SGC DSP Reference

SGC – The SSB People

SGC develops, manufactures, and sells high performance single sideband (SSB)
communications equipment. For more than 25 years, the company has sold to the
marine, military, aviation, and industrial markets world wide. Over these years,
SGC has earned an outstanding reputation for product reliability and for service
after sale.

On the cutting edge of technology, the company keeps pace with equipment
options, engineering developments, and design requirements. Its products are the
most competitive in the entire long distance communication market. SGC equip­ment is presently being used by the United Nations and international relief agencies
for inter-communications in developing countries throughout the world. Many
competitive racing vessels, as well as fishing boats, tugs, and commercial craft are
equipped with SGc equipment. In fact, an SGC radiotelephone provided the only
communication available on a recent Polar expedition by the National Geographic
Society.

SGC supplies U.S. Government agencies, foreign governmental agencies, and
major petroleum companies throughout Asia and Latin America. In addition, SGC
supplies equipment to major international geophysical corporations and exploration
crews.

All SGC equipment is designed and manufactured in the USA, with some compo­nents imported for different international suppliers and manufacturers. SGC has
qualified people ready to provide technical information, assistance in selecting
equipment, and recommendations for installations.

SGC welcomes your call to discuss your HF-SSB requirements.

Digital Signal

Processing

Facts and Equipment

Another

Informative Publication of

SGC, Inc.

Manufacturer of Advanced

Technology

ÒNo Compromise CommunicationsÓ

Table of Contents

Chapter 1

The idea of Digital Sound Processing 1

Understanding Sound 1
Hearing Sound 2

Frequency 2
Amplitude 3

Storing and Retrieving Sound 3

Storing sound 4
Retrieving sound 4

Transmitting and Receiving Sound by Radio 4

Modulation 5

Sidebands 6
Processing Sound Digitally 7
Recording on Compact Discs 7

Sampling 8

Volume 9

Compression 9

Chapter 2

The Idea of Analog Filtering 10

Analog Filters in Audio 10

Crossover Network 10

Woofers 10
Tweeters. 10
Midrange 10

Cutoff 11
Analog Filters in HF Radio 11

Symmetry 12

Crystal filters 12

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Mechanical filters 13

HF filters in practical applications 13

Wide bandpass 13
Medium bandpass 13
Narrow bandpass 13

Chapter 3

DSPs in HF Communications 15

DSP Flow Chart 15

Sample and Hold 16
Analog to Digital 16
DSP 1 7
Digital to Analog 17

Low-pass filter 17
DSP Evolution 18
DSPs in Transmitting Applications 18

DSPs in Speech Processing 18

DSP in SSB Generation 19

DSP in Phase Delay 19

Out-of-phase signal 19

Phase shifting networks 19
DSP in CW Modulation 20
DSPs in Receiving Applications 20

Standard DSP filters 20

Analog 21

Digital 21
Programming 21
Continuously Variable DSP Filters 22
RF Attenuator 23

ii

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P.O.Box 3526, 98009 Fax: [phone] or 746-7173 Tel: [phone] or 1-800-259 7331

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DSP Filters:

High-pass, Low-pass, and Bandpass 23
High-pass Filters 23
Low-pass Filters 24
Bandpass Filters 24

Notch filters 25

Band Interference 25
Heterodyne Interference 26

Digital AGC 26

Chapter 4

Available DSP HF equipment 28

The Digital Receiver 28
DSP Transceivers 28

SGC's SG-2000 PowerTalk 28

ADSP™ noise reduction 29
SNS™ noise reduction 29
First mobile DSP transceiver 30
Visual DSP filter display 30
Programmable digital filters 31
Pre-programmed filter settings 31
Notch filter 31
Variable Bandpass, low-pass,

and high-pass filters 31

Upgradable DSP head 31

Other Advantages 31

Removable Head 31
Simple design 32
High-power/small package 32
Tested for high quality 32

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SGC Inc. SGC Building,13737 S.E. 26th St. Bellevue, WA. 98005 USA

P.O.Box 3526, 98009 Fax: [phone] or 746-7173 Tel: [phone] or 1-800-259-7331

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Add-on DSP 33

Basic Features 33

Variable bandpass filters 34
Notch filter 34
Noise reduction 34

Advantages and disadvantages of

DSP add-ons 34
SGC's Add-on: PowerClear 35
Using DSP HF Equipment 36

Operating 36
Operating with DSP 36
Operating with PowerTalk 37

Chapter 5

The Future of DSP 39

HF Communications 39
New possibilities 39

Manipulation 39
Storage 40
Transmission 40
Digital transmission 40
Data to Computers 40
Other applications 41

Appendix A — Glossary 42
Appendix B — Further Reading 44
Subject Index 49

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The Idea of Digital Sound Processing

Introduction. Digital Signal Processing (DSP) may soon rev-

olutionize many aspects of the electronics industry. DSP will
have much the same effect on electronics that personal com­puters have had on everyday life since the early 1980s. And
part of that effect is due to the fact that DSP is computer­related.

