Round cord in a Square plug

July 1, 2004

"The good thing about standards is that there are so many to choose from."
-Andrew S. Tanenbaum

Converting a signal from one system to apply it to another is the daily routine of a radio facility technical manager, and creating the bridge between different systems can sometimes be a challenge. The interface might be as simple as an adapter. In other cases, it may be an active component that manipulates the signal to create something completely different. The devices used in your facility can range from very large to barely noticeable, but each one plays an important role. Without the smaller utility interfaces, many stations would not be on the air.

The Trends in Technology feature usually focuses on clearly defined systems and components that can be grouped into specific categories. However, we decided that it was time to highlight some of the less-obvious but no-less-important devices that serve essential functions. Call them widgets, gadgets, interfaces or problem-solvers, we assembled a list of 14 products that illustrate the utilitarian functions that simplify integrating systems.

Resource Guide

A sample of available widgets, gadgets, interfaces and problem-solvers.

While the Resource Guide is far from a complete list, it should provide enough basic information to help you get started.

Switching four stereo audio sources and controlling four relay closures from an auto-answer telephone line via DTMF commands is the purpose of the Conex Electro-systems DT-90. The unit can connect the selected audio source, the first channel audio source or a mic on the device to feed the telephone line for monitoring. In addition to the four audio sources, audio from the telephone line can be routed to the output. The four relays provide SPDT contacts and can be set as momentary or latching. Connections are made via plug-in terminal strips. An access code can be set to prevent unauthorized switching.

The SBS Guardian 2 is a stereo audio and composite signal-fail control unit. It automatically selects a main stereo audio input, alternate stereo audio input or a reserve stereo audio input on signal failure. It will also select a single audio channel if half a stereo pair is lost. Closures are available to activate a backup audio source, such as a CD player. It can also accept and switch between composite audio signals. Signals are filtered to prevent false triggering from line noises. Level thresholds can be set from the front panel. Available through Broadcasters General Store.

The RDL FP-UBC6 is part of the company's Flat-Pak line and provides six channels of unbalanced-to-balanced audio conversion. A gain trim pot on each channel fine-tunes the audio levels to convert an incoming signal ranging from -20dBV to -5dBV to provide a +4dBu output. The unit operates from a 24Vdc power supply. Input impedance is 10kohms, while the output impedance is 150ohms. One possible application is to convert multiple unbalanced audio channels from computer sound cards to feed a console or routing system. The unit can be attached with its mounting ears or mounted with the optional rack-mount adapter.

Building on the capabilities of the original Plan B Silence Eliminator, the Danagger Audio Works Plan B Plus silence sensor incorporates an additional level of audio failure detection and backup. An extra set of passively switched analog and digital audio inputs allows automatic connection to an alternate live program feed, such as an STL, dial-up codec or off-air receiver. If both incoming program feeds are down, an internal CD/MP3/DVD drive provides continuous replacement audio while a built-in voice remote control alerts station personnel. Delay range is four seconds to 10 minutes, and users can program a unique system ID number into each unit for multi-site installations.

The Circuitwerkes Prex manages and multiplies contact closures. It accepts a variety of input signals and converts them to contact closure outputs. The microprocessor allows each relay to be individually programmed and triggered by any number of inputs, combined together using basic logic functions. Relay operating modes include momentary, toggled, leading or trailing edge, pulse stretching as long as 4.5 hours, input debounce and maximum ontime. The Prex can be programmed using jumpers or through a computer serial port. The Prex configuration manager provides instant, graphical access to all of the setup commands.

The Aphex 228 converts eight channels of unbalanced -10dBu audio to balanced +4dBm audio. This allows multiple audio channels to be converted without the need for multiple discrete interfaces. The front-panel extended-range VU meter provides calibration and monitoring of each channel individually. Eight two-color signal presence/clip indicators sow signal presence and overload conditions. Front-panel gain trims for all eight channels are provided. The front-panel indicator lights can be dimmed if desired. All connectors are gold-plated. A test CD with commonly used reference levels is included to set levels for specific equipment.

The Titus Technological Laboratories Web-Rem controls and monitors the functions of Titus products or any other device via a local LAN or Internet connection. Several models are available that provide relays or open-collector outputs for control or TTL signals, and analog outputs for monitoring a remote device. Monitoring and control is via a Web page generated by the unit's own internal Web server. Connections to the remote device are via a db-style connector or plug-in terminal block. Each unit is powered by a 5Vdc supply, with less than 1A consumption. Metering shows link status, system activity, I/O status and power.

The Sine Systems MBC-1 replaces the basic signaling lights with a customized message on a Beta-Brite or other Adaptive Micro Systems LED message boards. Each MBC-1 can control several displays. The MBC-1 monitors as many as 15 control room devices and can display a unique message for each one. Any message that can be displayed on the message sign can be triggered by the MBC-1. Several messages are preprogrammed and can be changed. Inputs are individually programmed for a momentary or latched display. Latched messages are cleared with a common reset input.

