Applied Technology: Telex's BTR-800 wireless intercom system

Wireless intercoms have been a broadcast staple for more than 25 years, but the systems in use today bear little resemblance to their predecessors. The
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Wireless intercoms have been a broadcast staple for more than 25 years, but the systems in use today bear little resemblance to their predecessors. The first systems employed separate beltpacks for the transmitter and the receiver and were awkward to use, a drawback in broadcast production environments requiring mobility and ease of use. As wireless technology has evolved, systems have been designed that meet the real-life needs of the working broadcast professional.

Perhaps the biggest benefit of wireless intercoms is they allow users to "cut the cable" of hard-wired partyline systems and move about freely within the system's operational range. A wireless intercom is a partyline intercom system, and good quality systems can be seamlessly attached to existing hard-wired communications systems commonly used in broadcast facilities. Quality wireless intercoms offer a distinct advantage over traditional two-way radios in that they offer a more natural full-duplex operation. This enables all users on the system to speak and hear other users simultaneously without "covering" other users' transmissions.

The demands of modern broadcast productions make the full-duplex operation of wireless intercom systems an absolute necessity for stage managers, lighting and audio technicians, or any professional who has to deal with the breakneck speed and complexity of television productions.

The spread of digital television and the ever-increasing number of wireless users have made it more difficult to find channels for wireless intercoms in the available frequency spectrum. The spectrum has gotten a lot smaller, especially considering the re-allocation of four channels for public safety use and the upcoming re-allocation of UHF TV channels 60 through 69. Broadcast professionals have to consider such factors as the compatibility of frequencies with each other, as well as how to best avoid interference with local TV transmitters.

Unlike wireless microphones that operate only in one direction, wireless intercoms have more specific frequency spectrum requirements because of the relationship between the transmitter and receiver frequencies. Each intercom (if it is to be full-duplex) must have at least one system transmitter frequency that broadcasts to all beltpacks and one receiver frequency for each individual beltpack in the system. For a four-beltpack system (known as a four-up), this means a minimum of five frequencies.

Each beltpack must have a receiver set to the base transmit frequency and a transmitter set to its own unique receiver in the base. Due to a phenomenon called desensing, these two frequencies must have a fairly large frequency separation, typically at least 12MHz for UHF systems, or the transmitter will interfere with the receiver's operation.

The answer to the frequency problem is to utilize a digitally-synthesized, frequency-agile system. That may sound simple enough in theory but, in reality, designing such a product is a different matter. It must not only incorporate a superior design with high-quality filtering to withstand the rigors of an overcrowded frequency spectrum, but it must also offer an ergonomically designed user interface that allows ease of frequency selection and operation. End users must experience the same ease of operation they get from their existing two-wire beltpacks.

To date, the chief limitation to most wireless intercoms (other than finding available spectrum) has been that they are inherently one channel in nature while the most common hard-wired intercom system from RTS (used in virtually all TV broadcast trucks and facilities) is two-channel by nature. Two-channel operation allows users to switch easily from one intercom channel to another. This allows a stage manager, for instance, to communicate with the producer and then switch over to the director circuit as necessary. Two-channel operation has become the hard-wired industry standard, and users who have increasingly relied on wireless intercoms must be able to employ that technology in wireless form without having to deal with huge racks full of equipment.

Wireless intercom systems that can operate in high RF environments must not only offer interference-resistant operation, but must utilize design techniques that will not interfere with other wireless equipment like wireless microphones and IFBs. Also, one key to a wireless intercom's successful operation and coexistence with DTV will be its ability to avoid strong, local TV stations, as well as coordinate multiple frequencies. This holds true whether the system is VHF or UHF, fixed-frequency, frequency-agile or synthesized. Utilizing the minimum power necessary is absolutely critical if wireless intercoms are to coexist with other low-power wireless equipment. The utilization of intelligent systems that reduce beltpack transmitter power levels as they get closer to the base station can greatly decrease the harmful interference associated with wireless communications gear.

Future wireless intercoms will need to provide users with frequency agility, high-end filtering, RF power management, ease of use, two-channel operation, extended battery life, a small, lightweight beltpack and a user interface that allows operational and frequency parameters to be easily set and checked without the use of external equipment such as a laptop computer or special interface box.

The Telex BTR-800 system offers these features. It is able to draw on Telex's long history in the development of innovative wireless intercoms. For example, the Telex BTR500/600 wireless intercoms were designed to resist interference from broadband systems and are targeted toward fixed installations with high RF environments. The majority of the BTR-800's applications will be targeted toward the high-end television broadcast entertainment industry. Among the key benefits of the BTR-800 are UHF frequency agility, two-channel operation, a smaller beltpack, 1RU base station, reliable audio routing and, always important, a low cost.

As wireless intercom applications for broadcast professionals continue to grow more complex and challenging, the need for products that can meet these challenges will also grow accordingly. The Telex BTR-800 is just the latest example of this trend.

For more information on Telex's BTR-800, circle (450) on Free Info Card.