87
CHAPTER
5 Linear Systems
Most DSP techniques are based on a divide-and-conquer strategy called superposition. The
signal being processed is broken into simple components, each component is processed
individually, and the results reunited. This approach has the tremendous power of breaking a
single complicated problem into many easy ones. Superposition can only be used with linear
systems, a term meaning that certain mathematical rules apply. Fortunately, most of the
applications encountered in science and engineering fall into this category. This chapter presents
the foundation of DSP: what it means for a system to be linear, various ways for breaking signals
into simpler components, and how superposition provides a variety of signal processing
techniques.
Signals and Systems
A signal is a description of how one parameter varies with another parameter.
For instance, voltage changing over time in an electronic circuit, or brightness
varying with distance in an image. A system is any process that produces an
output signal in response to an input signal. This is illustrated by the block
diagram in Fig. 5-1. Continuous systems input and output continuous signals,
such as in analog electronics. Discrete systems input and output discrete
signals, such as computer programs that manipulate the values stored in arrays.
Several rules are used for naming signals. These aren't always followed in
DSP, but they are very common and you should memorize them. The
mathematics is difficult enough without a clear notation. First, continuous
signals use parentheses, such as: and , while discrete signals use x(t) y(t)
brackets, as in: and . Second, signals use lower case letters. Upper x[n] y[n]
case letters are reserved for the frequency domain, discussed in later chapters.
Third, the name given to a signal is usually descriptive of the parameters it
represents. For example, a voltage depending on time might be called: , or v(t)
a stock market price measured each day could be: . p[d]
The Scientist and Engineer's Guide to Digital Signal Processing 88
Continuous
System
Discrete
System
x(t) y(t)
x[n] y[n]
FIGURE 5-1
Terminology for signals and systems. A system is any process that generates an output signal in
response to an input signal. Continuous signals are usually represented with parentheses, while
discrete signals use brackets. All signals use lower case letters, reserving the upper case for the
frequency domain (presented in later chapters). Unless there is a better name available, the input
signal is called: x(t) or x[n], while the output is called: y(t) or y[n].
Signals and systems are frequently discussed without knowing the exact
parameters being represented. This is the same as using x and y in algebra,
without assigning a physical meaning to the variables. This brings in a fourth
rule for naming signals. If a more descriptive name is not available, the input
signal to a discrete system is usually called: , and the output signal: . x[n] y[n]
For continuous systems, the signals: and are used. x(t) y(t)
There are many reasons for wanting to understand a system. For example, you
may want to design a system to remove noise in an electrocardiogram, sharpen
an out-of-focus image, or remove echoes in an audio recording. In other cases,
the system might have a distortion or interfering effect that you need to
characterize or measure. For instance, when you speak into a telephone, you
expect the other person to hear something that resembles your voice.
Unfortunately, the input signal to a transmission line is seldom identical to the
output signal. If you understand how the transmission line (the system) is
changing the signal, maybe you can compensate for its effect. In still other
cases, the system may represent some physical process that you want to study
or analyze. Radar and sonar are good examples of this. These methods
operate by comparing the transmitted and reflected signals to find the
characteristics of a remote object. In terms of system theory, the problem is to
find the system that changes the transmitted signal into the received signal.
At first glance, it may seem an overwhelming task to understand all of the
possible systems in the world. Fortunately, most useful systems fall into a
category called linear systems. This fact is extremely important. Without the
linear system concept, we would be forced to examine the individual
Chapter 5- Linear Systems 89
System
System
x[n] y[n]
IF
THEN
k x[n] k y[n]
FIGURE 5-2
Definition of homogeneity. A system is said to be homogeneous if an amplitude change in
the input results in an identical amplitude change in the output. That is, if x[n] results in
y[n], then kx[n] results in ky[n], for any signal, x[n], and any constant, k.
characteristics of many unrelated systems. With this approach, we can focus
on the traits of the linear system category as a whole. Our first task is to
identify what properties make a system linear, and how they fit into the
everyday notion of electronics, software, and other signal processing systems.
Requirements for Linearity
A system is called linear if it has two mathematical properties: homogeneity
(hÇma-gen-~-ity) and additivity. If you can show that a system has both
properties, then you have proven that the system is linear. Likewise, if you can
show that a system doesn't have one or both properties, you have proven that
it isn't linear. A third property, shift invariance, is not a strict requirement
for linearity, but it is a mandatory property for most DSP techniques. When
you see the term linear system used in DSP, you should assume it includes shift
invariance unless you have reason to believe otherwise. These three properties
form the mathematics of how linear system theory is defined and used. Later
in this chapter we will look at more intuitive ways of understanding linearity.