You can expect DSP to affect applications as varied as med­ical electronics, diesel engine tune-ups, speech processing,
long-distance telephone calls, music processing and record­ing, and television and video enhancement. This book men­tions some of these applications, but it focuses mostly on the
products and techniques used in high frequency two-way
communications.

First, a few of the basics. We will discuss concepts of sound,
sound retrieval, and sound transmission by radio. Then we
will discuss how modern technology uses digital in accom­plishing these same tasks.

Understanding Sound

We feel the need to save our sense experiences. For instance,
we record photographs and video images, although we don’t
expect these mediums to reproduce exactly the original. The
photograph and video screen containing an image of a cloud
differ, of course, from a real cloud floating in the atmos­phere.

But sound, heard through one of our basic senses, holds a
special place in our lives because it allows us to communi­cate, protect ourselves from danger, and entertain ourselves.

And so, we save and retrieve our voices and our music on
tape and disc, and we transmit them to other parts of the
world via radio waves, wires, and cables. Anytime we trans-

1

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Chapter 1

mit, save, or retrieve a sound signal (which we call an audio
signal), that signal must be changed into a storable form and
then reconstituted into its former state so that we can
understand it and enjoy it.

Hearing sound

The sound of the rain hitting the ground is a physical pheno­menon. The rain drops hit the ground and cause air mole­cules to vibrate, to transmit through the air until their ener­gy dissipates. If your ear is within range of the vibrations,
the external parts of your ear will focus them so that they
will travel down the ear canals to the ear drum and bones in
the ears. Where the last bone connects to nerves, the physi­cal vibrations become neural impulses, and your brain sig­nals you that you hear the rain hitting the ground.

Those sound vibrations (called audio) travel in ripples, like
ripples in a pond when you toss in a rock. Ripples of water
will radiate out from the place that the rock splashed. The
height (amplitude) of the ripples will decrease as they move
farther away from the source of the splash. The amplitude of
the ripples represents the loudness of the sound.

Figure 1 — Simple ripple form

Frequency. The measure of each ripple from peak to peak

represents its frequency. The longer the measure, the lower
the frequency (and the deeper the sound pitch). The shorter
the measure, the higher the frequency (and the higher the
sound pitch).

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Figure 2 — Frequency of ripple from peak to peak

Amplitude. The measure of each ripple from peak to

trough represents its loudness (amplitude). In between the
peak and depth of the ripples, the level of the water is the
same as it is throughout the rest of the pond.

Figure 3 — Amplitude of ripple

from peak to trough

Complex audio signals, however, look much different from
those ripples on the pond. Whereas the pond ripples would
resemble single-tone audio signals (like ones from a tone
generator or tuning fork), complex sounds such as speech
and the sound of musical instruments comprise many differ­ent waves that overlap and mix together, a much more
jagged, complicated wave than any of those ripples on the
pond.

Storing and Retrieving Sound

When a microphone picks up a sound, it changes the sound
vibrations into electrical impulses. Inside the microphone,
the sound waves strike a thin element (typically a

diaphragm or ribbon). The movement of that element

3

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Frequency

Amplitude

through a magnetic field induces an electromagnetic signal
that will travel to an amplifier to boost the amplitude of the
tiny audio signals to a more usable level.

Storing sound. A phonograph record illustrates how the vi­brational pattern from the microphone/amplifier translates
those electromagnetic signals into physical vibrations. The
vibrations, cut into the grooves of a vinyl disc, match the vi­brations that the diaphragm made: waves that vary in
amplitude and frequency.

Figure 4 — Sound vibrations cut into the

sides of a long-play recording groove

Retrieving sound. To reproduce the sounds cut into the

vinyl record requires a phono cartridge very much like a
microphone: it contains an element that moves within an
electromagnetic field as the needle moves along in the

grooves. The width (amplitude) of the groove controls the
volume, and the rapidity (frequency) controls the pitch of
the sound.

