\tenrm
\magnification=\magstep1
\twocolumn
\centerline{DIGITAL SIGNAL PROCESSING}
\centerline{and Amateur Radio}
\centerline{by Dr. Thomas A. Clark W3IWI and Dr. Bob W. McGwier N4HY}
\bigskip
Tom calls the necessary ingredient magic, and Paul Rinaldo, W4RI, calls
the necessary ingredient an incubator[1][2].  Whatever you choose to call
it, we have lacked the necessary ingredient in ham radio to broadly apply
the collection of techniques known as {\it digital signal processing}
(DSP).  The magic in the amateur satellite service has been provided by
AMSAT[3].  In the digital communications modes, the incubator was
supplied by TAPR[4].  AMSAT and TAPR have joined to support an incubator
project to achieve the broad use of DSP in amateur radio.
\par
DSP is a field which has its roots in the mathematics of Newton, Gauss,
and Fourier.  These people lived hundreds of years ago.  Why are we
stessing these ancient techniques?  Primarily because a much newer
technological revolution has taken place which allows us to apply DSP
techniques in increasingly complex combinations.  Of course, we are
talking about computers in general and microprocessors in particular.
Until the very recent past, even microprocessor elements were primarily
used in proprietary special purpose devices with specific tasks in mind.
For example, there are DSP chip sets manufactured by several companies
that are totally dedicated to being Bel-212 modems for phone line use.
\par
We will not delve into the theory of Fourier analysis, linear
systems, ARMA models, and finite impulse response filters here.  However,
a little study of these technical topics reveals that we need only do a
handful of mathematical operations well.  The primary ones are multiply,
accumulate, and shift contiguous data locations in memory.  If we were to
design a processor that did these things well, and ignored a lot of the
things general purpose microprocessors are supposed to do, we would
open our systems to the power of these DSP techniques.  Fortunately for us,
we don't have to design these chips.  Possibly the best known family of
DSP chips today is the TMS320 family of DSP microprocessors (DSP engines).
In fact, the three operations above can be combined into {\it one machine
instruction} on the TMS320 family.
It is what makes these chips one of the idealized sets for most DSP operations.
There are others, including the Motorola DSP 56000 which have recently begun
to flow to users of DSP engines.  For others, and a general discussions of
these chips see[5].
\par
Why should all this excite us?  Maybe a few examples are in order.  A well
known novel, which used DSP techniques to the advantage of submariners,
was Tom Clancy's ``The Hunt for Red October.''
The signal processing done in that book  enabled the ``hunt'' to be sucessful.
The story was fiction but the techniques on which the plot was based are
very real.  CAT (computer aided tomography) scans
have greatly reduced the need for inherently dangerous exploratory surgery
thanks to DSP.  However, the funds for research and development have come
primarily from the telecommunications industry.  The ability to do very
complex DSP operations on small devices has greatly reduced the capital
costs of maintaining and growing the telecommunications that now surround
the globe.  The ISDN (integrated services digital network) will be a reality
in the near future thanks in a large part to digital techniques.
$$\hbox{{\bf BUT WAIT THERE'S MORE}}$$
\par
What has this to do with amateur radio?  Imagine that on Monday Bob wants
to talk to Tom on 20 meters and wants to use ACSSB (amplitude compandored
SSB) for a better signal to noise ratio.  On Tuesday Tom finds that 20 meters
is dead but needs to talk to Bob again, so he will use a voice encoding
scheme and send the digitized voice over the packet network between them.
But wait, there's more.  On Wednesday, Bob wants to transfer a large file
that resulted from the previous two days of conversation to Tom and wants
to do it at 9600 BPS.  But wait, these guys are really active, there's more.
On Thursday, Tom wants to gather weather maps from the NOAA satellites.  On
Friday, Bob wants to talk to Tom but apparently transmitter is acting up
and he needs an audio spectrum analyzer.  But wait there is more to ``DSP
week.''  On Saturday, it's an EME weekend and Bob and Tom want to try getting
each others echoes off the moon.  Alas!  Murphy has struck and both have
lost their high power amplifiers.  But wait there is more.  On Saturday, they
decide to try Fuji Oscar 12 PSK packet and its bulletin board in space.  A few
years ago, you would have said these guys are millionaires or loco since all
this can't be done anyway with what we have.  Each of these operations is
characterized by one important feature
$$\hbox{On a DSP chip {\bf IT'S ONLY SOFTWARE}.}$$
\par
ACSSB is almost too easy on DSP engines.  LPC-10 and ADPCM (linear predictive
coding and adaptive pulse coded modulation) have been around for a while
to do encoding of digitized voice.  The PSK modems and the WEFAX-APT
demodulation are both based on a phase locked loop (PLL) which is
straightforward on a DSP chip.  The spectrum analyzer and the EME weak
signal detection technique both use the same DSP process (FFT) as their
basis as decsribed below.
In addition to these examples, one you can make modules for all
the standard modems and some non-standard modems at least up to 2400 bps
on the least expensive of these chips (TMS320C10).  The heart of the
9600 bps demodulator can be done on this chip and the transmitter is
trivial.  On a single generation later the least expensive chip (and
still inexpensive) you can do the entire 9600 bps modem.  When Texas
Instruments finally releases the TMS320C30, you can do a 56 Kbps
MSK modem with coherent demodulation (optimal) and have microprocessor
power left over to do the entire AX25 protocol and HDLC.  This chip,
when coded carefully will sport 33 Megaflops (!!) and that is about
3/8-ths of the fastest code N4HY has ever been able to code on a Cray-1.
