Large Area Assistive Listening Systems: Review and Recommendations

2.3.3 Issues

* Up to now, 95 kHz has been the sub-carrier most frequently used by manufacturers of IR systems. When stereo reception was desired, a sub-carrier of 250 kHz was simultaneously employed. These frequencies reflected an informal agreement among manufacturers rather than any national or international standards (Lieske l994). Other frequencies are now being used as sub-carriers in ALS (300 kHz, 2.3 mHz and 2.8 mHz).

One reason for switching to a higher sub-carrier frequency is because of the electromagnetic interference at 95 kHz produced by the newly introduced T−12 fluorescent ballasts. Since these are more energy efficient than the older type of ballast (type T−8), their use is likely to increase in the future. In other words, the pre-eminence of the 95 kHz sub-carrier is no longer assured. While it does not appear to be on its way out, it may be employed in fewer locations. Compatibility between venues has been an advantageous feature of IR but, with the introduction of different sub-carriers, this advantage can no longer be taken for granted.

* The frequency of the sub-carrier is not the only uncontrolled factor in IR systems. Unlike RF fields, the strength of the IR field is left up to the manufacturers and installers. To reduce the interference effect of ambient light, a manufacturer may increase the radiated IR light level and decrease the sensitivity of the IR receiver. Other manufacturers may radiate less light energy from their emitters and depend upon the sensitivity and electronics of the receivers to detect an adequate signal. These factors help explain the variation in performance of different IR receivers in a specific venue, even when they all use the same sub-carrier frequency.

* IR receiver diodes generally detect a broad bandwidth of light, extending over 500 nm. Every receiver uses filters built into the detector diode that is designed to accept the transmitter frequency and block the other IR light (Laszlo l998). However, the filter characteristics cannot be too narrow, else slight transmitter "drifts", perhaps due to temperature effects may put the device outside the passband. This effect can be compensated for with more expensive electronics, but then the decision becomes a cost/benefit issue. The specific characteristics of these filters presumably differ among the receivers from different manufacturers and are another reason why, in spite of using the same sub-carrier frequency, some IR receivers are less compatible than others in the same location.

* IR light waves are considered "line of sight" transmission. That is, ordinarily the "eye" of the emitter should be facing the "eye" of the receiver, with no physical obstructions placed between these two devices. Turning one's back to the emitter, or placing an IR receiver in one's pocket, can either eliminate or distort the perceived audio signal. However, line of sight reception is affected by surface material (light surfaces reflect more light energy), the strength of the transmitted signal, and the geometric shape and number of listening levels (e.g. balconies) in the venue. In rooms with light covered surfaces, these reflections may enhance IR coverage by filling in gaps not covered by the primary signal. Conversely, with dark surface, or checkerboard patterns, may reduce or modify IR light reflections. Reportedly, a few recently introduced IR transmitters are capable of being employed in some outdoor venues (but not in direct sunlight).

* The emitters produce an ovoid IR light pattern that diminishes in strength following the inverse-square law. This means that a number of emitters must be used to ensure that all seats in a venue, such as in the corners or in the rear of the room, receive an "adequate" level of the light. Installers frequently "daisy-chain" emitters to guarantee that IR light is being directed at the audience from several directions. There are no standards which specify the radiated level of light required at seat locations. At least one manufacturer (Audex) makes a light meter available for installers. This particular lightmeter is designed only with the company's own products in mind, in that it reads only relative light intensity for determining whether the illumination levels are satisfactory. Installers also can, and usually do, listen through a receiver at various seat locations. This is a highly subjective judgment and offers hard of hearing listeners no assurance of adequate IR field strength at all seat locations in a venue. One of the recurring complaints made by the consumers in the focus group was the variations they experienced in IR field strength and the clarity of the IR signal in different venues and at different locations in the same venue.