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.ASSISTIVE LISTENING DEVICES
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These are special systems which are designed to assist people with hearing impairment for specific purposes such as using telephones, watching /listening to TV, doorbell, alarm clock etc. They by and large facilitate auditory activities of daily living.

They refer to system that

(a) Enhance only the signal (sound which is useful for the person) and not the noise  
     by transmitting amplified sound directly to the listener with hearing impairment.

(b) Transform sound into visual or vibrational signals

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SOUND ENHANCEMENT TECHNOLOGY

Sound Enhancement Technology systems used to assist in the reception of sounds. Eg. Personal and group hardwire systems, infrared systems, loop induction systems and frequency modulation systems.

In these systems, microphones picks up sound (incoming) and convert it into another form of energy. An amplifier then increase the intensity of this signal. Lastly a receiver converts the amplified signal back into acoustic (sound) energy so that the person with hearing impairment can hear it.

Induction Loop Systems

In Induction Loop Systems the amplified electrical signal is made to run through a loop of wire which may be enclosed in the walls of the room or worn as a neck-loop by the user. The electromagnetic field generated around the wire can be picked up by the hearing aid kept in T ( telecoil ) position and reconverted to amplified sound which the person with hearing impairment can hear.

These systems have been installed at ticket booking counters of railway stations in some of the major cities of India. This would enable the person with hearing impairment to hear announcements made over the microphone without being disturbed by the noise. The listener would have to wear his own hearing aid & set it to T position. Induction loop systems are also used commonly in special schools for the hearing impaired.

Group Hearing Aids

Group Hearing Aids are generally used in classrooms of special schools for children with hearing impairment. The teacher has microphone with him/her. The amplified signal is fed to earphones worn by the children sitting at their desks. Each earphone headset has a volume control adjustment on it or on the desk. There are microphones fixed to the desks so that whatever is spoken by one child can be heard by the others.

Frequency Modulated (FM Systems )

FM Systems are probably the best systems to combat the effects of noisy environment. The sound signal is picked up by the microphone (used by the person speaking) amplified and converted into a radio frequency modulated carrier waves. They are transmitted to a personal receiver worn by the person with hearing impairment. The receiver converts the signal to audible sound.

These systems provide very good sound clarity even in the presence of room noise. FM systems are however, expensive.

Infrared Systems

In Infrared Systems the sound picked up by the microphone is converted into infrared light waves, which are then dispersed throughout the listening environment. A special receiver is worn by the listener, which converts these light waves backs to sound. These systems give good sound clarity but have drawbacks including cost.

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TELEVISION ENHANCEMENT TECHNOLOGY

Television Enhancement Technology equipment is used to improve auditory perception of a televised signal. Also caption decoding can be done.

They include different types of devices. Some of them are targeted to improve sound quality for the listener. Infrared systems can be used for this purpose.

Closed caption decoders are also very useful as they convert all the spoken messages on TV into visible text/captions which can be read on the TV screen.

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TELECOMMUNICATION TECHNOLOGY

Telecommunication Technology systems used to enhance telephone communication. Eg; Text telephone, built -in portable amplifiers and induction loop systems.

These include systems designed to make communication over the telephone easier for the person with hearing impairment. A hearing aid user can keep his personal hearing aid on T position to listen over the telephone.

There are special hand sets which give amplified sound. Some manufacturers produce telephone couplers which can be attached to regular handsets to get amplified sound.

Text Telephones/Telephone devices/ Teletype writers -

Both parties communicating must have the instrument. One person types the message. The signal is transmitted through the telephone line and displayed on the screen of the person receiving the message.

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SIGNAL/ALERTING TECHNOLOGY

Signal/ Alerting Technology use devices which delivers signal through visual or tactile vibratory means so as to enable hearing impaired person to perceive them.

This includes technology to alert persons with hearing impairment to the presence of sound by converting the sound into another form of energy ( visual/ vibration) which she/he can perceive Eg. Alarm clock with vibrators, baby cry systems for hearing impaired parents, door bell/telephone alerts, smoke alarms.

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