Studio Technologies 72 Bedienungsanleitung

www.studio-tech.com

50023-0704, Issue 2

User Guide

Issue 2, July 2004

This User Guide is applicable for serial numbers:

M72-00151 and later

Model 72

Level Meter / Interface

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Model 72 User Guide Issue 2, July 2004

Studio Technologies, Inc. Page 3

Model 72

Level Meter / Interface

Introduction

The Model 72 Level Meter/Interface is

a compact, portable device that plugs

directly into an IFB or intercom circuit and

provides two useful functions: audio level

meters and line-level audio outputs. IFB

(interruptible foldback) circuits are used in

broadcast applications, providing one or

two channels of talent cue audio. Intercom

circuits are used by production personnel,

supplying one or two channels of “party-

line” communications.

Two 5-segment LED meters allow direct

observation of the audio signal levels

present on the connected IFB or intercom

circuit. The display range is optimized for

the signal levels found on typical “wet”

(DC with audio) IFB and intercom circuits,

rather than traditional “VU” scaling.

Two audio outputs provide transformer-

coupled “dry” signals, one output associ-

ated with each IFB or intercom channel.

These pro-audio-quality outputs are useful

for a variety of production and testing ap-

plications. For example, the outputs can

serve as the interface between a traditional

“wet” IFB system and a wireless in-ear

monitor or IFB system. The Model 72’s

outputs can also be connected to line-

level inputs on an audio console, allowing

IFB or intercom audio signals to be com-

bined and/or routed to other local or remote

talent or production personnel cue systems.

Other applications may arise where an am-

plified speaker needs to be used to monitor

an IFB or intercom circuit. The Model 72’s

audio outputs will make achieving this fast

and simple.

Power for the Model 72 is provided by the

connected IFB or intercom circuit. Active cir-

cuitry ensures accurate level metering and

excellent audio performance. The compact

device is housed in a rugged aluminum

enclosure. All inputs and outputs interface

using industry-standard 3-pin male and fe-

male XLR-type connectors. The connectors

are manufactured by Neutrik® and feature

gold-plated contacts and metal housings.

The Model 72 is compatible with virtually

all single- and dual-channel IFB and inter-

com circuits used in broadcast, theatrical,

and industrial applications. IFB circuits,

typically provided by Studio Technologies’

IFB interface units or the RTS® 4000-series,

can be directly connected. Industry-

standard party-line intercom circuits are

also compatible. Whether a single-channel

system from Clear-Com® or a dual-channel

TW-series system from RTS, direct connec-

tion to the Model 72 is supported.

View showing IFB/intercom input and power present

LED indicator

View showing level meters and audio outputs

Issue 2, July 2004 Model 72 User Guide

Page 4 Studio Technologies, Inc.

Model 72

Level Meter / Interface

The Model 72 provides both 3-pin male

and female XLR-type connectors for

direct interconnection with IFB and inter-

com circuits. The connector’s pins are

wired in parallel, allowing compatibility

with interconnecting cables having male

or female connectors. The two connectors

can also serve as a “loop through” func-

tion. Pin 1 of the connectors is used as

a common point for both DC power and

audio signals. Pin 2 of the connectors is

always used for DC power, typically in

the range of 25 to 30 volts. With dual-

channel IFB or intercom circuits pin 2 is

also used to carry one of the two audio

signals, typically designated as channel 1.

(This audio signal is superimposed onto

the DC voltage.) Pin 3 is always used to

carry an audio signal but never DC power.

In dual-channel IFB or intercom systems

pin 3 typically carries channel 2 audio.

In single-channel intercom systems pin 3

carries the one and only audio signal.

The Model 72 was developed out of

necessity, once again proving that the old

saying is correct; necessity was definitely

the “mother” when it came to the unit’s in-

vention. The genesis was very simple. Of-

ten when testing audio equipment out “in

the field,” Studio Technologies personnel

felt “blind” when interfacing with IFB and

intercom circuits. Typically it was difficult

to obtain “clean” balanced line-level audio

signals, and never possible with a nomi-

nal level approaching +4 dBu. Worse yet,

confirming the signal levels of an IFB or

intercom circuit was virtually impossible.

It often seemed that the signal levels were

either too low or too “hot,” but there was

no simple way available to establish that

fact. With the Model 72 these problems

should disappear. Monitoring of the audio

levels is clear and accurate, and balanced

line-level outputs are directly available.

Operation

The only action required for initial opera-

tion of the Model 72 is connecting an IFB

or intercom circuit. If the audio outputs are

going to be utilized interconnecting cables

must also be connected. No external

power source is necessary and the unit

contains no configuration switches or user

controls.

Connecting an IFB or

Intercom Circuit

Two 3-pin XLR-type connectors, one male

and one female, are associated with the

Model 72’s IFB/intercom input. The three

pins on each connector are wired in par-

allel, allowing the mating connector on

the interconnecting cable to be of either

gender. In addition to providing interfacing

flexibility, the two connectors can be used

in a loop-through arrangement. This may

eliminate the need in some applications

to use a “Y” or splitter cable.

