FM system
As FM system are wireless signal transmission systems referred to, the signals with the frequency-modulated transmit radio signals (FM). The term is mainly used for sound transmission systems for the hard of hearing .
Occasionally, infrared hearing aids are also referred to as “FM systems” if frequency modulation is also used for the infrared light used for transmission.
Basic principle
The device configuration consists of a transmitter and a receiver . The desired audio signals are recorded on the "transmitter" (for example with a microphone ), converted into modulated electrical radio signals and broadcast. The person with hearing impairments in turn carries the receiver, which either converts the radio signal back into sound waves , which are conducted to the ear with their own earphones , or via cables to a connected, independent hearing aid .
For both the direct transmission from the FM receiver module to the ear and the “indirect” transmission to the hearing aid, there is actually no “ amplification ” of the sound, but only the attenuation of the sound at a greater distance is bypassed by the transmission process .
Instead of the wired transmission with the so-called " audio shoe " of the hearing aid technology, a transmission from the receiver using the Bluetooth technology can also take place if the hearing aid is equipped with this technology.
Execution for conference or school class systems
Channel
A microphone is connected to the transmitter of the system, which picks up the audio signals that are emitted by the transmitter as FM radio signals in the ultra-short wave range.
Different transmitter versions can either be designed for stationary operation e.g. B. with an audio system as a connected sound source or carried as a mobile by a speaker who also moves around the room (e.g. teacher in the school class).
In order to ensure trouble-free operation of several separate systems in different rooms of a building (e.g. in schools ), the systems are configured to be multi-channel and switchable.
receiver
A receiver that the person with hearing loss wears on their body or that is attached to their seat can pick up and convert these signals. Depending on the device configuration, the receiver can use headphones or a connection to the independent individual hearing aid. In the latest developments, the receiver is integrated with highly miniaturized electronics in the audio shoe , which was previously only designed as a pure cable plug-in connection and which can be attached to behind-the-ear hearing aids and also to the "speech processors" of cochlear implants worn on the outside of the ear .
Depending on the device configuration, it is also possible to use an induction loop around the neck instead of the headphones, the hearing aid cable connection or the audio shoe equipped with electronics . The sound signals are transmitted to the hearing aid by electromagnetic induction . The hearing aid must have a so-called telecoil .
Typical parameters
- The radio transmission frequency is in the ultra-short wave range, depending on the product, between around 200 and 800 MHz
- The pitch of the transmitted sound is in the range of human sound perception between 80 and 16000 Hz
- Values between 15 and 25 dB are given as the dynamic gain compared to hearing aid only use
- The transmission range is on the order of up to 30 meters
Version for radio or television sound
An additional transmitter on the radio or television set picks up the sound and transmits it in the form of FM radio signals to a receiver module that the hearing impaired person wears on their body.
Due to its shape, the receiver is often called the “under-the-chin receiver”. It has two ear pieces with mini speakers on each short plastic tube. The lower ends of these tubes are connected by a more or less rod-shaped "bracket" housing that is worn hanging below the chin. This is where the electronics are located that convert the received FM signal back into sound waves. As with the conference or school class systems, an induction loop placed around the neck in conjunction with the individual, independent hearing aid can be used here (depending on the product) instead of the acoustic headphones .
Features of use
Several people equipped with receivers can hear at different volumes within the transmission range without influencing each other.
The FM receivers of the participating persons must belong to the same product family as the transmitting device. There is no cross-product standard here.
In principle, freedom of movement is possible within the range of the radio transmitter without interruption of reception, whereby, in contrast to infrared hearing systems, there is no shielding or " radio shadow " from objects or people standing in front of it.
See also
Web links
- System description Phonak FM systems
- Which hearing system? Tips for parishes (on Schwerhoerigenseelsorge.de)
- Guide to hearing and communicating barrier-free in the world of work : Systems for technical hearing support The Hörkomm.de project supports the inclusion of hard-of-hearing employees.