Here is a copy of my final project:
Digital
Signal Processing for Communications Systems
Introduction
Early signal
processing techniques used from 1930 to the 1970's took the form of
specific modules and circuits designed to handle an area of
interference, noise, or to address an audio quality issue. For
example, engineering handbooks of this era treated impulse noise with
fast attack limiters and filters were available to restrict bandwidth
with the intent of providing a better signal to noise ratio and
audible signal to the user.
In the late 1970's
a transition from analog to digital took place in the communications
field. The process took several iterations from early bit slice
processing techniques to today's popular 360 megahertz Digital Signal
Processor (DSP) system.
With built-in
Analog to Digital Converter input and Digital to Analog Converter
output, DSP processor chips have become the focal point of
significant improvements in communications and the benefits haven't
stopped. Encoding speech, where the communication path takes
advantage of the power to digitally process signals with voice
equalization, compression techniques and forward error correction
have all improved the quality of transmissions. The race is on to
implement new features every day with customizable firmware upgrades
just a download away.
History
and Progress
The microprocessor
appeared in a significant way about 1975. With that early advent, the
processing power was a 4 bit chip and speeds measured in the
sub-megahertz range (.6 MHz). While not a fast enough product for
on-the-fly digital processing needs, the American Telephone
and Telegraph phone
people were quick to realize the importance of digital switching. “In
1965, digital
appeared in
Electronic Switching System One. By 1975, Electronic Switching System
four in Chicago was introduced as the new digital system with DSP”
(A Brief History 1).
Digital became a
useful and efficient method to handle call routing but must have
impressed the
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company with the
possibility of actually processing the voice aspect of calls using
DSP methods.
Prior to this time,
physical limits of processing power, equipment size and length of
time to build facilities were roadblocks to effective DSP tools.
Computers were only available to large corporations, researchers, and
the military. About 1978, a new product appeared that would change
that; DSP on a chip had arrived.
While we might
think of telephone communications as a wired system because of its
visibility as the last mile factor between the home or business and
the central office, microwave and radiotelephone or even satellite
form a great portion of the long distance paths. While reasonable
stable within their respective domains, the push to maximize the
number of callers per circuit is always the goal. For the telephone,
a major concern is noise introduced by external interference,
component generated noise and miles to thousands of miles of wire.
For the radio communications people, some of the same issues affected
their signal as well. The weaker the signal, the harder it is to fix
the noise problem.
Earlier attempts
to use the available form of microprocessor with its low speed and
general purpose instruction set proved problematic as a real time
solution. Bit slice processors became popular due to their fixed
purpose instruction set and parallel operation with a fundamental
goal of short latency. Latency being the problem where the signal
processing takes too much time to accomplish, thereby causing an
inappropriate delay in the signal. The then common Z-80
microprocessor was expected to take 16 milliseconds for an
operation with conventional construction and programming
practices whereas a bit slice design with internal microcode was
doing the job in .5 microseconds. That was the type of improvement
needed to make DSP acceptable. (Bishai 2)
Signals in the
analog domain are input to a DSP system through an analog to digital
converter.
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A
sample represents a portion of the overall signal, broken into
segments that allow handling by the
processor. Once
stored in memory, the sample might be directly manipulated by a
mathematical equation or additional samples gathered for averaging
purposes. In a general way, an algorithm or set of computer
instructions treat the sample content or may use several samples in
an averaging process to improve the signal quality and might provide
one approach to improve the signal to noise ratio. After the sample
is processed, it is output through a digital to analog converter and
sent to the next stage. For example, all this must happen quickly in
a telephone system where delays in conversation impact the quality of
communication. Speed is an important consideration in the DSP
process. Conversions in the audio spectrum pose less difficulty than
wide band video. Slower DSP designs might be placed in the audio
chain where the frequency spectrum and levels are less demanding.
Fast DSP chips are better for streaming video content.
The TMS32010 DSP
chip produced by Texas Instruments in 1983, proved a big success. It
was based on the Harvard architecture and had separate instruction
and data memory. With a special instruction set, like
load-and-accumulate or multiply-and-accumulate, it could work on
16-bit numbers and needed only 390 nanoseconds for a multiply–add
calculation . About five years later, the second generation of DSP
chips appeared. “The new product had additional memories for
storing two operands simultaneously and included hardware to
accelerate tight loops. They also had an addressing unit capable of
loop-addressing with some operating on 24-bit variables and required
only 21 nanoseconds for an operation” (A Brief History 2). That is
worth repeating: 16 milliseconds to 21 nanoseconds!
