Page 453 - Complete Wireless Design

Wireless Issues

452 Chapter Ten

placed into a transmitter’s modulator. Speech compression and companding
are the most common of these processing techniques. A special form of speech
processing, called automatic level control (ALC), affects the RF, instead of the
audio, of a transmitter system.
Considering that the modulation frequency of FM, SSB, and AM transmit-
ters will affect the transmitted bandwidth, a method to limit the maximum
baseband frequency must be utilized. This can be accomplished by an active
low-pass filter placed within the audio sections.
Speech compression prevents a much wider bandwidth from forming outside
the desired AM passband because of the deleterious effects of overmodulation,
which produces spectral splatter. Splatter is constructed of the additional har-
monics created in the baseband signal by overmodulation, which further mod-
ulates the carrier, causing extra sideband components and a widening of the
bandwidth. An overdriving of the IF and/or RF amplifiers of the transmitter
may also occur, creating IMD. Splatter and IMD generate adjacent channel
interference (ACI) and a less intelligible baseband signal. A speech compres-
sion circuit decreases these negative effects by amplifying a signal normally
up to only a predetermined level, but then will begin to reduce gain by 1 dB
for every 2 dB of audio input signal. These basic speech compression circuits
will simply confine the maximum AM or SSB audio amplitude to some maxi-
mal value, while dynamic compression helps intelligibility by increasing the
smaller baseband amplitude levels as well. Compression schemes in general
are quite capable of raising the average output power of an SSB transmitter,
while decreasing distortion and splatter and limiting modulation to 100 per-
cent or below. Compression is so effective at increasing the average transmit
power because human speech has voice amplitudes that are highly complex
and irregular, yet the transmitter must be prepared to send out the highest
voice peak with low distortion and splatter—a peak that may be 10 to 12 dB
higher than the average energy contained within the entire waveform. Thus,
compression smooths over much of these amplitude variations in order to
allow a much higher average output power, and thus an increase in the range
of the wireless link. Simply, basic speech compression acts in the same way as
standard AGC, but is located in the audio stages (Fig. 10.25).
The technique called companding can take compression to outrageous levels
by almost completely compressing all of the peaks to be at close to the same
level as the speech’s valleys, and then expanding them back to normal ampli-
tudes at the receiver (Fig. 10.26). This permits a high dynamic range, better
signal-to-noise ratio, and much higher average output powers.
Many FM, and some AM and SSB, voice transmitters may even employ a
form of processing called speech clipping. This clipper circuit actually hard-
limits the voice signal if it reaches a certain high amplitude. A low-pass filter
at the output of the audio section not only removes the harmonics produced by
this clipping action, but also limits the maximum frequency possible of the
baseband signal. A similar concept is an audio clipper circuit (Fig. 10.27),
which can provide a degenerative out-of-phase feedback signal for any audio

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