The connected IFB or intercom circuit

must have common on pin 1, DC power

(and possibly audio) on pin 2, and audio

on pin 3. For correct Model 72 operation

the DC power on pin 2 must be within

the range of 22 to 32 volts and capable

of supplying 42 milliamperes of current.

Once an IFB or intercom circuit is connect-

ed the red LED, labeled PIN 2 POWER,

should light.

Level Meters

Any time that an IFB or intercom circuit

is connected to the Model 72 the two

5-segment LED meters will be active.

Model 72 User Guide Issue 2, July 2004

Studio Technologies, Inc. Page 5

Model 72

Level Meter / Interface

They display the level of audio signals

present on pins 2 and 3 of the connected

circuit. The meters are calibrated to clearly

display the signal levels found on typi-

cal IFB and intercom circuits. The meters

display the actual signal levels that are

present on the pins in dBu. (This relates to

the voltage level, in dB, as referenced to

0.775 volts RMS.) The ballistics of the me-

ter is a cross between that of a VU and a

peak (PPM) meter. (We affectionately refer

to it as a “PU” meter!) The way the LEDs

“move” in response to signals should be

comfortable for most users to observe.

Green LEDs are used to display the me-

ters’ lowest four level steps. They light

to indicate signal levels that are typically

present on a correctly functioning IFB or

intercom circuit. The LED that displays

the highest value on the meter scale

(–4 dBu) is yellow in color, indicating that

a relatively “hot” signal is present. While

not necessarily a problem, having the

yellow LED lighting steadily would typi-

cally indicate a signal level that may be of

concern.

Audio Outputs

Two transformer-balanced line-level audio

outputs are provided. One output is as-

sociated with the audio signal that may

be present on pin 2 of the connected IFB

or intercom circuit. The other output is

associated with the audio signal that is

present on pin 3 of the connected circuit.

Whether signal is available on both audio

outputs will depend on the type of IFB or

intercom circuit that is connected. The

actual level on the audio outputs is 14 dB

above its source, i.e., there is 14 dB of

input-to-output gain. This was selected so

that connecting a typical IFB or intercom

circuit that has a nominal level of –10 dBu

will result in a +4 dBu nominal level on the

audio output connectors.

Each audio output is connected to exter-

nal devices using a 3-pin male XLR-type

connector. Mating connectors (females)

should be prepared so that signal high

(+ or hot) is on pin 2 and signal low (– or

cold) is on pin 3. The cable’s shield can

be connected to pin 1, but it will have no

function. To limit the chance of ground-

ing interaction between the Model 72 and

connected equipment, the pin 1 connec-

tions on both audio output connectors are

isolated from any point in the Model 72.

The fact that pin 1 “floats” will minimize

the chance of hums, noise, or buzzes be-

ing present on the equipment connected

to the main output.

For full electrical isolation, the audio out-

puts are transformer isolated. To provide

protection against accidental connec-

tion to cables that have DC power pres-

ent, the audio outputs are also capacitor

coupled. In series with the output leads

are 300 ohm resistors, making the effec-

tive output impedance 600 ohms. These

resistors serve several purposes. They

will limit the maximum amount of output

current that can be drawn in the event of a

cable short. In addition, they allow multiple

audio outputs to be connected together,

creating a passive summing network. By

using a simple “Y” cable the two audio

outputs can be passively summed. A side

effect from using this passive summing

technique is that signal attenuation will oc-

cur. The audio quality won’t suffer, but this

passive summing method does create an

audio “pad.” If the two audio outputs are

connected together a signal attenuation

of 6 dB can be expected.

Issue 2, July 2004 Model 72 User Guide

Page 6 Studio Technologies, Inc.

Model 72

Level Meter / Interface

Technical Notes

Channel Assignments

By design, the Model 72 doesn’t define

what signals will be present on the pins

of the connected IFB or intercom circuit.

This is because of the many different ways

signals and signal names can be assigned

to the same type of circuit. For univer-

sality, the meters and audio outputs are

referenced to the pins on the IFB/intercom

input, rather than having assigned names.

This naming method leads to one level

meter and one audio output being asso-

ciated with pin 2. The second meter and

audio output is associated with pin 3. For

reference it may be worthwhile to list some

standard pin assignments:

• In broadcast IFB applications pin 2 is

often defined as the “interrupt” or the

“program-with-interrupt” channel. It

may also be referred to as IFB channel

1. Pin 3 is often defined as “program”

or “program-only.” It may also be re-

ferred to as IFB channel 2.

• Many broadcast applications use

TW-type intercom systems from RTS.

In these cases pin 2 will be intercom

channel 1; pin 3 will be intercom

channel 2.

