Solving Interference Problems Affecting
Infrared Assistive Listening Devices
(Source: Steve Unger, B.A.Sc.)
Over the last ten years, infrared technology has been successfully used in communication aids for hard of hearing people. Such devices employ infrared light to carry the desired sound from the source (a person speaking or music) via a transmitter to hard of hearing listeners wearing receivers. In this way, background noise, echoes and other acoustical interference are greatly reduced and the listeners hear a clear reproduction of the desired sound. However, infrared light itself may be susceptible to non-acoustical interference.
In this Application Note we discuss the three major sources of interference affecting infrared assistive listening systems: sunlight, certain types of indoor lighting and electromagnetic emissions. We also make some recommendations on minimizing the effects of interference created by such sources.
The Sun emits all types of electromagnetic radiation including infrared. In fact, it is the most intense source of infrared light around. Sunlight is not modulated (infrared emitted by assistive listening systems is) and therefore, will not be picked up by infrared receivers as a signal. However, the infrared light from the Sun is so intense that it will over-power or swamp the signal coming from an infrared transmitter. This is analogous to using a flashlight to send Morse code on a bright, sunny day. Therefore, it is common "technical wisdom" that infrared assistive listening systems cannot be used outdoors. This, however, is not entirely true, and infrared assistive listening systems can be used outdoors under certain circumstances.
Our testing indicates that solar interference on infrared assistive listening systems is mainly limited to direct sunlight, causing a 50 to 60% reduction in the range of the system. Infrared systems always require line-of-sight between receiver and transmitter/emitter. However, in sunlight the sensitivity for misaligning the receiver with the transmitter/emitter increases significantly, that is, the receiver must always be pointed directly toward the transmitter/emitter. If good alignment and good "lock" are maintained, the sound quality of the system is acceptable. The actual degree of reduction in range will vary with time of day, season, weather and latitude.
Test results in shade or on a cloudy/overcast day are quite different. Here, the infrared system operates with only marginal reductions in range and sensitivity. Therefore, infrared assistive listening systems can be used outdoors in shaded areas or on cloudy days.
While it is not recommended, if it is necessary to use a system in direct sunlight the range of the system should be considered a third of the normal. In other words, use three to four times as many emitters as would normally be indicated. Note, that if the system is used indoors in a room with large windows, the same rules regarding direct sunlight apply. In such circumstances, use curtains or sheers to stop the direct sunlight from entering the room; indirect or filtered sunlight is fine.
All indoor types of lighting (incandescent, fluorescent, halogen and others) emit some infrared light. Like sunlight, such infrared is not modulated. The intensities of such infrared light sources are many magnitudes smaller than the Sun´s, and therefore, have little or no effect on the performance of infrared systems. However, certain types of lighting can cause interference. High intensity mercury-vapour or sodium lights may produce effects similar to the Sun. Recently, the new electronic ballast fluorescent lights have emerged as substantial sources of interference.
Electronic Ballast Fluorescent Lights
Today, there is push toward using energy-efficient lights, and rightly so. Many of these new energy- efficient lights are fluorescent fixtures using electronic ballasts. This applies to both standard fluorescent fixtures using tubes and to the new compact fluorescent replacements for incandescent lights. The electronic ballast operates at a frequency much higher than the old style magnetic ballasts and thus more efficiently excite the phosphorus inside the tubes. Electronic ballast equipped fixtures tend to be 10 to 15% more efficient.
Whereas the old magnetic ballasts operated at 50/60 Hz (line voltages), electronic ballasts operate between 20 and 30 kHz depending on the model. The ballast causes the tubes to flicker at, and at double the operating frequency. In other words, a ballast operating at 60 Hz causes the tubes to flicker at both 60 and 120 Hz. This is too high in frequency to be seen (by most people) and too low in frequency to interfere with most electronic devices. Similarly, with an electronic ballast operating at 30 kHz, the tubes flicker at 30 and 60 kHz creating a modulated infrared light source. In this case, however, the flickering or modulated light will interfere with other electronic devices that communicate using infrared light, such as remote controls and infrared assistive listening systems. As an illustration, the TV may suddenly change channels, or a VCR may suddenly start playing a tape, both triggered by infrared signals from electronic ballast fixtures.
The original frequency used for the transmission of infrared light in assistive listening systems is 95 kHz; close to the flicker frequencies of electronic ballast fixtures. An infrared receiver will interpret the flickering fluorescent light as a signal and may lock onto it instead of the infrared assistive listening transmitter. In fact, it is the relative intensity of the light sources that determines which signal the receiver locks onto. In other words, if you point the infrared receiver toward the transmitter, you will hear the desired sound, if you point the receiver toward the lights, you will hear only static from the lights. The lights interfere with the infrared transmission.
The easiest fix is to turn off the lights. However, this may not be practical. Alternately, the lights can be covered with an infrared blocking filter, eliminating the modulated infrared light at the source.
Another solution is to operate the assistive listening system at a higher frequency, further away from the flicker frequencies of electronic ballast fixtures. Therefore, a new 250 kHz industry standard for infrared assistive listening systems has arisen. 250 kHz is far enough from 60 kHz that there is virtually no interference.
This solution will work well as long as electronic ballast manufacturers keep their operating frequencies below 70 kHz. The energy efficiency of fluorescent lights does not increase above 12 to 15 kHz, therefore the only reason to go higher is to reduce ballast size. For example, in Japan smaller electronic ballasts operate at 40 to 50 kHz (double is 80 to 100 kHz). As yet, there is no official standard for the operating frequencies of electronic ballasts, so ballast manufacturers are free to use whatever frequency they desire. ANSI (American National Standards Institute ) has a draft electronic ballast standard, but unfortunately it mentions nothing about operating frequency.
Sources of electromagnetic interference (EMI) are everywhere in our modern electronic age. They originate from office equipment, computers, power lines, and cellular phones to mention a few. This is particularly important for "T-coil" or "T-switch" equipped hearing aid users. The T-coil is a magnetic field antenna and as such it is susceptible to pick up EMI. The EMI usually manifests itself at the output of the hearing aid as a background hum, buzz, or static sound. Therefore, an infrared (or other) assistive listening system may be working perfectly, yet a hum or buzz is being heard.
The best solution to solve this problem is to set up the assistive listening system in a room with little or no electrical equipment and/or turn off unneeded electrical equipment. Often electrical distribution panels, and in-wall or ceiling power lines are the sources of the problem. Here, the best course of action is to move the system to another room or have the users sit as far as possible from the wall in which the interference is emanating. To find which positions are acceptable, a MagnatelTM magnetic field meter may be used to measure the level of EMI.