For now, let's go through these formal mathematical properties.
As illustrated in Fig. 5-2, homogeneity means that a change in the input signal's
amplitude results in a corresponding change in the output signal's amplitude.
In mathematical terms, if an input signal of results in an output signal of x[n]
, an input of results in an output of , for any input signal and y[n] k x[n] k y[n]
constant, k.
The Scientist and Engineer's Guide to Digital Signal Processing 90
System
System
IF
THEN
System
AND IF
x1[n] y1[n]
x2[n] y2[n]
y1[n]+y2[n] x1[n]+x2[n]
FIGURE 5-3
Definition of additivity. A system is said to be additive if added signals pass through it
without interacting. Formally, if x1[n] results in y1[n], and if x2[n] results in y2[n], then
x1[n]+x2[n] results in y1[n]+y2[n].
A simple resistor provides a good example of both homogenous and nonhomogeneous
systems. If the input to the system is the voltage across the
resistor, , and the output from the system is the current through the resistor, v(t)
, the system is homogeneous. Ohm's law guarantees this; if the voltage is i(t)
increased or decreased, there will be a corresponding increase or decrease in
the current. Now, consider another system where the input signal is the voltage
across the resistor, , but the output signal is the power being dissipated in v(t)
the resistor, . Since power is proportional to the square of the voltage, if p(t)
the input signal is increased by a factor of two, the output signal is increase by
a factor of four. This system is not homogeneous and therefore cannot be
linear.
The property of additivity is illustrated in Fig. 5-3. Consider a system where
an input of produces an output of . Further suppose that a different x1[n] y1[n]
input, , produces another output, . The system is said to be additive, x2[n] y2[n]
if an input of results in an output of , for all possible x1[n] % x2[n] y1[n] % y2[n]
input signals. In words, signals added at the input produce signals that are
added at the output.
Chapter 5- Linear Systems 91
System
System
x[n] y[n]
x[n+s] y[n+s]
IF
THEN
FIGURE 5-4
Definition of shift invariance. A system is said to be shift invariant if a shift in the input
signal causes an identical shift in the output signal. In mathematical terms, if x[n]
produces y[n], then x[n+s] produces y[n+s], for any signal, x[n], and any constant, s.
The important point is that added signals pass through the system without
interacting. As an example, think about a telephone conversation with your
Aunt Edna and Uncle Bernie. Aunt Edna begins a rather lengthy story about
how well her radishes are doing this year. In the background, Uncle Bernie is
yelling at the dog for having an accident in his favorite chair. The two voice
signals are added and electronically transmitted through the telephone network.
Since this system is additive, the sound you hear is the sum of the two voices
as they would sound if transmitted individually. You hear Edna and Bernie,
not the creature, Ednabernie.
A good example of a nonadditive circuit is the mixer stage in a radio
transmitter. Two signals are present: an audio signal that contains the voice
or music, and a carrier wave that can propagate through space when applied
to an antenna. The two signals are added and applied to a nonlinearity,
such as a pn junction diode. This results in the signals merging to form a
third signal, a modulated radio wave capable of carrying the information
over great distances.
As shown in Fig. 5-4, shift invariance means that a shift in the input signal will
result in nothing more than an identical shift in the output signal. In more
formal terms, if an input signal of results in an output of , an input x[n] y [n]
signal of results in an output of , for any input signal and any x[n% s] y[n% s]
constant, s. Pay particular notice to how the mathematics of this shift is
written, it will be used in upcoming chapters. By adding a constant, s, to the
independent variable, n, the waveform can be advanced or retarded in the
horizontal direction. For example, when , the signal is shifted left by two s' 2
samples; when , the signal is shifted right by two samples. s' &2
The Scientist and Engineer's Guide to Digital Signal Processing 92
Shift invariance is important because it means the characteristics of the
system do not change with time (or whatever the independent variable
happens to be). If a blip in the input causes a blop in the output, you can
be assured that another blip will cause an identical blop. Most of the
systems you encounter will be shift invariant. This is fortunate, because it
is difficult to deal with systems that change their characteristics while in
operation. For example, imagine that you have designed a digital filter to
compensate for the degrading effects of a telephone transmission line. Your
filter makes the voices sound more natural and easier to understand. Much
to your surprise, along comes winter and you find the characteristics of the
telephone line have changed with temperature. Your compensation filter is
now mismatched and doesn't work especially well. This situation may
require a more sophisticated algorithm that can adapt to changing
conditions.
