Instruction manual

ACU-1000 Operations Manual
4-12 Interoperability Now
the minimum necessary to pass the information. This type of noise reduction is most effective
on purely random noise, such as white or pink noise, and less effective on impulse noises. The
noise reduction value allows the amount of noise reduction to be set in ten steps from off to
maximum. Increasing the level provides more actual noise reduction, but may give a “surging”
quality to the recovered audio depending on its frequency content. Reducing the level lowers
the noise reduction but may provide the best sounding audio in some cases. The best setting in
a particular application depends on the noise level and represents a balance between noise
reduction amount and ultimate audio quality.
The only method to find the correct amount of Noise Reduction to apply is to listen to the
received signal as the level is changed; this is best done using the HSP Handset so that you can
be sure that all noise heard is from the radio’s received signal. Do not use the HSP speaker or a
cross-connected radio. A little Noise Reduction goes a long way, and too much will give the
received signal a fuzzy, artificial sound. It may be advantageous to attempt to improve the
signal quality by other means (such as improving antenna placement) before adding Noise
Reduction.
Noise Reduction Procedure
• The default setting for Noise Reduction is Off (no reduction). While listening to the
received signal, increase the Noise Reduction setting one step at a time until the best
signal quality is reached.
• If possible, listen to the receive signal from several different sources and determine the
Noise Reduction setting that works for most.
• If the signal quality is later improved, revisit the Noise Reduction setting.
• Click “Apply” to save the setting.
4.8.3 Audio Equalization
In most communications links, audio frequency shaping takes place in multiple places.
Typically the microphone, associated amplifiers, modulation filters such as bandpass and
preemphasis, receive deemphasis, receive audio amplifiers, and finally, the speaker or handset
device. In general, the communications channel tries to optimize the 300 to 3000Hz range as
this is where most of speech information is contained. In FM radios, in order to reduce noise,
the higher frequencies are preemphasized (boosted) prior to transmission and then
deemphasized (rolled off) after the discriminator audio output. The deemphasis not only brings
the frequency shaping back to normal, but it has the advantage of rolling off any high
frequency noise picked up during transmission.
Typically, the audio takeoff point for interface of an FM type receiver into a DSP module
would either be at the discriminator output or the speaker output. If taken off at the
discriminator output, it will still have the high frequency preemphasis which will give it a very
tinny sound. Conversely, if taken off at the speaker, the audio shaping of a particular brand of
radio may produce a more muffled sound that is hard to understand because it is lacking in high
frequency information.