The electrical impulses from the phono cartridge travel to an
amplifier, from which the strengthened signals travel to a
speaker to be reproduced again as vibrations in the air. The
electrical impulses cause the speaker voice coil to pump in
and out, causing the speaker cone to vibrate just as the
microphone element did, transmitting those vibrations
through the air—to your waiting ear.

Transmitting and Receiving Sound by Radio

This book concerns DSP in radio technology, transmitting and
receiving audio signals via the radio. This technology must
address how to transmit a radio frequency signal that also

4

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conveys an audio message. Consider that the typical voice
signal ranges from about 100 to 5000 Hz (.1 to 5 kHz) while
a typical radio signal might be transmitted on 7,200,000 Hz
(7200 kHz—in the 40-meter amateur band). Somehow, the
two signals have to be mixed together.

Modulation. One of the most common means to impress an
audio signal on a radio signal is amplitude modulation (AM).
The first component of the AM signal is the carrier. Just an
“empty” radio signal that contains no audio, the carrier is
called that because its only purpose is to carry an audio sig­nal to receivers. A good way to hear a carrier is to tune in to
the AM broadcast band and tune in to a radio station. When
there is no audio and no static, you are hearing the carrier.

Figure 5 — A carrier signal without modulation

The amplitude-modulated signal has three basic compo­nents: the carrier, its upper sideband, and its lower side­band. When audio signals are added to an AM signal, the
carrier frequency remains at the exact frequency of the
radio signal.

Figure 6 — A carrier signal with modulation

The two audio signals, known as the upper sideband and the
lower side-band, appear on either side of the carrier. The

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SGC Inc. SGC Building,13737 S.E. 26th St. Bellevue, WA. 98005 USA

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upper sideband audio signal appears above the center of the
carrier, and the lower sideband audio signal appears below
the center of the carrier. As a result, if you tune your radio
to the center of an AM radio station, the audio often won’t be
as strong as if you tune slightly to either side of the center.

Sidebands. If you look at one of the sidebands on an oscillo­scope (a video presentation of signal shapes), it will look
quite a bit like an actual voice signal. In single-sideband
(SSB) radio transmission, the carrier and one of the side­bands are filtered out of the AM signal and eliminated. All
that is transmitted is one of the audio sidebands.

Figure 7 — All the energy is concentrated

in the upper sideband (righthand diagram)

SSB transmission is important for two-way communications
in the HF band. All of the power that once was used to
amplify the carrier and two sidebands in an AM transmitter
can now concentrate in the remaining single sideband. And
now the SSB transmission requires only half the channel
width. As a result, an SSB signal sounds almost 10 times
louder than an equivalent AM signal. Because of its efficien­cy, ease of use, and good voice intelligibility, SSB is by far the
most-used radio transmission on the HF bands.

The modulated signal moves from the transmitter out
through the antenna and into the air. It travels through the
atmosphere for dozens or even thousands of miles. When it
is received by an antenna, the tiny radio signal passes into
the receiver. In the receiver, the signal is amplified, filtered,
and the audio deciphered. The deciphered audio signal goes
through the same processes described in Storing and

Retrieving Sound.

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Processing Sound Digitally

The sound processing we have discussed so far is called ana­log, a system in which audio and radio waves mimic the

sound waves they represent.
Digital signal processing changes analog audio signals into

digital impulses, that is into millions of numbers which
describe audio signals. The most common example of digital
technology is the compact disc (CD). Every wave of sound is
converted into binary code (1s and 0s). These numbers are
transmitted in such a way that the audio wave is “built” from
blocks of these numbers.

One way to think of these wave representations is to draw a
mountain on a sheet of paper. That’s the analog signal. For
the digital representation of this paper mountain, place the
wooden squares from a Scrabble game in rows over top of
the paper. With the wooden squares, you can represent the
mountain that you drew on the paper, except that the edges
of the block representation are blocky, not smooth. In actual
digital audio, the numeric building blocks are so tiny that

any blocky edges in the digital audio wave are undetectable.

