The design of an intercom system, like any key system, must begin with a definition of individual requirements including operational, environmental and budgetary considerations. Though not very glamorous, intercom systems—a critical, “behind the scenes” component of any facility—are usually left to last when building new or upgrading.
Initially, intercoms were do-it-yourself projects based on telephone technology which led to vast variations in performance and a complete lack of standardization. Different industries developed different terminologies and operational methods. In the last twenty years, however, reliable, high-performance systems have become available from several manufacturers, giving end-users considerable options.
Because the history of modern intercom systems is relatively short, differences in terminology exist from user to user and manufacturer to manufacturer. As productions and facilities increase in complexity, so do demands on communications systems. An intercom system for a sizable mobile unit can price in the $200,000 range with considerably higher costs for large studio and plant operations. Tight integration between the primary facility’s audio infrastructure and communications system, however, will significantly streamline operations and result in substantial cost reductions.
First we must develop a common language and define what the customer needs. The following discussion will serve to clarify terminology and present some basic principles of system design.
Communications systems typically used in the entertainment industry can be grouped into three major categories.
PARTY LINE
The first category is commonly referred to as party line, P.L. or conference line. This is the simplest form of system and is typically used for teamwork activities such as television production, theater and live show communications, and aircraft testing. These are generally two-wire in nature, meaning that the talk and listen signals are carried on the same conductors. Party line systems can be implemented using four-wire techniques but are more complex and difficult to expand. Power for the individual stations may be carried on the same conductors as the audio or on separate conductors.
In a party line system, all operators share a common channel or number of channels. There is no privacy between operators except when provided by multiple channels or by special limited access “iso” channels. Some signaling can be furnished but usually not to the extent that every operator can selectively signal every other operator. Current party line systems offer high performance, low to moderate cost, and can use one or multiple channels with the more popular systems using from one to 12. The advantages of two-wire party line systems include simple wiring, easy expansion to additional stations, very little need for central equipment, low cost per station and simple operation for team type activities.
The disadvantages of two-wire systems are their reliance on hybrids (two- to four-wire converters) which are required for interfacing between systems and cause significant degradation of system performance, the difficulty encountered in interfacing them to other two-wire systems that have different characteristics, their lack of selective calling to multiple stations and their limited privacy capability.
The first all digital party line system has recently been released by Riedel Communications. A significant departure from traditional designs, it promises performance advantages along with the convenient implementation of typical analog two wire systems.
THE MATRIX
The second category is commonly referred to as matrix or point-to-point. These are four-wire systems in which the talk and listen signals to and from each station are carried on separate paths. The paths may be wires, RF carriers, digital links or optical carriers. In a matrix system, each station can selectively call the other stations and a tally display at the receiving station specifies the caller. Matrix systems provide complete privacy between stations and generally provide custom programming capability from a computer. They are implemented in analog, digital and hybrid configurations.
Advantages of four-wire systems include simple interfacing to outside world systems, lack of hybrid null problems, selective calling to all stations, call tally, privacy and easy interface to other audio equipment. They are easily expandable to very large sizes.
Disadvantages include significant central equipment requirements, high cost, expense of expansion and increased wiring needs when compared with two-wire systems. In large systems, these issues are outweighed by flexibility and ease of connectivity.
IFB
The third category is IFB (interrupt fold back). These systems are used to cue talent. IFB systems are also called interrupt feedback, program-interrupt, director interrupt or director cue. They can be viewed as one-way intercom systems. With the rise of matrix systems as the dominant type used in broadcast, the IFB function is an integral part of the main intercom.
In an IFB system, program audio is fed to the talent (usually through an in-the-ear earphone). The program audio can be interrupted or attenuated by an operator (usually the director or producer) to give verbal cues to the talent. Typical systems have control keys for the producer, director, assistant director, and audio operator. IFB systems can be quite extensive, involving multiple program inputs and links to off-premises locations. Their heaviest use is in sports and news operations.
Dave Brand has been active in the professional audio community since 1967.
His extensive experience in product design, system design and system implementation in the communications arena has positioned him as an asset to all major suppliers of intercom products. Brand, a seasoned industry consultant, currently serves as general manager of Riedel Communications’ US operation located in Burbank, CA. He may be reached at 818-744-9134.
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