Why do homogeneity and additivity play a critical role in linearity, while shift
invariance is something on the side? This is because linearity is a very broad
concept, encompassing much more than just signals and systems. For example,
consider a farmer selling oranges for $2 per crate and apples for $5 per crate.
If the farmer sells only oranges, he will receive $20 for 10 crates, and $40 for
20 crates, making the exchange homogenous. If he sells 20 crates of oranges
and 10 crates of apples, the farmer will receive: . This 20×$2 % 10×$5 ' $90
is the same amount as if the two had been sold individually, making the
transaction additive. Being both homogenous and additive, this sale of goods
is a linear process. However, since there are no signals involved, this is not
a system, and shift invariance has no meaning. Shift invariance can be thought
of as an additional aspect of linearity needed when signals and systems are
involved.
Static Linearity and Sinusoidal Fidelity
Homogeneity, additivity, and shift invariance are important because they
provide the mathematical basis for defining linear systems. Unfortunately,
these properties alone don't provide most scientists and engineers with an
intuitive feeling of what linear systems are about. The properties of static
linearity and sinusoidal fidelity are often of help here. These are not
especially important from a mathematical standpoint, but relate to how humans
think about and understand linear systems. You should pay special attention
to this section.
Static linearity defines how a linear system reacts when the signals aren't
changing, i.e., when they are DC or static. The static response of a linear
system is very simple: the output is the input multiplied by a constant. That
is, a graph of the possible input values plotted against the corresponding
output values is a straight line that passes through the origin. This is shown
in Fig. 5-5 for two common linear systems: Ohm's law for resistors, and
Hooke's law for springs. For comparison, Fig. 5-6 shows the static
relationship for two nonlinear systems: a pn junction diode, and the
magnetic properties of iron.
Chapter 5- Linear Systems 93
Voltage
low
resistance
high
resistance
a. Ohm's law
Force
weak
spring
strong
spring
b. Hooke's law
FIGURE 5-5
Two examples of static linearity. In (a), Ohm's law: the current through a resistor is equal to the
voltage across the resistor divided by the resistance. In (b), Hooke's law: The elongation of a spring
is equal to the applied force multiplied by the spring stiffness coefficient.
Current
Elongation
Voltage
a. Silicon diode
0.6 v
b. Iron
H
FIGURE 5-6
Two examples of DC nonlinearity. In (a), a silicon diode has an exponential relationship between
voltage and current. In (b), the relationship between magnetic intensity, H, and flux density, B, in
iron depends on the history of the sample, a behavior called hysteresis.
Current
B
All linear systems have the property of static linearity. The opposite is
usually true, but not always. There are systems that show static linearity,
but are not linear with respect to changing signals. However, a very
common class of systems can be completely understood with static linearity
alone. In these systems it doesn't matter if the input signal is static or
changing. These are called memoryless systems, because the output
depends only on the present state of the input, and not on its history. For
example, the instantaneous current in a resistor depends only on the
instantaneous voltage across it, and not on how the signals came to be the
value they are. If a system has static linearity, and is memoryless, then the
system must be linear. This provides an important way to understand (and
prove) the linearity of these simple systems.
The Scientist and Engineer's Guide to Digital Signal Processing 94
An important characteristic of linear systems is how they behave with
sinusoids, a property we will call sinusoidal fidelity: If the input to a
linear system is a sinusoidal wave, the output will also be a sinusoidal
wave, and at exactly the same frequency as the input. Sinusoids are the
only waveform that have this property. For instance, there is no reason to
expect that a square wave entering a linear system will produce a square
wave on the output. Although a sinusoid on the input guarantees a sinusoid
on the output, the two may be different in amplitude and phase. This
should be familiar from your knowledge of electronics: a circuit can be
described by its frequency response, graphs of how the circuit's gain and
phase vary with frequency.
