FM Systems - how do they work

A traditional FM System comprises of two main parts – a radio transmitter and radio receiver. The transmitter captures sound via a microphone or direct connection to a sound source and transmits to the receiver.

Transmitters and receivers vary dependent on the manufacturer. Some have manual controls only while others have manual controls with a small screen for menu selection to set the transmission frequency between the transmitter and receiver and select the use of the audio input. The screen may also show the battery level, frequency channel and direction of the microphone.

Receivers

Receivers have different physical outlines, functions and methods of working with hearing aids or cochlear implants. One functional difference is that some receivers have an inbuilt microphone which depending on the situation can be used without the transmitter.

The different methods of receivers working with hearing aids or cochlear implants are:

• Neckloop

The most common is the receiver with a Neckloop that is worn around the neck. The receiver feeds the transmission into the neckloop which is your own personal induction loop and the hearing aid(s) or cochlear implant processor is set to the “T” position. The telecoil, in the hearing aid or implant processor, picks up the magnetic field generated by the neckloop and the electronics in the hearing aid converts the magnetic field back to sound. The sound level received can be controlled by both the receiver and hearing aid/implant processor volume controls.

• Direct Audio Input FM Receiver 

If the hearing aid(s) or cochlear implant processor also have a Direct Audio Input a miniature receiver can be connected directly via an audio shoe (adaptor). The miniature receiver is powered by the hearing aid battery and the sound level received controlled by the hearing aid volume control. The “T” position is not used.

• Direct Audio Input lead

If the hearing aid(s) or cochlear implant processor have a Direct Audio Input the receiver can be connected by a connecting lead directly to the hearing aid via an audio shoe (adaptor) rather than using a neckloop. The cable is normally supplied by the hearing aid manufacturer. The sound level received can be controlled by both the receiver and hearing aid volume controls.

• Inductive Earhooks

Earhooks can replace a neckloop though not as common, especially when wearing glasses. The combination of hearing aids, glasses and earhooks can be uncomfortable for people. The sound level received can be controlled by both the receiver and hearing aid volume controls.

People who do not wear hearing aids can use the neckloop receivers with headphones or earphones. Some manufacturer’s now have a receiver, with volume control, that attaches to the ear. A miniature speaker is positioned over the outside of the ear canal or in the ear canal.

Transmitters

Transmitters on the majority of equipment have an integral microphone, an audio input and frequency selection. As systems have developed, in particular those using digital transmission, there is a selection of transmitters for the user to select from to provide a solution for their individual lifestyle. There are microphones that pick up from greater distances with functions that give the user more control over the quality and clarity of sound they receive and others with features that include Bluetooth and a Transmitter Receiver. 

• Bluetooth® connects the FM System to a Mobile Phone that also has Bluetooth®.

The caller’s speech is transmitted from the mobile to the FM Transmitter and from the FM Transmitter to the FM Receiver. The receiver’s speech is picked up by the microphone on the FM System and transmitted to the mobile.

• The microphones usually have two or three positions

Omnidirectional – where the sound (speech) pick up is 360°

Directional

Directional with zoom

• Audio Input - Line Input

Allows the transmitter to be connected to sound sources (TV, Radio, PC, MP3, Amplifiers etc.)

3.5mm stereo socket

• Audio Input – Microphone Input

Allows an external microphone to be connected via the 3.5mm stereo socket

• Frequency Selection

Dependent on the manufacturer some systems only have one transmission frequency

More than one frequency can be very useful as it provides you with the option to change to a different frequency if you are in a situation, where the sound you are receiving is noisy, possibly due to another person or organisation using the same frequency or one that is close.

• Transmitter Receiver allows two microphones to be used to transmit simultaneously to receivers.

One microphone transmits to the other microphone, which contains a receiver, and this microphone transmits the sound from both microphones to the receivers.

 

Thanks to ihear for supplying the content on this page.

Images are shown courtesy of Comfort Audio AB.

Photos thanks to Phonak.