See Oppenheim and Schafer, Second Edition pages 8–93, or First Edition pages
8–79. 1 Discrete-time signals. A discrete-time signal is represented as a
sequence of numbers: .... so the real and imaginary parts are exponentially
weighted sinusoids. When j˛j D 1 the ..... conditions, and a unique solution can be
obtained.
Discrete-time signals and systems
x[−3] x[−2] x[−4] x[−1]
See Oppenheim and Schafer, Second Edition pages 8–93, or First Edition pages 8–79.
x[4]
x[0] 1
... −4
−3
−2
... 2
3
−1 0
4 x[1]
1 Discrete-time signals
n
x[3] x[2]
A discrete-time signal is represented as a sequence of numbers: x D fxŒng;
(This can be plotted using the MATLAB function stem.) The value xŒn is undefined for noninteger values of n.
1 < n < 1:
Here n is an integer, and xŒn is the nth sample in the sequence. Discrete-time signals are often obtained by sampling continuous-time signals. In this case the nth sample of the sequence is equal to the value of the analogue signal xa .t / at time t D nT : xŒn D xa .nT /;
Sequences can be manipulated in several ways. The sum and product of two sequences xŒn and yŒn are defined as the sample-by-sample sum and product respectively. Multiplication of xŒn by a is defined as the multiplication of each sample value by a. A sequence yŒn is a delayed or shifted version of xŒn if
1 < n < 1:
yŒn D xŒn
The sampling period is then equal to T , and the sampling frequency is fs D 1=T .
with n0 an integer. The unit sample sequence
xa .1T / ...
n0 ;
1 ... t
x[1]
is defined as
For this reason, although xŒn is strictly the nth number in the sequence, we often refer to it as the nth sample. We also often refer to “the sequence xŒn” when we mean the entire sequence. Discrete-time signals are often depicted graphically as follows:
1
n
0
ıŒn D
8 1 the sequence grows in magnitude as n increases. n