Now for the reverse question: If a system always produces a sinusoidal output
in response to a sinusoidal input, is the system guaranteed to be linear? The
answer is no, but the exceptions are rare and usually obvious. For example,
imagine an evil demon hiding inside a system, with the goal of trying to
mislead you. The demon has an oscilloscope to observe the input signal, and
a sine wave generator to produce an output signal. When you feed a sine
wave into the input, the demon quickly measures the frequency and adjusts his
signal generator to produce a corresponding output. Of course, this system is
not linear, because it is not additive. To show this, place the sum of two sine
waves into the system. The demon can only respond with a single sine wave
for the output. This example is not as contrived as you might think; phase lock
loops operate in much this way.
To get a better feeling for linearity, think about a technician trying to determine
if an electronic device is linear. The technician would attach a sine wave
generator to the input of the device, and an oscilloscope to the output. With a
sine wave input, the technician would look to see if the output is also a sine
wave. For example, the output cannot be clipped on the top or bottom, the top
half cannot look different from the bottom half, there must be no distortion
where the signal crosses zero, etc. Next, the technician would vary the
amplitude of the input and observe the effect on the output signal. If the system
is linear, the amplitude of the output must track the amplitude of the input.
Lastly, the technician would vary the input signal's frequency, and verify that
the output signal's frequency changes accordingly. As the frequency is
changed, there will likely be amplitude and phase changes seen in the output,
but these are perfectly permissible in a linear system. At some frequencies, the
output may even be zero, that is, a sinusoid with zero amplitude. If the
technician sees all these things, he will conclude that the system is linear.
While this conclusion is not a rigorous mathematical proof, the level of
confidence is justifiably high.
Examples of Linear and Nonlinear Systems
Table 5-1 provides examples of common linear and nonlinear systems. As you
go through the lists, keep in mind the mathematician's view of linearity
(homogeneity, additivity, and shift invariance), as well as the informal way
most scientists and engineers use (static linearity and sinusoidal fidelity).
Chapter 5- Linear Systems 95
Table 5-1
Examples of linear and nonlinear systems. Formally, linear systems are defined by the properties
of homogeneity, additivity, and shift invariance. Informally, most scientists and engineers think
of linear systems in terms of static linearity and sinusoidal fidelity.
Examples of Linear Systems
Wave propagation such as sound and electromagnetic waves
Electrical circuits composed of resistors, capacitors, and inductors
Electronic circuits, such as amplifiers and filters
Mechanical motion from the interaction of masses, springs, and dashpots (dampeners)
Systems described by differential equations such as resistor-capacitor-inductor
networks
Multiplication by a constant, that is, amplification or attenuation of the signal
Signal changes, such as echoes, resonances, and image blurring
The unity system where the output is always equal to the input
The null system where the output is always equal to the zero, regardless of the input
Differentiation and integration, and the analogous operations of first difference and
running sum for discrete signals
Small perturbations in an otherwise nonlinear system, for instance, a small signal being
amplified by a properly biased transistor
Convolution, a mathematical operation where each value in the output is expressed as the
sum of values in the input multiplied by a set of weighing coefficients.
Recursion, a technique similar to convolution, except previously calculated values in the
output are used in addition to values from the input
Examples of Nonlinear Systems
Systems that do not have static linearity, for instance, the voltage and power in a
resistor: , the radiant energy emission of a hot object depending on its temperature: P' V2R
, the intensity of light transmitted through a thickness of translucent material: R ' kT 4
, etc. I ' e &"T
Systems that do not have sinusoidal fidelity, such as electronics circuits for: peak
detection, squaring, sine wave to square wave conversion, frequency doubling, etc.
Common electronic distortion, such as clipping, crossover distortion and slewing
Multiplication of one signal by another signal, such as in amplitude modulation and
automatic gain controls
Hysteresis phenomena, such as magnetic flux density versus magnetic intensity in iron,
or mechanical stress versus strain in vulcanized rubber
Saturation, such as electronic amplifiers and transformers driven too hard
Systems with a threshold, for example, digital logic gates, or seismic vibrations that are
strong enough to pulverize the intervening rock
The Scientist and Engineer's Guide to Digital Signal Processing 96
x[n] y[n]
IF
THEN
x[n] y[n]
System System
A B
System System
B A
FIGURE 5-7
The commutative property for linear
systems. When two or more linear systems
are arranged in a cascade, the order of the
systems does not affect the characteristics
of the overall combination.
