ADAAG Manual

FM Systems

FM systems use a short-range radio transmitter which converts sound to a radio signal sent to small FM radios tuned to the same channel usually worn around the neck. The radio receiver can be connected to a choice of couplers. FM systems are highly portable and flexible, subject to requirements for frequency control, and provide an alternative for facilities with multiple venues which can provide the requisite support. An FM system is simple to install and easily integrated with existing sound systems. There are few constraints on transmitter location since FM broadcasts are not constrained to the line of sight like infrared systems. Signal range is usually determined by the size of the antenna and the power of the transmitter but in most cases only one transmitter is needed (typical ALS range is roughly 300 feet). This, plus the fact that sunlight does not interfere with the signal, make it the best choice for large outdoor assemblies such as open-air arenas.

Each listener using the system must have an FM receiver but can move around more freely than with other wireless systems (loop, infrared). A choice must be made between providing fixed channel receivers (usable only with a compatible transmitter) and tunable receivers (more flexible, but also more expensive). Some people with hearing impairments have difficulty tuning a receiver, especially when multiple programs are being conducted.

The fact that FM signals can spillover into adjacent rooms or buildings can present problems. If confidentiality is required then an FM system is likely not the best choice. Use of an FM broadcast might also raise copyright issues for certain performances since anyone with a properly tuned FM receiver can intercept the broadcast. Some FM systems may experience signal drift (depending on the quality of the system), radio wave interference (depending on the environment), and interference from other FM devices using adjacent channels, such as pagers. Frequency management is an important consideration. However, most institutions that support multiple broadcasts are likely to have an audiovisual support staff which can assume responsibilities for this important task. The use of tunable receivers simplifies the problem of frequency control, but puts the responsibility on the listener. Implementation plans should include instructions and tuning assistance for users available where receivers and couplers are issued.

Multiple frequencies are possible and can support various programs such as audio description for people who are blind and simultaneous translation in multiple languages. However, a separate transmitter is needed when simultaneous programs are conducted within a facility, or even in nearby facilities.

Three formats are available within the frequency range (72 - 76 mHz) for ALS applications:

• 40 channels for the educational band (used in educational settings, this allows many simultaneous classes or events)

• 10 channels for wide band transmissions (popular in single channel applications)

• 17 channels for narrow band transmissions (useful where multiple FM systems are used in close proximity, such as a theater complex)