type so that applying
anything between 100 and 230 volts,
50/60Hz is acceptable. Whenever mains
power is connected the front-panel
Model 42 User Guide Issue 1, July 2004
Studio Technologies, Inc. Page 11
power present LED should be lit. This LED
is powered by the internal 36 volt DC
power supply that is used by the circuitry.
If the LED is not lit confirm that AC mains
power is active (“hot”) and that the cord
is securely mated with the inlet connector
on the Model 42’s back panel.
In all foreseeable situations, both normal
and abnormal, the front-panel LED should
be lit. However, it’s possible that if all four
IFB circuits 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 the LED
will not light, or will light intermittently. If
the LED presents this scenario, even after
confirming that mains power is correctly
being applied, try removing the loads
from the IFB circuits. The easiest way to
do this is to remove the 3-pin female XLRtype connectors that are plugged into the
Model 42’s back panel. If after a few seconds the LED again lights, carefully check
the IFB circuit wiring for fault conditions.
If the LED still doesn’t light, even after
confirming that mains power is present
and that the IFB circuits 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 mains power. This fuse will
open (“blow”) only if a serious failure
occurs inside the unit. The fuse is not
field-replaceable. The Model 42 must be
returned to the factory, or an authorized
service location, for review and repair.
Maintaining Correct Input
Signal Levels
The Model 42’s four 2-channel audio
inputs are designed for nominal signal
levels of +4dBu. Applying signal levels
significantly lower than +4dBu will 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 +4dBu will reduce
the headroom and greatly increase the
chance of reaching audio “clipping.”
Obviously, these cautions are not unique
to the Model 42, but apply to most audio
equipment.
The easiest means of confirming that the
Model 42 is being presented with the
correct audio levels is to use a 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”
line-level audio outputs are provided.
The Model 72 should prove to be very
useful, both during initial Model 42
installation and routine system testing.
Complete information on the Model 72
is available on the Studio Technologies
website.
Maintaining Correct IFB
Circuit Current Draw
Each of the four IFB circuits is designed
to provide up to 200 milliamperes of DC
current. By design, the IFB circuits are
protected so that an overload condition,
or even a complete short circuit, should
not cause damage. Exceeding 200 milliamperes will cause the protection circuitry
to come into play. An overload condition
will cause the output voltage to shut off
continually or intermittently. The exact
Issue 1, July 2004 Model 42 User Guide
Page 12 Studio Technologies, Inc.
action will depend on the specific overload condition that is present. In general,
the more extreme the overload condition,
the sooner normal operation will cease.
Restoring the output load to be within the
rated 200 milliamperes will allow the
output to again operate normally. A few
seconds may be required from the time
an overload condition is removed and
when normal operation again takes place.
Please don’t test the Model 42’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.
If there is concern that excessive loads
are being placed on one or more of the
IFB circuits, performing a simple test is
recommended. This 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-type
connector associated with the IFB circuit
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-type 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 circuit.
That is, measure the worst-case condition
and ensure that the load is within the rated
200 milliamperes output. If possible,
leaving a 10 or 20% reserve margin is
a good practice.
Technical Notes
Cable Length
There are no hard and fast rules defining
the maximum cable length possible when
connecting user devices to Model 42 IFB
circuits. 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 standard 20 AWG
microphone-type 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 200 Announcer’s Console
being connected to a Model 42 IFB circuit.
We’ll select Belden 8412 as the interconnecting cable. For correct operation, the
Model 200 needs at least 24 volts DC
between pins 1 and 2 of its IFB input
connector. It has a current draw of 95
milliamperes. The Model 42’s IFB circuit
presents an output voltage of 30 volts
across pins 1 and 2 and can supply a
maximum current of 200 milliamperes.
(As the Model 200’s current draw is well
within the Model 42’s capability, this is not
a limiting factor.) The difference between
the voltage supplied by the Model 42
(30 volts) and the voltage required by
Model 42 User Guide Issue 1, July 2004
Studio Technologies, Inc. Page 13
the Model 200 (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 0.095 amperes equals
63 ohms. And finally, with 8412’s 21.8
ohms (total) per 1000 feet of cable, a
maximum of 2890 feet of cable can be
used and still be less than or equal to
63 ohms. Using this example as a guide,
entering the appropriate values will allow
you to determine the maximum cable
length for your application.
audio) connections. While one wire from
the second pair carried the pin 1 (DC and
audio common) connection. The interchannel crosstalk in the voice audio band
was on the order of –45dB. 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 ear pieces.