Special Properties of Linearity
Linearity is commutative, a property involving the combination of two or
more systems. Figure 5-10 shows the general idea. Imagine two systems
combined in a cascade, that is, the output of one system is the input to the
next. If each system is linear, then the overall combination will also be linear.
The commutative property states that the order of the systems in the cascade
can be rearranged without affecting the characteristics of the overall
combination. You probably have used this principle in electronic circuits. For
example, imagine a circuit composed of two stages, one for amplification, and
one for filtering. Which is best, amplify and then filter, or filter and then
amplify? If both stages are linear, the order doesn't make any difference and
the overall result is the same. Keep in mind that actual electronics has
nonlinear effects that may make the order important, for instance: interference,
DC offsets, internal noise, slew rate distortion, etc.
Figure 5-8 shows the next step in linear system theory: multiple inputs and
outputs. A system with multiple inputs and/or outputs will be linear if it is
composed of linear subsystems and additions of signals. The complexity does
not matter, only that nothing nonlinear is allowed inside of the system.
To understand what linearity means for systems with multiple inputs and/or
outputs, consider the following thought experiment. Start by placing a signal
on one input while the other inputs are held at zero. This will cause the
multiple outputs to respond with some pattern of signals. Next, repeat the
procedure by placing another signal on a different input. Just as before, keep
all of the other inputs at zero. This second input signal will result in another
pattern of signals appearing on the multiple outputs. To finish the experiment,
place both signals on their respective inputs simultaneously. The signals
appearing on the outputs will simply be the superposition (sum) of the output
signals produced when the input signals were applied separately.
Chapter 5- Linear Systems 97
FIGURE 5-8
Any system with multiple inputs and/or
outputs will be linear if it is composed
of linear systems and signal additions.
System System
A B
System System
System
C
D E
x1[n]
x2[n]
x3[n]
y1[n]
y2[n]
y3[n]
Linear
constant
x[n]
y[n]
x1[n]
x2[n]
y[n]
a. Multiplication by a constant
Nonlinear
b. Multiplication of two signals
FIGURE 5-9
Linearity of multiplication. Multiplying a signal by a constant is a linear operation. In
contrast, the multiplication of two signals is nonlinear.
The use of multiplication in linear systems is frequently misunderstood. This
is because multiplication can be either linear or nonlinear, depending on what
the signal is multiplied by. Figure 5-9 illustrates the two cases. A system that
multiplies the input signal by a constant, is linear. This system is an amplifier
or an attenuator, depending if the constant is greater or less than one,
respectively. In contrast, multiplying a signal by another signal is nonlinear.
Imagine a sinusoid multiplied by another sinusoid of a different frequency; the
resulting waveform is clearly not sinusoidal.
Another commonly misunderstood situation relates to parasitic signals added
in electronics, such as DC offsets and thermal noise. Is the addition of these
extraneous signals linear or nonlinear? The answer depends on where the
contaminating signals are viewed as originating. If they are viewed as coming
from within the system, the process is nonlinear. This is because a sinusoidal
input does not produce a pure sinusoidal output. Conversely, the extraneous
signal can be viewed as externally entering the system on a separate input of
a multiple input system. This makes the process linear, since only a signal
addition is required within the system.
The Scientist and Engineer's Guide to Digital Signal Processing 98
decomposition
synthesis
x[n]
x0[n]
x1[n]
x2[n]
FIGURE 5-10
Illustration of synthesis and decomposition of
signals. In synthesis, two or more signals are
added to form another signal. Decomposition is
the opposite process, breaking one signal into
two or more additive component signals.
Superposition: the Foundation of DSP
When we are dealing with linear systems, the only way signals can be
combined is by scaling (multiplication of the signals by constants) followed by
addition. For instance, a signal cannot be multiplied by another signal. Figure
5-10 shows an example: three signals: , , and are added to form x0[n] x1[n] x2[n]
a fourth signal, . This process of combining signals through scaling and x [n]
addition is called synthesis.