Recording on Compact Discs

Although CD audio isn’t directly related to DSPs in high fre­quency radio use, CDs do offer a familiar example of digital

Figure 8 — Drawing of a mountain outlined in game

tiles makes a blocky pattern

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audio in the home. The music that is to be recorded onto a
compact disc—simply a thin disc of aluminum that is encased
in a plastic laminate to protect the recording—must be in a
digital medium; that is, it must be converted into massive
numbers of 1s and 0s. When the disc is recorded, much
error-correcting data and system information (like track
information and markers) also go onto the disc along with
the music. All of this data must be retrievable, so the alu­minum disc is etched with minuscule pits. The pitted and
unpitted areas translate as the 1s and 0s that represent the
data.

In place of needle and cartridge of the analog record player,
a laser optical assembly retrieves the audio in a compact disc
player. This low-powered laser fires at the tracks of the disc.
The unpitted areas of the disc reflect its light back, but the
pitted areas reflect almost nothing. This tremendously fast
flickering of light is received by a photodetector that
changes the light flickers into binary electrical impulses.
These are then converted into analog impulses, which can be
amplified and converted into sound by the speakers.

Sampling. Of course the analog-to-digital and digital-to­analog processes are extremely complicated—especially when
you consider that such things as coding and sampling must
also occur in the system. Sampling is the process by which
the compact disc player retrieves an analog sound, then
checks the digital source for its accuracy, then plays another
sound. This cycling occurs 44,100 times per second (44.1
kHz), although many players now sample several times more
than that per second to make sure that the information being
received is accurate and not error-ridden. Such sampling at
harmonic frequencies is known as over-sampling. Many of
the high-cost compact disc players sample up to eight times
the standard sample frequency.

Volume.Relative sound volume also needs to be considered.
Every audio wave-form has a peak-to-peak length (the fre-

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quency of the sound), which determines the pitch of the
sound, and a height (the amplitude of the sound), which
determines its volume. In order for the compact disc player
to accurately reproduce music and not end up reproducing all
of the frequencies at the same volume, the sound samples
are quantified to a 16-bit number between 0 and 65,535.
Every tiny piece of audio can be reproduced by the compact
disc at any one of 65,536 different volume levels.

Compression. These codes that determine various aspects of
the compact disc’s sound and technical operations all require
a vast amount of information. A full compact disc of approx­imately 74 minutes requires in the neighborhood of 34 mil­lion bits of information to produce. If this information was all
held on a standard computer floppy disc, the selection would
have to be placed on 48 5.25” discs or 25 3.5” discs. Using a
compression code makes it possible for digital tapes and
MiniDiscs to be digital and hold as much music as they do.

Conclusion

You have seen how a complex radio carrier wave and its
audio signal can be filtered so only a sideband remains in
use. And you have seen how audio signals can be converted
to digital signals, in such forms as CDs.

In the next chapter, we look at the idea of filters that can
make changes in waves—whether those waves are sound
waves or radio frequency waves. And in Chapter 3, we look
at how digital signals can be processed for radio transmitting

and receiving.

9

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The Idea of Analog Filtering

Analog filters are used for a wide variety of applications in
electronics. One familiar application illustrates how filters
work: speaker crossover networks.

Analog Filters in Audio

Speaker crossovers usually consist of three different types of
filters that combine to channel audio to the proper speakers.
The typical speaker arrangement comprises a woofer (low­frequency speaker), a midrange speaker, and a tweeter
(high-frequency speaker) for each channel of a sound system.
Filters make sure the appropriate audio frequencies at appro­priate volume reach each speaker.

Crossover Network. The crossover consists of low-pass,
high-pass, and bandpass filters at the speaker inputs. Each
filter crops out certain frequencies and passes other frequen­cies.

Woofers. Most woofers are most effective in the several hun-

dred Hz range, so the low-pass filter might be set at 500 Hz.
All frequencies below 500 Hz (but little above that frequen­cy) will pass to the woofer.

Tweeters. Similarly, most tweeters are effective above about

4 kHz, so the high-pass filter might be set at this frequency.
All frequencies above 4 kHz (but little below that frequency)
will pass to the tweeter.

Midrange. Midrange speakers use a more complicated filter—

a bandpass filter, which combines high-pass and low-pass fil­ters to set both a high-frequency and a low-frequency limit
on the audio that passes through. This bandpass filter would
pass all frequencies that were in an audio band above 400 Hz
and below 4 kHz.

As a result of such filtering, these speakers produce good-

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P.O.Box 3526, 98009 Fax: [phone] or 746-7173 Tel: [phone] or 1-800-259 7331

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© 1997 SGC Inc

Chapter 2