Military has often
provided a path for eventual consumer use. While the scope of DSP may
have found initial use in radar display systems and battlefield
communications, eventually, spill-over occurs and the general public
benefits from the technology improvements. For example, commercial
radio
communications
manufactures often have a hand in both high tech military and
business radio
applications at the
same time. This dual role benefits the consumer with lessons learned
during the
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research on
military applications.
With industry
emphasis on DSP, new tools and libraries are appearing in the field
for designers. Chip manufacturers routinely offer at least some free
tool sets but may require payment for the high end variety. Compilers
provide specialized instruction sets to deal with internal signal
handling. Below is a short piece of coding and note that the code
itself refers to DSP operations and will perform the following
equation:
L-1
y(n)
=
ai
x(n-i)
i=0
And may be coded
below for use in a DSP processor:
For (n=0;
n
{
s=0;
for 9i=0; i
{
s
+= a[i] * x[n-i];
}
y[n] = s;
}
Equation and code
sample (Crawford 26).
Hyper-jump to the
specialized DSP microprocessor of 2012 and we find availability of
chips that represent a flood of possibilities in speeds and
processing power. Prices range from $1.95 to $10.00 for devices that
have been fine-tuned in capability and price. For power users and
video needs, Texas Instrument lists DSP chips in muli-processor
configurations with speeds up to 1.5 Gigahertz and prices in the
$200.00 range. Most of today's products provide custom software to
work with and a rich
instruction set
provided for direct control of the DSP process, to the degree that
specialized functions
provide fast product
turnaround and integration. Specialized chip architecture and
functionality for the
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intended audio,
video and processing environment make drop-in placement possible.
Conclusion
An amateur radio
equipment manufacture states.
“The model K3
now sports two 32-bit digital signal processors to provide true
software- defined features to handle signal processing tasks and
operating modes. The operator has full control over any operating
situation, with 8-band receive and transmit equalization, stereo
speaker / sound-card outputs, binaural effects, and advanced noise
reduction. Also included are built-in digital modes, continuous
wave Morse code on screen display, and teletype decoding and
encoding, so the operator can enjoy the excitement of data
communications with or without a computer” (Elecraft 1).
This equipment is
primarily an audio and control situation. While the use of two DSP
chips might seem excessive, it does provide a responsive environment
for the operator. This particular 100 Watt radio transceiver might be
more accurately described as a software define radio with the ability
to be controlled from a computer or remotely via the internet. An
interesting new addition to the accessories for the radio now
includes a remote front panel that connects to the radio body
remotely whether a room away or a thousand miles away providing the
look and feel of being at the remote site.
While
communications systems gain a great deal from DSP in general, the
benefits to other areas are evident. The use of DSP in hearing aid
technology provides a refinement for those individuals with hearing
loss. To be able to fine tune the amplification process on short
notice and through a nearby wireless connection, is simply amazing.
Advances in the field provide multi-band processing and adaptive
technology to remove noise and unwanted frequencies from being passed
on the the ear. A secondary benefit of the system is an increase in
battery life to 165 hours.
Another area where
DSP is evident, is the music industry, “with systems to create
effects and
new sounds for the
guitar player as well as keyboard users” (Hunter 114). This new
inroad provides the
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artist with new
tools that were previously unavailable and packaged for portable use
The last mention
for DSP is the medical field. Without the ability to filter noise and
extraneous
signals, the body
scans now available would not provide the resolution and detail
needed for accurate diagnosis. This is a high dollar environment
where DSP could exist outside the chip size product and became
integrated into the overall system in a physical large way. And the
processing did not initially need to be instantaneous because the
doctor could wait for the scan to be rendered. With modern DSP, the
improved information allows early treatment options for better
survivability.
Wherever signals
are heard or seen, whether it is on a military radar display, the
homeowners 7.1 surround sound system, or the latest iPhone, DSP will
be in more future products.
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Works
Cited
"A
Brief History - DSP." ATT.com.
Bell Labs, 3 Mar. 2012. Web. 24 Apr. 2012.
Bishai,
S., and M. S. Metwally. "Digital Signal Processing."
Engineering
Journal of Qatar University
1 (1988): 1-2. Web. Apr. 2012.
Crawford, David. EEE
Strath. Epson UK, Crawford, David and R. W. Stewart. Web. 26 April
2012
Elecraft. "Elecraft
K3 Radio." (2012): 1-2. Print.
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