Frequency Modulated Systems(FM Systems)

Catherine V. Palmer, PhDUniversity of Pittsburgh

Pittsburgh, Pennsylvania USApalmercv@upmc.edu

What is an FM system?

Wireless type of Assistive Listening Device (ALD)

Voice from speaker is picked up via a wireless microphone located near mouth, acoustic signal is then converted to an electrical waveform and transmitted via FM to a receiver tuned to the same frequency. The electrical signal is then amplified, converted back to an acoustic waveform, and conveyed to the listener through coupling to the ear or hearing aid

Types of FM systems

Microphone and transmitter worn by the speaker with the body worn receiver worn by the listener (connected to hearing aid through neckloop/telecoil or direct audio input)

FM boot for a BTE or speech processor Hearing Aid with built in BTE FM receiver Open ear receiver (no hearing aid) Sound Field FM system

FM Boot System (Phonic Ear)

“Lexis” Hand held transmitter Receiver connects to BTE

via Direct Audio Input (DAI)

4 microphone array plus omni directional mic

Digital Signal Processing 50dB signal-to-noise ratio Manually adjustable gain

control (14dB range)

BTE FM (Phonic Ear)

“Sprite” Nonlinear programmable

BTE with built in FM receiver

Linear or Wide Dynamic Range Compression versions of DSL, NAL-RP and POGO II

Can be used with all hearing levels

Choice of FM only, Fm and t-coil

BTE FM (Phonak)

“EduLink” Free-style synthesized FM

receiver Great for very young

children with unilateral/minimal hearing loss

Multi-frequency Programmable frequency

and gain level

Sound Field (Phonic Ear)

“Radium” Speaker/receiver for team

teaching Multi-channel Dual receiver Frequency synthesizer Wide band transmission Freq. range of 50 Hz to

12K Hz

Why and where to use an FM system?

Why: To improve signal to noise ratio, to reduce the effects of noise, reverberation, and distance

Where: Classroom, indoors, outdoors, church, meetings, lecture halls, restaurant, car, when one on one communication is warranted, movie theatre, when ambient noise is present, when background noise is present, in reverberant rooms

Who can use an FM system?

Anyone who wants improved signal to noise ratio

Children/adults Individuals with hearing loss Individuals without hearing loss Individuals with auditory processing

problems Individuals with cochlear implants

Adjusting the FM receiver output controls

When coupling the FM to the hearing aid you want the receiver to output the same signal level and spectrum when the transmitter is used as when the hearing aid alone is used– Assuming the hearing aid has been fit correctly, the

hearing aid response should become the target

– Input test signal for setting the ALD = 80-85 dB SPL

– Record output response and compare it to the hearing aid response with a 50-60 dB SPL input signal

Transmitter

Receiver

Receiver

Adjust gain and/or tone controls on FM receiver or in the “FM hearing aid program” until the response out of the hearing aid connected to the FM system matches the response obtained for the hearing aid alone – Note: this procedure is for FM only (not an

activated environmental microphone)– Set an environmental microphone response

lower than the FM microphone response in order to maintain the signal-to-noise advantage

Conclusion

The most important aspect of an FM system may be the training of individuals to troubleshoot the system. – Teachers– Parents– Students