Cabling Issues – Crosstalk
The Model 42’s IFB circuits 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
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.
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 42 IFB circuit with an IFB
user device. One pair carried the pin 2 (DC
with channel 1 audio) and pin 3 (channel 2
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 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 user guide.
Superior Power Delivery and
Audio Quality
As previously discussed, one of the Model
42’s strengths is its ability to very effectively deliver energy to the connected IFB
Issue 1, July 2004 Model 42 User Guide
Page 14 Studio Technologies, Inc.
user devices. This allows more devices
0
5
10
15
20
25
30
35
0 50 100 150 200 250
Current (mA)
Voltage (V)
4010
Model 42
to be supported over longer cable runs.
How does the Model 42 accomplish
this? Simply by having circuitry that
is superior to that used in most of the
“industry-standard” 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% 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 circuit
voltage begins to drop problems with user
device performance can occur. Contrast
this situation with the performance provided by the Model 42. The DC voltage
supplied by its IFB circuits won’t “poop
out” when loaded over its 0 to 200 milliamperes range. This will allow IFB belt
pack and announcer’s console devices to
work correctly in many more applications.
Figure 3 shows the IFB circuit voltagecurrent curves for the RTS 4000-series and
the Model 42 Interface. The performance
differences are quite interesting.
Figure 3. IFB Circuit Voltage-Current Curves for RTS 4000-Series and Model 42 Interface
Model 42 User Guide Issue 1, July 2004
Studio Technologies, Inc. Page 15
It’s interesting to note the reason why
typical IFB circuit 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.
Specifications
IFB Circuits: 4
Type: “wet” (DC power with two channels of
unbalanced audio)
Connections: common on pin 1, DC (+30V
nominal) modulated with channel 1 audio (–10dBu
nominal) on pin 2, and channel 2 audio (–10dBu
nominal) on pin 3
Maximum Level:
Pin 2 Outputs (DC with Channel 1 Audio): +9dBu
(+23dBu on audio input)
Pin 3 Outputs (Channel 2 Audio): +14dBu (+28dBu
on audio input)
Connectors:
Audio Inputs: 1, 25-pin, female, D-subminiature,
4-40 threads
IFB Outputs: 4, 3-pin, male, XLR-type
AC Mains: 3-blade, IEC 320 C14-type (mates with
C13-type socket)
AC Mains Requirement:
100-230V, 50/60Hz, 0.7A maximum @ 100V, 0.4A
maximum @ 230V
General Audio:
Frequency Response:
Pin 2 Outputs (DC with Channel 1 Audio):
20Hz-20kHz ±2.5dB (80Hz-20kHz ±0.25dB)
Pin 3 Outputs (Channel 2 Audio): 20Hz-20kHz
±0.25dB
Distortion (THD+N): pin 2 outputs (DC with
channel 1 audio) 0.01%, ref +4dBu in, measured
at 1kHz
S/N Ratio: pin 2 outputs (DC with channel 1
audio) 80dB, ref +4dBu in, 20Hz-20kHz
Crosstalk: 81dB, typical, ref +4dBu in,
20Hz-20kHz
Audio Inputs: 8, organized as four 2-channel
inputs
Type: transformer balanced, capacitor coupled,
compatible with balanced or unbalanced sources
Impedance: 10k ohms, nominal
Nominal Level: +4dBu
Dimensions (Overall):
19.00 inches wide (48.3cm)
1.72 inches high (4.4cm)
9.58 inches deep (24.3cm)
Mounting: one space in a standard 19-inch rack
Weight: 6.4 pounds (2.9kg)
Specifications and information contained in this
User Guide subject to change without notice.
Issue 1, July 2004 Model 42 User Guide
Page 16 Studio Technologies, Inc.
Appendix A
Interfacing Riedel Artist™ Matrix Intercom Systems with the Model 42 Interface
Information courtesy of
Riedel Communications Inc.
Model 42 User Guide Issue 1, July 2004
Studio Technologies, Inc. Page 17
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Issue 1, July 2004 Model 42 User Guide
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