• Often theatrical and industrial applica-

tions use single-channel intercom sys-

tems from Clear-Com. In these cases

pin 2 will only provide DC power, hav-

ing no audio source associated with

it. Pin 3 will provide intercom audio or

intercom channel 1.

Power LED and Input Voltage

It’s important to note that the LED labeled

PIN 2 POWER will light essentially any

time a positive DC voltage is present on

pin 2 of either of the IFB/intercom input

connectors. Any DC that’s above approxi-

mately 3 volts is sufficient to light the LED.

This is by design as the LED is really just

intended to indicate that a “wet” circuit

has been connected. For full Model 72

operation the DC input needs to be in the

range of 20 to 22 volts.

IFB Audio Levels

The nominal level of audio signals associ-

ated with IFB circuits is typically –10 dBu.

This is the case with the interface units

from Studio Technologies, as well as such

systems as the RTS 4000-series. But actu-

ally having the correct level present

on an IFB circuit is often a “hit-or-miss”

proposition. During field testing Studio

Technologies’ personnel found that a

wide range of nominal audio levels were

present on “real-world” IFB circuits. Many

were fine, being reasonably close to the

desired –10 dBu. But some were much too

low, while others were much too “hot.” We

observed one unfortunate baseball “color”

commentator being sent interrupt audio

signals so “hot” relative to program audio

as to almost make his ears bleed! This

situation should not have been allowed

to happen.

In defense of field technical personnel,

measuring the audio level of an IFB circuit

hasn’t traditionally been an easy proposi-

tion. But that situation has now changed

with the Model 72. Using the unit will allow

level controls on the source equipment to

be adjusted as required.

Model 72 User Guide Issue 2, July 2004

Studio Technologies, Inc. Page 7

Model 72

Level Meter / Interface

Intercom Audio Levels

In North America the two most common

intercom systems are those from RTS

and Clear-Com. From tests performed

in Studio Technologies’ lab the nominal

RTS TW-series audio level was approxi-

mately –10 dBu. The dynamic range

control provided by belt-packs such as

the MP-325 was very good, limiting the

maximum level to at most 10 dB above

the nominal. The nominal audio level as-

sociated with a single-channel Clear-Com

system was harder to characterize. It ap-

peared to be a few dB less than –10 dBu,

but with a much larger dynamic range.

Level peaks of 10 to 20 dB over nominal

were easy to produce.

The contents of this section, along with the

previous one, may elicit howls of protest

from a host of engineers and intercom-

system experts. But for years we’ve heard

differing reports as to the actual nominal

audio levels for RTS and Clear-Com

systems. The “in-the-know cats” agreed

that the nominal level of RTS TW-series

intercom (and 4000-series IFB) was

–10 dBu, a value that we confirmed in our

tests. But the nominal level for Clear-Com

party-line systems was variously reported

as –20, –15, –12, –10, and “you know,

the Clear-Com level!” It’s most likely that

early Clear-Com systems did use a nomi-

nal level in the –20 dB range. But after

making controlled tests, their contempo-

rary equipment seemed to be much

closer to –10 dBu. And with the limited

dynamic-range control that we experi-

enced, the actual level during operation

may vary widely.

In conclusion, the engineers at Studio

Technologies are always open to learning

more. Additional information from the field

concerning IFB and intercom system lev-

els would be welcomed. Stopping by our

offices for an in-person chat would be also

great. (And bringing along pizza and beer

for a tech-talk session would certainly get

our attention!) Just park the production

trailers on the street!

Issue 2, July 2004 Model 72 User Guide

Page 8 Studio Technologies, Inc.

Model 72

Level Meter / Interface

Specifications

General Audio:

Frequency Response: 20 Hz-20 kHZ, ±0.5 dB

Distortion (THD+N): 0.02%, measured at 1 kHz,

–10 dBu in/+4 dBu out

S/N Ratio: 93 dB, referenced to –10 dBu in/

+4 dBu out

Connectors:

IFB/Intercom Input: 3-pin male and female

XLR-type

Audio Outputs: 3-pin male XLR-type

IFB/Intercom Input:

Type: single- or dual-channel, unbalanced (pin 1

common; pin 2 DC (or DC with audio); pin 3 audio)

Impedance: 20 k ohms, nominal

Level: –10 dBu, nominal

Compatibility: virtually all single- and dual-channel

IFB and intercom circuits

Audio Outputs:

Type: balanced, transformer-coupled with series

capacitors and isolation resistors

Impedance: 600 ohms, nominal

Nominal Level: +4 dBu, measured with –10 dBu

IFB/intercom input level

Maximum Level: +16 dBu into 2 k ohms

Gain: 14 dB, input-to-output

Power Requirement:

22-32 Vdc, 42 mA (provided by IFB/intercom input)

Dimensions (Overall):

4.2 inches wide (10.7 cm)

2.0 inches high (5.1 cm)

4.7 inches deep (11.9 cm)

Weight: 0.8 pounds (0.35 kg)

Specifications and information contained in this

User Guide subject to change without notice.

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