Narrow-Band Basics
Less Interference
All FM receivers are tuned to a specific carrier
frequency. The selectivity of the receiver determines
how close nearby radio signals can be to this carrier
frequency before interference occurs. True narrow-
band receivers like the Williams Sound R19 use
dual-conversion circuitry to achieve a very narrow
“window” of acceptance for radio signals. This
allows them to operate in areas that have a lot of
different radio signals with less likelihood of
receiving interference.
More Simultaneous Channels
This narrow window of acceptance means more
channels can be squeezed into the designated
frequency band. Instead of dividing the Auditory
Assistance Band (72-76MHz) into 10 wide channels
(wide-band), the same band can be divided into 40
closely spaced (narrow-band) channels. The
selectivity of the R19 dual conversion receiver
allows you to operate on these closely-spaced
channels without interference from the next channel.
Greater Operating Range
Another measure of FM receiver performance is
sensitivity, measured by the weakest radio signal that
the receiver can respond to and still operate properly.
All hearing assistance transmitters are limited by
FCC Rules to the same transmitted signal strength,
so you can’t boost transmitter power or antenna
efficiency to get more operating distance. The
farther you move from the transmitting antenna, the
weaker the signal becomes. A very sensitive receiver
like the R19 will allow greater operating distance.
Because of its excellent operating distance, the
PPA System 400 can be used in situations where
maximum coverage area is needed.
Sound Quality
Narrow-band systems give up some frequency
response (extended highs and lows) and noise
performance compared to wide-band systems.
This is usually not a drawback for spoken word
presentations, but may not be optimal for programs
which are primarily music.
Manufacturers often list a minimum signal strength
needed to maintain an acceptable noise level in the
receiver before it “squelches” or goes quiet. The
W
illiams Sound R19 receiver is designed to offer
maximum sensitivity with minimum noise and a
reasonable squelch action.
Overload Protection
Making a receiver sensitive for greater reception
range of weak signals can create a problem when the
receiver is used in an area that has strong radio
signals. Unless the receiver has an superior dynamic
range, it can be overloaded by the strong signal
and become noisy, distorted, or vulnerable to
interference. The Williams Sound R19 receiver uses
state-of-the-art circuitry for improved RF dynamic
range.
Capture Effect
When an FM receiver is presented with two radio
signals on the same channel, it will “lock on” to
whichever signal is stronger. This is called the
“capture effect.” Because signal strength is related
to distance, this usually means that the closest
transmitter antenna will produce the strongest signal
and “capture” the receiver.
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