Studio Technologies 42A Users Manual

Introduction

The Model 42A is designed to create broadcast-standard IFB circuits from line-level 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 provides four independent IFB circuits. For
convenience, the four circuits can be connected by way of eight 3-pin male XLR-type connectors; four
of which are located on the back panel and four on the front. Each IFB circuit 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 over-current and short-circuit 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-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 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
circuits are often used in the broadcast industry for talent cueing applications, both in studio and fi eld
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 nominal level
in the range of 0 to +8 dBu. 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 nominal level of the DC power source is typically in the range of 30
to 32 volts. The audio is unbalanced with a typical nominal level of –10 dBu. The Model 42A implements
wet IFB circuits. As such, in this user guide the term IFB will always represent this type of circuit.

Model 42A Front Panel

Highlights

• Transformer-coupled inputs

• Excellent audio quality

• Superior power-feed performance

• Front- and back-panel IFB output connectors

• Single rack-space mounting

• Universal mains input powering

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. Using 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 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 fi xed
location or as part of a remote facility, the unit is capable of providing excellent performance. In addition, the
Model 42A is applicable 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 belt packs, 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 Circuits

The Model 42A supplies four independent IFB circuits. 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 user cue signals.
Generally one source is confi gured in the matrix intercom system as “interrupt” while the other is confi gured as
“program.” An alternate term often used for the “interrupt” channel is “program-with-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 confi guration
may prove useful in radio broadcasting, audio-with-picture, or recording studio applications. The specifi c 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 program-with­interrupt audio.

Maintaining excellent audio performance was a major Model 42A design goal—the hiss, hum, and noise associ­ated with typical IFB circuits 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 signal.

Each Model 42A IFB circuit provides DC power and two channels of unbalanced audio by way of two 3-pin male
XLR-type connectors. The DC output is nominally 30 volts with a maximum rated current of 200 milliamperes. A
major strength of the Model 42A 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 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

Model 42A Back Panel

Studio Technologies, Inc. Model 42A Interface, Issue 1, Page 2

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 confi rm 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 maintained. A headphone output is also provided, allowing “real-world” check­ing of IFB audio quality. In addition to being able to manually select the IFB circuit to be monitored by the meters
and headphone output, an “auto scan” mode is also included. This allows each of the four IFB circuits to be
monitored in a continuously repeating sequence.

DC Monitoring and Fault Shut Down

To prevent possible damage to the Model 42A’s IFB output circuitry, the DC output voltage present on each IFB
circuit is continually monitored. If the voltage on an IFB circuit falls below 24 volts its associated LED will “fl ash”
as an error indication. If this under-voltage condition continues for more than 4 seconds a shut-down mode will
be entered, turning off power to the IFB circuit. 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 Clear­Com®, Drake, Telex®/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 might be gained by confi guring the
nominal audio level of the intercom system’s “virtual” IFB output ports to match the Model 42A’s +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 circuits allow virtually every IFB user device to be supported. These include the Model
30-series listen-only 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. Industry-standard 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 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 circuits can be connected to listen­only belt packs, several models of which are available from Studio Technologies. For example, the Model 35
Talent Amplifi er will allow one or two pairs of stereo headphones to be supported. Each of the Model 42A’s
four IFB circuits will support up to six Model 35 Talent Amplifi ers.

Studio Technologies, Inc. Model 42A Interface, Issue 1, Page 3

Specifi cations

General Audio:

Frequency Response:

Pin 2 Outputs (DC with Channel 1 Audio): 20 Hz-20 kHz ±2.5 dB (80 Hz-20 kHz ±0.25 dB)
Pin 3 Outputs (Channel 2 Audio): 20 Hz-20 kHz ±0.25 dB

Distortion (THD+N): 0.01%, measured at 1 kHz, +4 dBu, pin 2 outputs (DC with channel 1 audio)

S/N Ratio: 80 dB, ref +4 dBu out, 20 Hz-20 kHz, pin 2 outputs (DC with channel 1 audio)

Crosstalk: 81 dB, typical, ref +4 dBu in, 20 Hz-20 kHz

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 Output Circuits: 4

Type: DC power with two channels of unbalanced audio

Connections: common on pin 1, DC (+30 V nominal) modulated with channel 1 audio (–10 dBu nominal) on pin 2, and channel 2 audio

(–10 dBu nominal) on pin 3

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: 200 milliamperes 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 volts peak-to-peak, 100 ohm load

Monitor Section – Output Voltage Detection:

Measures DC voltage level directly on pin 2 of IFB circuit output connector pairs. Status LEDs fl ash when level is less than approximately
24 volts DC.

Connectors:

Audio Inputs: 25-pin D-subminiature female, 4-40 threads

IFB Outputs: 8 (two per IFB circuit), 3-pin male XLR-type

Headphone Output: ¼-inch 3-conductor phone jack

AC Mains: 3-blade, IEC 320 C14-compatible (mates with IEC 320 C13)

AC Mains Requirement: 100-230 volts, 50/60 Hz, 0.7 amperes maximum @ 100 volts, 0.4 amperes maximum @ 230 volts

Dimensions (Overall):

19.00 inches wide (48.3 cm)

1.72 inches high (4.4 cm)

9.58 inches deep (24.3 cm)

Mounting: one space in a standard 19-inch rack

Weight: 6.8 pounds (3.1 kg)

Specifi cations subject to change without notice.
© by Studio Technologies, Inc., April 2008.

Studio Technologies, Inc.

Telephone [phone]
www.studio-tech.com

50128-0408, Issue 1