Large Area Assistive Listening Systems: Review and Recommendations

2.2.2 Standards

2.2.2.1 FCC Regulations

In 1982, the Federal Communications Commission authorized the use of frequencies within the 72‒76 MHz band as the designated radio frequencies that could be used by people with hearing loss. This is non-exclusive, unlicenced band. Other users, such as pagers, emergency vehicles, etc. are also permitted to transmit radio signals on frequencies within this band. ALS manufacturers differ in how they allocate this band. For example: Phonic Ear makes 40 narrow-band (50 kHz) channels available; Gentner uses 37 narrow-band (50 kHz) channels; Telex, 16 narrow-band (50 kHz) channels; Drake, 10 wider (150 kHz) channels; Williams, 10 wide (200 kHz) or 40 narrow-band (50 kHz) channels, while Comtek employs 10 wide-bands (200 kHz) and narrow-band (50 kHz) channels both in the 72‒76 MHZ and in the 216 to 217 MHZ bands. Some of the frequencies used by different manufacturers may be identical, while others may differ. The FCC also permits the use of the 216‒217 MHz band as a low power radio source for auditory assist ive devices and several manufacturers are now marketing large area transmitter using this higher frequency band. ALS in really large venues, such as football stadiums, can also employ one of the commercial FM frequencies as long as they meet the power requirements designated by the FCC.

In terms of the permitted maximum power of ALS, the FCC limits it to no more than 80 millivolts per meter at 3 meters. This can provide for an effective transmission range between 300 and 500 feet. Larger antennas can extend this range to l000 feet; however, the maximum strength cannot be exceeded regardless of antenna design. The 216‒217 MHz band does permit higher signal strength and can provide greater operating distances. Within the transmission range, the field-strength of the transmitted radio signal should be adequate and equal at all seat locations within a venue. ALS are "low-power" devices that are not likely to interfere with other permitted user in the same channels (i.e. paging devices, emergency vehicles). No priority is given assistive listening devices. When interference occurs from other radio sources, the onus is on the ALS user to switch to another channel within the same frequency band.

2.2.2.2 New York State Standards

As far as can be determined, New York is the only state that has developed written standards for large area FM assistive listening systems. Enforcement of the standards is the option of the local building code inspectors (who have, literally, thousands of items to verify on a checklist code). Information and educational sessions can help ensure their understanding and compliance with the ALS standards (Johnstone l997). The New York State standards reflect the combined operation of both the transmitter and the receiver using various coupling options.

* Given an appropriate input signal to the transmitter (as specified by the transmitter's specifications), the frequency response at the output of the receiver and transducer should not vary more than +/- 5 dB over the frequency range 100 to 8000 Hz from the value at 1000 Hz. (Note: This appears to be an overstatement; it is unlikely that such transducers as miniature earphones, as well as silhouettes and neckloops working through hearing aids would be capable of meeting this standard at the higher frequencies).

* The minimal signal to noise ratio at the receiver's output should not be less than 35 dB. Harmonic distortion shall not be more than 10%. The minimum sensitivity of the receiver shall be no greater than 1 uV at 12 dB SINAD. The maximum RF signal generated by the transmitter shall not exceed 8000 uV/meter at 30 meters (which accords with the FCC regulations).

* When using a neckloop as a transducer, it should generate an average magnetic field strength of 150 mA/meter, with peaks no more than 600 mA/meter. Measurements shall be made in the center of the neckloop. If using a silhouette transducer, the average magnetic field strength should be 50 mA/meter, with peaks no more than 200 mA/meter. Measurements should be made at a distance 10 cm from the silhouette. (Note: 10 mm would appear to be a more realistic distance.) If using a miniature earphone as the output transducer, the output shall be at least 80 dB SPL and no more than 130 dB SPL.

* All systems must be capable of accepting input signals at line or microphone levels and must be capable of interfacing with existing PA systems.