The Henry Engineering Matchbox HD is a bi-directional balanced-to-unbalanced interface. It offers a 100dB S/N ratio, with 22dB of headroom to yield 122dB of dynamic range. The four direct-coupled, independent amplifiers convert stereo inputs and outputs from unbalanced -10dBv levels to +4dBm balanced 600ohms lines. All four outputs can be adjusted to accommodate a range of operating levels. The Matchbox HD also features a high gain mode to properly match the low-level unbalanced outputs of computer sound cards. The unit features an internal ac power supply. It is 1/3 rack width and can be mounted using a rack shelf, or wall mounted with wall/cabinet mounting brackets.

JK Audio manufactures a wide range of telephone audio interfaces, and the That-2 might be considered the premium unit of the passive adapters available. Designed to provide audio in and out of a standard telephone set, the unit connects between the telephone and handset. Audio can be taken from and fed into the phone simultaneously. Audio connections are made via RCA or XLR connectors and can be adjusted with separate level controls. A selector switch configures the handset jack for use with different types of phone systems. A handset switch disconnects the handset mic when feeding audio.

The ATI Matchmaker BI400 is a bi-directional RCA to XLR audio converter. The unbalanced connections are made via -10dBu, 10kohm RCA connectors and +4dBm, 600ohm XLRs. All connectors are rear-mounted. The front-panel controls allow output level matching as needed. Companions to this unit include the BU400, which has four balanced XLR inputs to four unbalanced RCA outputs, and the UB400, which features four unbalanced RCA inputs to four balanced XLR outputs. All RCA connectors are gold-plated.

Broadcast Tools, a prolific manufacturer of utility devices, offers the AVR-8, a voice remote control system that reports changes detected on any of its eight digital inputs to a remote telephone or pager. After delivering a greeting, the AVR-8 then speaks a unique message for each input. Messages come factory programmed but can be rerecorded with customized messages. After reporting, the AVR-8 can be given commands via the telephone keypad to report on the input state of any of the eight digital inputs or controlling one of the four relays. Voice confirmation is provided after a command is executed. Each input stores as many as eight, 16-digit numbers and one 32-digit phone number.

The Radio Systems B.O.B. (break-out box) for Audio Science sound cards provides a simpler way to make audio connections in and out of the computer system. It is designed for use with the ASI4300 and ASI6000 sound cards. The rack mount units are available in XLR and RJ-45 Studio Hub versions and allow access to all analog and digital audio I/O, as well as clocking and sync signals. One multi-pin connector connects the unit to the audio card. Multiple B.O.B.s can be ganged when access to all eight stereo analog input and output channels is needed. Connections to the audio adapter are via a 50-pin, high density SCSI-type connector for analog connections and a 26-pin, high-density connector for digital (AES/EBU) connections.

The Presonus Central Station is a studio-monitoring interface that features three sets of stereo analog inputs on two sets of TRS-balanced inputs and an unbalanced RCA input with level trim control. It provides two digital inputs via S/PDIF or TOSLINK with D/A conversion up to 24-bit/192kHz rates. It features three sets of monitor outputs, each with its own set of passive trim controls. The monitoring section also provides mute, dim and mono switches, and a set of cue outputs that can feed headphone amplifiers and a separate stereo main line level output. The audio switching paths are completely passive and use sealed relays. An optional remote control is available.

While this is not really an interface, it is a handy gadget that caught our eye. The Middle Atlantic Products Rack Ruler tape measure is unique in that it is marked in inches and in rack units. The Mylar-coated, retracting steel tape extends to 96" and is housed in an ABS case with a thumb lock and belt clip. The back of the tape includes audio tables, charts and pinouts, or data tables, charts and contact wiring depending on the model.

Conversion between AES3 standards

There are two versions of the AES3 digital audio standard. One is a balanced signal passed over a twisted-pair wire with a characteristic impedance of 110ohms. Its complete name is AES3-1992. The other is an unbalanced signal passed over coaxial cable with a characteristic impedance of 75ohms. This is called AES3-ID-1995. Both formats are commonly referenced without the last four digits, which indicate the year the standard was adopted.

Apart from the voltage level and ground reference, the formats are identical. A passive circuit can be used to convert signals from one format to the other. An example of this is available at int he Engineer's Notebook. The example circuits account for the difference in impedance. The balanced-to-unbalanced converter also reduces the voltage level by 14dB.

Because these are passive circuits, the unbalanced-to-balanced converter cannot increase the voltage level. This circuit is better suited to applications where a long signal path is needed and an amplifier can be placed at the receiving end.

To build your own passive convertors, follow this link to the Engineer's Notebook.

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