1. signal and systems basics

A Presentation on Basics of
Signal & Systems
By:-
Sunil Kumar Yadav (2013UEE1176)
Pawan Kumar Jangid (2013UEE1166)
Manish Kumar Bagara (2013UEE1180)
Chandan Kumar (2013UEE1203)
Subject:- Network, Signal & Systems

What is a Signal?
• A signal is a pattern of variation of some form
• Signals are variables that carry information
Examples of signal include:
Electrical signals
– Voltages and currents in a circuit
Acoustic signals
– Acoustic pressure (sound) over time
Mechanical signals
– Velocity of a car over time
Video signals
– Intensity level of a pixel (camera, video) over time

How is a Signal Represented?
Mathematically, signals are represented as a function of
one or more independent variables.
For instance a black & white video signal intensity is
dependent on x, y coordinates and time t f(x,y,t)
On this course, we shall be exclusively concerned with
signals that are a function of a single variable: time
t
f(t)

Example: Signals in an Electrical Circuit
The signals vc and vs are patterns of variation over time
Note, we could also have considered the voltage across the resistor or
the current as signals
+
-
i vcvs
R
C
)(
1
)(
1)(
)(
)(
)()(
)(
tv
RC
tv
RCdt
tdv
dt
tdv
Cti
R
tvtv
ti
sc
c
c
cs
=+
=
−
=
• Step (signal) vs at t=1
• RC = 1
• First order (exponential)
response for vc
vs,vc
t

Continuous & Discrete-Time Signals
Continuous-Time Signals
Most signals in the real world are
continuous time, as the scale is
infinitesimally fine.
Eg voltage, velocity,
Denote by x(t), where the time
interval may be bounded (finite) or
infinite
Discrete-Time Signals
Some real world and many digital
signals are discrete time, as they
are sampled
E.g. pixels, daily stock price
(anything that a digital computer
processes)
Denote by x[n], where n is an integer
value that varies discretely
Sampled continuous signal
x[n] =x(nk) – k is sample time
x(t)
t
x[n]
n

Signal Properties
On this course, we shall be particularly interested in signals with
certain properties:
Periodic signals: a signal is periodic if it repeats itself after a fixed
period T, i.e. x(t) = x(t+T) for all t. A sin(t) signal is periodic.
Even and odd signals: a signal is even if x(-t) = x(t) (i.e. it can be
reflected in the axis at zero). A signal is odd if x(-t) = -x(t).
Examples are cos(t) and sin(t) signals, respectively.
Exponential and sinusoidal signals: a signal is (real) exponential if it
can be represented as x(t) = Ceat
. A signal is (complex) exponential
if it can be represented in the same form but C and a are complex
numbers.
Step and pulse signals: A pulse signal is one which is nearly
completely zero, apart from a short spike, δ(t). A step signal is zero
up to a certain time, and then a constant value after that time, u(t).
These properties define a large class of tractable, useful signals and
will be further considered in the coming lectures

What is a System?
• Systems process input signals to produce output
signals
Examples:
– A circuit involving a capacitor can be viewed as a
system that transforms the source voltage (signal) to
the voltage (signal) across the capacitor
– A CD player takes the signal on the CD and transforms
it into a signal sent to the loud speaker
– A communication system is generally composed of
three sub-systems, the transmitter, the channel and the
receiver. The channel typically attenuates and adds
noise to the transmitted signal which must be
processed by the receiver

How is a System Represented?
A system takes a signal as an input and transforms it
into another signal
In a very broad sense, a system can be represented as
the ratio of the output signal over the input signal
That way, when we “multiply” the system by the input
signal, we get the output signal
This concept will be firmed up in the coming weeks
System
Input signal
x(t)
Output signal
y(t)

Example: An Electrical Circuit System
Simulink representation of the electrical circuit
+
-
i vcvs
R
C
)(
1
)(
1)(
)(
)(
)()(
)(
tv
RC
tv
RCdt
tdv
dt
tdv
Cti
R
tvtv
ti
sc
c
c
cs
=+
=
−
=
vs(t) vc(t)
first order
system
vs,vc
t

Continuous & Discrete-Time
Mathematical Models of Systems
Continuous-Time Systems
Most continuous time systems
represent how continuous
signals are transformed via
differential equations.
E.g. circuit, car velocity
Discrete-Time Systems
Most discrete time systems
represent how discrete signals
are transformed via difference
equations
E.g. bank account, discrete car
velocity system
)(
1
)(
1)(
tv
RC
tv
RCdt
tdv
sc
c
=+
)()(
)(
tftv
dt
tdv
m =+ ρ
First order differential equations
][]1[01.1][ nxnyny +−=
][]1[][ nf
m
nv
m
m
nv
∆+
∆
=−
∆+
−
ρρ
First order difference equations
∆
∆−−∆
=
∆ ))1(()()( nvnv
dt
ndv

Properties of a System
On this course, we shall be particularly interested in
signals with certain properties:
• Causal: a system is causal if the output at a time, only
depends on input values up to that time.
• Linear: a system is linear if the output of the scaled
sum of two input signals is the equivalent scaled sum of
outputs
• Time-invariance: a system is time invariant if the
system’s output is the same, given the same input
signal, regardless of time.
These properties define a large class of tractable, useful
systems and will be further considered in the coming
lectures

1. signal and systems basics

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