Decomposition is the inverse operation of synthesis, where a single signal is
broken into two or more additive components. This is more involved than
synthesis, because there are infinite possible decompositions for any given
signal. For example, the numbers 15 and 25 can only be synthesized (added)
into the number 40. In comparison, the number 40 can be decomposed into:
or or , etc. 1% 39 2% 38 &30.5% 60% 10.5
Now we come to the heart of DSP: superposition, the overall strategy for
understanding how signals and systems can be analyzed. Consider an input
Chapter 5- Linear Systems 99
System
x[n]
x0[n]
x1[n]
x2[n]
y0[n]
y1[n]
y2[n]
y[n]
System
System
The Fundamental
Concept of DSP
decomposition
synthesis
FIGURE 5-11
The fundamental concept in DSP. Any
signal, such as , can be decomposed into x [n]
a group of additive components, shown here
by the signals: and . Passing x1[n], x2[n], x3[n]
these components through a linear system
produces the signals, and . y1[n], y2[n], y3[n]
The synthesis (addition) of these output
signals forms , the same signal produced y [n]
when is passed through the system. x [n]
signal, called , passing through a linear system, resulting in an output x[n]
signal, . As illustrated in Fig. 5-11, the input signal can be decomposed y[n]
into a group of simpler signals: , , , etc. We will call these the x0[n] x1[n] x2[n]
input signal components. Next, each input signal component is individually
passed through the system, resulting in a set of output signal components:
, , , etc. These output signal components are then synthesized y0[n] y1[n] y2[n]
into the output signal, . y[n]
The Scientist and Engineer's Guide to Digital Signal Processing 100
Here is the important part: the output signal obtained by this method is
identical to the one produced by directly passing the input signal through the
system. This is a very powerful idea. Instead of trying to understanding how
complicated signals are changed by a system, all we need to know is how
simple signals are modified. In the jargon of signal processing, the input and
output signals are viewed as a superposition (sum) of simpler waveforms. This
is the basis of nearly all signal processing techniques.
As a simple example of how superposition is used, multiply the number 2041
by the number 4, in your head. How did you do it? You might have imagined
2041 match sticks floating in your mind, quadrupled the mental image, and
started counting. Much more likely, you used superposition to simplify the
problem. The number 2041 can be decomposed into: . Each of 2000 % 40 % 1
these components can be multiplied by 4 and then synthesized to find the final
answer, i.e., . 8000 % 160 % 4 ' 8164
Common Decompositions
Keep in mind that the goal of this method is to replace a complicated problem
with several easy ones. If the decomposition doesn't simplify the situation in
some way, then nothing has been gained. There are two main ways to
decompose signals in signal processing: impulse decomposition and Fourier
decomposition. They are described in detail in the next several chapters. In
addition, several minor decompositions are occasionally used. Here are brief
descriptions of the two major decompositions, along with three of the minor
ones.
Impulse Decomposition
As shown in Fig. 5-12, impulse decomposition breaks an N samples signal into
N component signals, each containing N samples. Each of the component
signals contains one point from the original signal, with the remainder of the
values being zero. A single nonzero point in a string of zeros is called an
impulse. Impulse decomposition is important because it allows signals to be
examined one sample at a time. Similarly, systems are characterized by how
they respond to impulses. By knowing how a system responds to an impulse,
the system's output can be calculated for any given input. This approach is
called convolution, and is the topic of the next two chapters.
Step Decomposition
Step decomposition, shown in Fig. 5-13, also breaks an N sample signal into
N component signals, each composed of N samples. Each component signal
is a step, that is, the first samples have a value of zero, while the last samples
are some constant value. Consider the decomposition of an N point signal,
, into the components: . The component x[n] x0[n], x1[n], x2[n], þxN&1[n] k th
signal, , is composed of zeros for points 0 through , while the xk[n] k&1
remaining points have a value of: . For example, the x[k] & x[k&1] 5 th
component signal, , is composed of zeros for points 0 through 4, while x5[n]
the remaining samples have a value of: (the difference between x[5] & x[4]
Chapter 5- Linear Systems 101
Impulse
Decomposition
Step
Decomposition
x[n]
x0[n]
x1[n]
x2[n]
x27[n]
x[n]
x0[n]
x1[n]
x2[n]
x27[n]
FIGURE 5-12
Example of impulse decomposition. An N
point signal is broken into N components,
each consisting of a single nonzero point.
FIGURE 5-13
Example of step decomposition. An N point
signal is broken into N signals, each consisting
of a step function
sample 4 and 5 of the original signal). As a special case, has all of its x0[n]
samples equal to . Just as impulse decomposition looks at signals one point x[0]
at a time, step decomposition characterizes signals by the difference between
adjacent samples. Likewise, systems are characterized by how they respond to
a change in the input signal.
