Large Area Assistive Listening Systems: Review and Recommendations

4.2 Functional Comparisons

The advantages, problems and limitations of IL systems were reviewed above, in the description and issues concerning such systems. While these systems have a continued contribution to make in providing auditory access for people with hearing loss, they rarely appear in direct competition with IR and FM systems in terms of new installations. Unless more hard of hearing people can be convinced (or their hearing aid dispensers) to include telecoils in their hearing aids, this situation is unlikely to change. This section, therefore, will concentrate on comparing the advantages and disadvantages of FM and IR systems.

It is easier to ensure that a transmitted signal is adequate and equal throughout any size venue, including outdoors, with an FM rather than an IR system. An FM transmitter can be hooked into the sound system and, in most instances, that appears to be the extent of the necessary "installation". FM systems, however, are susceptible to outside radio interference or may cause such interference to other nearby users of radio equipment. There appears to be an adequate number of channels available to ensure coverage in multiplex cinemas or multiple adjacent auditoriums. Since the FM transmission is not contained within the facility, an FM ALS is not appropriate if privacy is a concern.

An IR signal, on the other hand, stays contained within a facility, thus ensuring privacy. Any number of systems can be used in adjacent auditoriums without interference or spillover. Most venues with IR systems currently use the same sub-carrier (95 kHz), thus providing compatibility between different locations. However, this is an informal practice, and other sub-carrier frequencies can and are now being used in some places. The use of other and different sub-carrier frequencies would require either that receivers be designed only for particular transmitters, or that multiple-frequency IR receivers be deployed.

IR signals are affected by sunlight and, less severely, by fluorescent lights. It is possible to install more emitters in a facility, and overcome all but the most unfavorable natural or artificial light conditions (like direct sunlight). Ensuring adequate light coverage in a facility requires a skilled installer, a fair amount of "trial and error", and a willingness to verify the field strength at all likely seat positions in a facility. IR systems are considered "line of sight", in that the "eye" of the receiver must be in line with an emitter. However, because of the undefined light reflection patterns in different rooms, this may not be strictly applicable; in some rooms and at some use positions, strict "line of sight" reception is not necessary. Reception, therefore, may be uncertain and has to be empirically determined in each venue. Covering the light receiving diode with clothing (as in a pocket) or turning one's back to an emitter will most likely interfere with reception.

IR receivers tend to be simpler than FM receivers, though this may be less true as additional sub-carrier frequencies become used. In this case, multiple frequency IR receivers may become common. In some places and for some populations, this can be a problem; simpler is better. FM receivers vary from a fixed, single frequency reception to units with the capacity to detect many ALS channels. Both FM and IR receivers can be used with the same types of coupling possibilities, including personal hearing aids.