Preview and Introduction - Digital Communication: Principles and System Modelling - page 5

only, whereas the two tone (DSB-SC type) signal generates a pair of impulses. These
representations of math models are satisfying the definition of signal but can only
be considered as an example of a special class of signal, which is modeled as func-
tion of time or frequency as f(t) or F ( ). Let's consider now an image shown in the

following Fig. 1.4 . To represent the image as a mathematical function, we have to
first divide the entire image into a number of unit squares of homogeneous intensity
value to each square. These smallest picture elements are called as pixels. Intensity
or color of each smallest square block is now can be represented as function of ordi-
nate and abscissa, i.e., function of space where the entire image plane is considered
as a Cartesian co-ordinate system.

ω

Fig. 1.4 Image
representation as a function
of 2D space

Origin

x

f(x,y)

y

Similarly, video images can be represented functionally as a multivariate func-
tion, where image signal is represented as I(t,x,y), i.e., function of both time and
space.
As per the discussion above, now we can characterize a signal mathematically as
a function of independent variable/variables.
Now from the above three properties, we can define signal as, A physical quan-
tity used for carrying information from one entity to another and mathematically
modeled as a function of independent variable/ variables .
Our discussion would be mainly focused on the signals which can be modeled as
a function of time. In the next section, we'll discuss the time-value definition and
classification of signals. Hence, we'll clearly understand the time-value concept of
analog and digital signals. Also we can find the step-by-step procedure of conversion
of analog signal to digital signal.