The Scientist and Engineer's Guide to Digital Signal Processing 102
xE [n ] ' x [n] % x [N&n]
2
xO [n ] ' x [n] & x [N&n]
2
EQUATION 5-1
Equations for even/odd decomposition.
These equations separate a signal, , x [n]
into its even part, , and its odd part, xE [n]
. Since this decomposition is based xO [n]
on circularly symmetry, the zeroth
samples in the even and odd signals are
calculated: , and . xE [0] ' x [0] xO [0] ' 0
All of the signals are N samples long,
with indexes running from 0 to N-1
Even/Odd Decomposition
The even/odd decomposition, shown in Fig. 5-14, breaks a signal into two
component signals, one having even symmetry and the other having odd
symmetry. An N point signal is said to have even symmetry if it is a mirror
image around point . That is, sample must equal , N/2 x[N/2% 1] x[N/2& 1]
sample must equal , etc. Similarly, odd symmetry occurs x[N/2% 2] x[N/2& 2]
when the matching points have equal magnitudes but are opposite in sign, such
as: , , etc. These definitions x[N/2% 1] ' &x[N/2&1] x[N/2% 2] ' &x[N/2&2]
assume that the signal is composed of an even number of samples, and that the
indexes run from 0 to . The decomposition is calculated from the N&1
relations:
This may seem a strange definition of left-right symmetry, since N/2& ½
(between two samples) is the exact center of the signal, not . Likewise, N/2
this off-center symmetry means that sample zero needs special handling.
What's this all about?
This decomposition is part of an important concept in DSP called circular
symmetry. It is based on viewing the end of the signal as connected to the
beginning of the signal. Just as point is next to point , point x[4] x[5] x[N&1]
is next to point . Picture a snake biting its own tail. When even and odd x[0]
signals are viewed in this circular manner, there are actually two lines of
symmetry, one at point and another at point . For example, in an x[N/2] x[0]
even signal, this symmetry around means that point equals point x[0] x[1]
, point equals point , etc. In an odd signal, point 0 and x[N&1] x[2] x[N&2]
point always have a value of zero. In an even signal, point 0 and point N/2
are equal to the corresponding points in the original signal. N/2
What is the motivation for viewing the last sample in a signal as being next to
the first sample? There is nothing in conventional data acquisition to support
this circular notion. In fact, the first and last samples generally have less in
common than any other two points in the sequence. It's common sense! The
missing piece to this puzzle is a DSP technique called Fourier analysis. The
mathematics of Fourier analysis inherently views the signal as being circular,
although it usually has no physical meaning in terms of where the data came
from. We will look at this in more detail in Chapter 10. For now, the
important thing to understand is that Eq. 5-1 provides a valid decomposition,
simply because the even and odd parts can be added together to reconstruct the
original signal.
Chapter 5- Linear Systems 103
Even/Odd
Decomposition
Interlaced
Decomposition
x[n]
xE[n]
xO[n]
x[n]
xE[n]
xO[n]
0 N/2 N
even symmetry
odd symmetry
even samples
odd samples
FIGURE 5-14
Example of even/odd decomposition. An N
point signal is broken into two N point signals,
one with even symmetry, and the other with
odd symmetry.
FIGURE 5-15
Example of interlaced decomposition. An N
point signal is broken into two N point signals,
one with the odd samples set to zero, the other
with the even samples set to zero.
Interlaced Decomposition
As shown in Fig. 5-15, the interlaced decomposition breaks the signal into two
component signals, the even sample signal and the odd sample signal (not to
be confused with even and odd symmetry signals). To find the even sample
signal, start with the original signal and set all of the odd numbered samples
to zero. To find the odd sample signal, start with the original signal and set all
of the even numbered samples to zero. It's that simple.
At first glance, this decomposition might seem trivial and uninteresting. This
is ironic, because the interlaced decomposition is the basis for an extremely
important algorithm in DSP, the Fast Fourier Transform (FFT). The procedure
for calculating the Fourier decomposition has been known for several hundred
years. Unfortunately, it is frustratingly slow, often requiring minutes or hours
to execute on present day computers. The FFT is a family of algorithms
developed in the 1960s to reduce this computation time. The strategy is an
exquisite example of DSP: reduce the signal to elementary components by
repeated use of the interlace transform; calculate the Fourier decomposition of
the individual components; synthesize the results into the final answer. The
The Scientist and Engineer's Guide to Digital Signal Processing 104
results are dramatic; it is common for the speed to be improved by a factor of
hundreds or thousands.