\par
We have emphasized some fairly advanced applications.
In fact, these chips lend themselves very well to the more mundane tasks
of filters of many types: complex bandpass, lowpass, and highpass filters
of many types are very straightforward on these chips.  An optimal and
adaptive woodpecker filter that can ``tune'' to different pecking rates
is also achievable on this type of chip.  The list of things
you can do on these chips is enough to write an entire textbook on digital
signal processing and there are books available on these topics[6].
$$\hbox{{\bf A Little Magic}}$$
\par
To begin this project we are negotiating for 20-25 boards manufactured
by Delanco-Spry in Silver Spring, Md.  On them is the Texas Instruments
TMS320C10-25.  These boards fit into a standard long expansion slot on almost
MS-DOS based machine (see figure 1).  Our final BUT WAIT THERES MORE is that
all the
functions mentioned in the DSP week above can be done on these boards with
high level support from the DOS machine.  Some of the software exists and
has been tested and is being improved.  If it were all done there would be
no need for a project and we would concentrate on getting wealthy.  We have
concentrated our initial efforts
on the spectrum analyzer and weak signal work with a great deal of
success.  The spectrum analyzer is the functional equivalent (with a few less
bells and whistles) of a \$10000 machine manufactured by a reputable company
here in the U.S.
When most of us think of a spectrum analyzer, we envision a device
which is a tunable receiver slaved to an oscilloscope.  The frequency
of the receiver is swept and the receiver's detector output is displayed.
At any one instant, the receiver is tuned to only one frequency.  If the
receiver has a given detector bandwidth and we are using some total sweep
width, then we can define a useful ratio N to be
$$\hbox{N = sweep width/bandwidth}$$
where N is typically 100-200 and is the number of independent frequency
channels that are being scanned.  On any one channel we spend only
$1/Nth$ of the time and our ability to see a weak signal is severely
comprimised.
In the case of a spectrum analyzer done with DSP techniques, we can
choose to have N independent detectors, each of which operates all the time.
To do this, we have to take at least 2N data samples of the wideband input
signal (corresponding to the sweep width) with a high-speed A/D converter,
and then do mathematical magic on the 2N numbers; the magic most often
applied is the Fast Fourier Transform
(FFT).  You make N be some power of 2 (like 128, 256, 512, 1024).  With the DSP
chips and boards we can now buy for less than the price of a weekend
in Los Angeles or an 8877 on 2 meters, N=512 with a channel bandwidth
of a few Hz and a total spectral bandwidth of a few kHz is feasible.
See figure 2 for a pictorial representation of the two types of analyzers.
\par
If you add up the cost of using special purpose
hardware to do the other tasks in the DSP week, you can see that the savings
are tremendous and indeed make some of these things possible where they were
at best difficult before.  The primary example is the EME without high power
amplifiers.  We hope this project will produce the other things in our
DSP week.  The structure of the project will follow a set of rough
guidelines.  We are looking for a few people who are capable of writing (or
learning to write) TMS320 assembler to write the functional modules that
will be used as building blocks.  The applications in this project will
be controlled in a high level language on the PC.  We have tentatively
settled on ``C'' as the natural system language for these applications.
Already in use are Borland's TurboC$^{tm}$ and Microsoft's MSC-5.0$^{tm}$ and
Aztec ``C''.  We are looking for people who can program ``C'' to help
produce these neat applications for the modules on the board.  We also
have a place for a few people who will be strictly ``users'' to critique
the efforts of the people doing the coding.  As applications become available,
we will either publish them or make them available to the amateur community
through the commercial vendors in those instances where the vendor either
paid for the development by funding the work or acquires a license from
the project participants.  The commerical route is for those
who have (say) a C-64 and want a widget that plugs into their computer
and provides these neat applications.  In the case of ACSSB, DSP will provide
an easy way to get pilot tone coded compression without having to adapt
someone else's reject boards or modifying your equipment.  For packet radio,
for the first time we will have adaptive equalization working for us.
If you are
interested in participating in this project, write or call us and we'll be
glad to put you to work!  We believe this project will provide exciting new
capabilities $$\hbox{{\bf REAL SOON NOW!}}$$
\vfill\eject
\hangindent .2in
\noindent
[1] Thomas A. Clark, a talk given at the 1987 Trenton Computer Festival.\par
\hangindent .2in
\noindent
[2] Paul Rinaldo, W4RI, ``Empirically Speaking'', {\it QEX}, April 1987.\par
\hangindent .2in
\noindent
[3] In the U.S., AMSAT, Inc., P.O. Box 27, Washington, D.C. 20044\par
\hangindent .2in
\noindent
[4] TAPR, Inc., P.O. Box 22888, Tuscon, Arizona 85734. \par
\hangindent .2in
\noindent
[5] Aliphas, Amnon and Feldman, Joel, ``Digital Signal
Processing Chips'', {\it IEEE Spectrum}, June 1987, pp. 40-45
\par\hangindent .2in\noindent
[6] Rabiner, R. and Gold, B., {\it Digital Signal
Processing}, Prentice-Hall, Englewood Cliffs, N.J., 1975.\par
\end
