2006 Fall Welcome to Signals and Systems xiexiaochun@gnnu.edu.cn

2006 Fall

Welcome toWelcome to

Signals and Systems

xiexiaochun@gnnu.edu.cn

2006 Fall

Grading PolicyGrading Policy

Homework – 10%Lab – 20%Midterm Exam – 20%Final Exam – 50%Project

2006 Fall

What will we learn?What will we learn?This course is about using mathematical

techniques to help analyze and synthesize systems which process signals.

We will learn an analytical framework:A language for describing signals and

systems;A set of tools for analyzing signals and

systemsProblems of signal and system analysis:Analyzing existing systems;Designing systems.

2006 Fall

Examples of signals Electrical signals --- voltages and currents in a

circuit Acoustic signals --- audio or speech signals

(analog or digital) Video signals --- intensity variations in an image

(e.g. a CAT scan) Biological signals --- sequence of bases in a gene Economical signals ---price of stocks

…

we will treat noise as unwanted signals

2006 Fall

Examples of systemsExamples of systems

Electrical systems --- amplifer circuitComputer systems --- mp3 playerControl systems --- automobile Economical systems --- stock market

2006 Fall

Signals and SystemsSignals and Systems

Signals : functions of one or more independent variables.

Systems : respond to particular signals by producing other signals or some desired behavior.

As functions, they should have have define domain and range.

2006 Fall

Signal ClassificationTypes of Independent Variable

Time is often the independent variable. Example: the electrical activity of the heart recorded with chest electrodes –– the electrocardiogram (ECG or EKG).

2006 Fall

The variables can also be spatial

Eg. Cervical MRI– In this example, the

signal is the intensity as a function of the spatial variables x and y.

2006 Fall

Independent Variable Dimensionality

An independent variable can be 1-D (t in the EKG) or 2-D (x, y in an image).

We focus on 1-D for mathematical simplicity but the results can be extended to 2-D or even higher dimensions. Also, we will use a generic time t for the independent variable,whether it is time or space.

2006 Fall

Continuous-time (CT) Signals and Discrete-time (DT) Signals

A continuous-time signal will contain a value for all real numbers along the time axis.

X(t)

A discrete-time signal will only have values at equally spaced intervals along the time axis.

X[n] Why DT ? x(t)---sampling--->x[n]

Can be processed by modern digital computers and digital signal processors (DSPs).

2006 Fall

Signal Energy and PowerSignal Energy and Power

Total Energy– Total energy over the time interval

– Total energy over an infinite time interval

2

1

2)(

t

tdttx

2

1

2][

n

n

nx

dttx

2)(

n

nx2

][

Time-Averaged Power （平均功率）– Average power over the time interval– Average power over an infinite time interval

Instantaneous power（瞬时功率）

2006 Fall

Signal Energy and PowerSignal Energy and Power

Energy signal

0 t

tx

0 t

tx

Power signal

0 t

tx

Neither energy,nor power signal

2006 Fall

Right- and Left-Sided Signals

A right-sided signal is zero for t < T and a left-sided signal is zero for t > T, where T can be positive or negative.

2006 Fall

Bounded and Unbounded Signals

Whether the output signal of a system is bounded or unbounded determines the stability of the system.

2006 Fall

Signal OperationSignal OperationShifting y(t)=x(t-t0)

– t0>0 (delay)– t0<0(advance)

Reflecting y(t)=x(-t)Time-Scaling y(t)=x(at)

– a>1(compress) – a<1(expand)

It is rarely use the time-scaling operation when dealing with discrete waveform.

2006 Fall

Signal OperationSignal Operation

)]([)(a

btafbatf

)]([)(a

btafbatf

ab

(1)Contract by a;(2) Shift right with – sign, shift left with + sign.

ab

ab

(1)Reflect and contract by a;(2) Shift right with – sign, shift left with + sign.

ab

2006 Fall

Signals with symmetryPeriodic signals

– CT x(t) = x(t + T)– DT x[n] = x[n + N]

If T1/T2=q/r, where q and r are integers, then there is T=rT1=qT2, which makes

Demo: sum of periodic signals

)()()()( 2121 txtxTtxTtx

2006 Fall

Signals with symmetry (continued)

Even and odd signals– Even x(t) = x(-t) or x[n] = x[-n]– Odd x(t) = -x(-t) or x[n] = - x[-n]

x(0) = 0 or x[0] = 0

Any signals can be expressed as a sum of Even and Odd signals. That is:

.2)]()([)(

,2)]()([)(

)()()(

txtxtx

txtxtx

where

txtxtx

odd

even

oddeven

2006 Fall

Summary What we have learned in this lecture?

– Examples and Classification of Signal– Signal operations – Signals with symmetry

What was the most important point in the lecture? What was the muddiest point? What would you like to hear more about?

2006 Fall

ReadlistReadlist

Signals and Systems – 1.3,1.4,2.5, Mathematical Review (P53)

Question:– Periodic of DT Signals– Euler Relation

2006 Fall

Problem SetProblem Set

1.21(a),(c)1.22(b),(d)1.23(a)1.24(b)