Fourier Decomposition
Fourier decomposition is very mathematical and not at all obvious. Figure
5-16 shows an example of the technique. Any N point signal can be
decomposed into signals, half of them sine waves and half of them N%2
cosine waves. The lowest frequency cosine wave (called in this xC0 [n]
illustration), makes zero complete cycles over the N samples, i.e., it is a DC
signal. The next cosine components: , , and , make 1, 2, xC1 [n] xC2 [n] xC3 [n]
and 3 complete cycles over the N samples, respectively. This pattern holds
for the remainder of the cosine waves, as well as for the sine wave
components. Since the frequency of each component is fixed, the only
thing that changes for different signals being decomposed is the amplitude
of each of the sine and cosine waves.
Fourier decomposition is important for three reasons. First, a wide variety
of signals are inherently created from superimposed sinusoids. Audio
signals are a good example of this. Fourier decomposition provides a direct
analysis of the information contained in these types of signals. Second,
linear systems respond to sinusoids in a unique way: a sinusoidal input
always results in a sinusoidal output. In this approach, systems are
characterized by how they change the amplitude and phase of sinusoids
passing through them. Since an input signal can be decomposed into
sinusoids, knowing how a system will react to sinusoids allows the output
of the system to be found. Third, the Fourier decomposition is the basis for
a broad and powerful area of mathematics called Fourier analysis, and the
even more advanced Laplace and z-transforms. Most cutting-edge DSP
algorithms are based on some aspect of these techniques.
Why is it even possible to decompose an arbitrary signal into sine and cosine
waves? How are the amplitudes of these sinusoids determined for a particular
signal? What kinds of systems can be designed with this technique? These are
the questions to be answered in later chapters. The details of the Fourier
decomposition are too involved to be presented in this brief overview. For
now, the important idea to understand is that when all of the component
sinusoids are added together, the original signal is exactly reconstructed. Much
more on this in Chapter 8.
Alternatives to Linearity
To appreciate the importance of linear systems, consider that there is only one
major strategy for analyzing systems that are nonlinear. That strategy is to
make the nonlinear system resemble a linear system. There are three common
ways of doing this:
First, ignore the nonlinearity. If the nonlinearity is small enough, the system
can be approximated as linear. Errors resulting from the original assumption
are tolerated as noise or simply ignored.
Chapter 5- Linear Systems 105
FIGURE 5-16
Illustration of Fourier decomposition. An N point signal is decomposed into N+2 signals, each
having N points. Half of these signals are cosine waves, and half are sine waves. The frequencies
of the sinusoids are fixed; only the amplitudes can change.
Fourier
Decomposition
x[n]
xC1[n]
xC2[n]
xC8[n]
xS1[n]
xS2[n]
xS8[n]
xC0[n] xS0[n]
xC3[n] xS3[n]
cosine waves sine waves
The Scientist and Engineer's Guide to Digital Signal Processing 106
Second, keep the signals very small. Many nonlinear systems appear linear if
the signals have a very small amplitude. For instance, transistors are very
nonlinear over their full range of operation, but provide accurate linear
amplification when the signals are kept under a few millivolts. Operational
amplifiers take this idea to the extreme. By using very high open-loop gain
together with negative feedback, the input signal to the op amp (i.e., the
difference between the inverting and noninverting inputs) is kept to only a few
microvolts. This minuscule input signal results in excellent linearity from an
otherwise ghastly nonlinear circuit.
Third, apply a linearizing transform. For example, consider two signals being
multiplied to make a third: . Taking the logarithm of the a[n] ' b[n] × c[n]
signals changes the nonlinear process of multiplication into the linear process
of addition: . The fancy name for this log(a[n]) ' log(b[n]) % log(c[n] )
approach is homomorphic signal processing. For example, a visual image can
be modeled as the reflectivity of the scene (a two-dimensional signal) being
multiplied by the ambient illumination (another two-dimensional signal).
Homomorphic techniques enable the illumination signal to be made more
uniform, thereby improving the image.
In the next chapters we examine the two main techniques of signal processing:
convolution and Fourier analysis. Both are based on the strategy presented in
this chapter: (1) decompose signals into simple additive components, (2)
process the components in some useful manner, and (3) synthesize the
components into a final result. This is DSP.