Radio Frequency (RF) Systems:
The third type of wireless listening system uses radio
frequency (RF) technology to
transmit the audio to the listener. This system is simply a
low power variation on the commercial FM radio services that bring
music and news program into your home or auto. The signal is
"broadcast" on radio waves by a frequency modulation (FM)
transmitter connected to a tuned
antenna in the
listening area. The radio waves radiating from the antenna are
picked up by listeners using an "FM radio" receiver tuned to the
transmitting frequency ( See our TechNote
About Radio Transmission for more
information on how RF transmitters and receivers work).
The Federal Communications
Commission (FCC) allows the use of two bands of frequencies for
these RF transmissions:
(1) The first, provided by
the FCC, is located
between 72 and 76 MHz. ( Actually only three sub-bands are
authorized 72.0 - 73.0, 74.6 - 74.8 & 75.2 - 76.0 MHz. ).
Transmissions on these bands are covered by Title 47 Part 15.237
for "auditory assistance devices" See
About FCC Regulations for more information.
regulations do not specify exact transmission frequencies only a
maximum of 200 kHz bandwidth. Most manufactures divide the
available spectrum in to10 - 200 kHz.
( audio bandwidth to 15 kHz. ) wideband channels or 40 - 50 kHz. (
audio bandwidth to 10 kHz ) narrowband channels. The
terms wideband and narrowband refer to the amount of available RF
spectrum each channel can occupy (
Bandwidth, 200 kHz or
50 kHz respectively. See our TechNote
for more detailed information on RF bandwidth). As you might
expect the wider bandwidth channels produce a higher fidelity ( Less
noise [ Technically, a
higher S/N ratio ] and
extended flat frequency
response ) audio signal at the expense of having fewer available
channels ( Simultaneous transmissions ) available from the allocated
band of frequencies. See the
72 MHz Compatible Chart for a detailed listing of the
channels available from various manufactures.
however, several limitations to the 72 - 76 MHz band.
first is a result of the FCC regulations which limit the maximum
radiated transmitter power in terms of field strength, not power (
Watts ), as is most often the case for RF transmitters. The rule
reads: 'The field strength of any emissions within the permitted 200 kHz band shall not exceed 80 millivolts/meter at 3 meters."
This is a extremely limiting regulation on the area that we can
cover effectively. If we improve the antennal the out put power must
be reduced to keep the field strength within the regulation at 3
meters form the antenna. In the absence of any local
interference ( not often the case ) the legal field strength is
enough to cover school rooms and most churches adequately. The
largest of churches may have to resort to locating the transmitter
away form the audio control point to put the transmitting antenna
closer the the users in the pews. Larger facilities such as
arenas and large auditoriums will not be adequacy covered by the
standard non-licensed hearing impaired transmitters.
Fortunately the most of these larger facilities can be covered with
special licensed transmitters. SHOWORKS can facilitate the
application, licensing and installation of these higher powered
transmitters for qualified customers.
second limitation is that interference form local transmitters
licensed for other services is very common ( especially in large
cities ) on these frequencies. There are many potential
sources of such interference. These include local TV stations
on channel 4 or 5, beeper transmitters, emergency call box systems
and airport approach warning beacons. To make the situation
worse, section 15.5 of the FCC regulations states: "Operation of
an intentional, unintentional, or incidental radiator is subject to
the conditions that no harmful interference is caused and that
interference must be accepted that may be caused by the operation of
an authorized radio station, by another intentional or unintentional
radiator, by industrial, scientific and medical (ISM) equipment, or
by an incidental radiator." In other words, when a
authorized frequency in this band is rendered unusable by a
interference there is nothing to do but look for the availability of
additional frequencies that are not interfered with.
third consideration for this band of frequencies is the maximum
number of simultaneous channel transmissions possible with out
interference. This can be a bit confusing because the various
manufactures have differing statements on the subject....
(2) The second band
of frequencies, referred to by the FCC as the Low Power Radio Service (LPRS),
is located between 216 and 217 MHz. Federal regulations
provide for 19 - 50 kHz. ( audio bandwidth to 10 kHz. ) narrowband channels or
38 - 25 kHz. ( audio bandwidth to 6 kHz ) very narrowband channels.
This is a little confusing because the wider of the authorized
channels on the 216 MHz band is the same bandwidth as the narrower
bandwidth channels on 72 MHz.
Availability of transmitters on
the 216 Band can also be an issue. Both Listen and Williams offer portable body
pack transmitters in the 216 band, but only Listen offers a
permanently install transmitter ( Rack mount and 115V AC wall powered
) in the 216 MHz. band. The Williams TGS-100 belt-pack transmitter offers a
choice between 4 narrowband channels (E, G, J & L). Listen
offers both portable and permanent installation transmitters that
transmit on all 57 authorized frequencies. See the
216 MHz Compatible Chart for a detailed listing of the
channels available from various manufactures.
multi-path interference problems and number of available channels]
RF systems generally
have the following advantages:
Can be used
indoors or outdoors
affected by sunlight or weather
cover large areas ( 216 covers larger areas ( up to 3000 Ft. from
the antenna ) and repeater transmitters can be used for extremely
large coverage areas like golf courses.
Easiest technology to provide even coverage at all seat locations.
Not interrupted by physical objects in room.
Portable battery operated transmitters practical for tour guide and
other mobile applications.
Good fidelity available using 72 MHz. wide band channels.
Not susceptible to local electrical interference from electric
motors or power lines.
Not susceptible to IR spectrum output from high efficiency
fluorescent lighting devices.
Many options of frequency and band width to tailor system to
customer's exact requirements.
Can be used for meeting monitoring outside of meeting room.
Lowest cost of installation.
disadvantages of RF systems include:
Restricted access, receiver is necessary for each user.
Compromised Confidentiality, complete privacy is not possible.
FM signals pass through walls and can be monitored from a
considerable distance with sensitive receiving equipment and
Susceptible to radio frequency interference from local sources or
other radio services such as beeper transmitters and certain TV