Data stored in a computer in the form of 0s and 1s. To be carried from one place to another
(inside or outside the computer), data are usually converted to digital
signals. This is called digital signal
conversion or encoding digital data into a digital signal.
Digital to
Digital Conversion- Digital to
Digital encoding or conversion is representation of digital information by the
digital signal. For Example, when you
transmit data from your computer to your printer, both the original data and he
transmitted data are digital. In this type of encoding, the binary ‘1s’ and
‘0s’ generated by a computer are translated in to a sequence are voltage pulses
that can be propagated over a wire.
Of the many mechanism for Digital to
Digital encoding, we will discuss only those most useful for data
communication. These fall in to three
broad categories Unipolar, Polar and By Polar.
Unipolar- Digital transmission system work by sending voltage pulses
along a medium link usually a wire or cable in most type of encoding one
voltage level stands for binary ‘0’ and another level stands for binary
‘1’. The polarity of a pulses refers to
whether it is positive of negative.
Unipolar encoding is so named because it uses only one polarity. This polarity is assigned to one of the two
binary states, usually the 1. The other
state, usually the ‘0’ is represented by ‘0’ voltage.
DC
Component- The average amplitude of a Unipolar
encoded signal is nonzero. This creates
what is called a direct current (DC) component (A component with zero frequency)
when signal contains a DC component. It
cannot travel through media that cannot handle DC components.
Polar- Polar encoding uses who voltage levels, one positive and one
negative by using both levels, in most popular encoding methods the average
voltage level on the line is reduced and the DC component problem of unipolar
encoding is alleviated.
Polar encoding uses two levels (Positive and negative)
of amplitude.
Non Return
to Zero- The NRZ encoding the level of the
signal is always either of positive or negative. The two most popular methods of NRZ
transmission are discussed blow.
NRZ-L- An NRZ-L encoding the level of the signal depend on than type
of bit it represents. A positive voltage
usually mean s the bit is a “0”. And a
negative voltage means the bi is a 1 (or vice versa); thus the level of the
signal is dependent upon the state of the bit.
In NRZ-L the level of the signal is dependent upon the
state of the bit.
NRZ-I- In NRZ-I an inversion of the voltage
level represents a 1 bits, It is the
transition between a positive and negative voltage not the voltage themselves
that represent a 1 bit is encountered.
The existence of 1s in the data stream allows the receivers to resynchronize
its timer to actual arrival of the transmission. A string of ‘0s’ can still cause
problems. But because ‘0s’ are not as
likely, they are less of a problem.
In NRZ-I the signal is inverted if a 1 is encountered.
Bipolar- The ‘1s’ are represented by
alternating positive and negative voltages.
If the first ‘1’ bit is
represented by the negative amplitude, the third by the positive amplitude and
so on.
In bipolar encoding we use three levels positive zero
and negative.
Bit Rate and Baud Rate-
Two terms use
frequently to data communication are bit rate and baud rate. “Bits rate is
number of bits transmitted during one second.
Baud rate is
refer to the number of signal units per second Baud rate is less than or equal
to the bit rate.
Example-
An analog signal carries four bits in each signal element. If 1000 signal elements are sent per second
find the baud rate and the bi rate.
Baud Rate= Number of
signal element x 1000 bits per seconds
Bit Rate= Baud Rate X Number of bits per signal
element
1000 X 4 =4bps
Example-
The bit rate of a signal is 3000.
If each signal element carries 6 bits what is the baud rate.
Baud Rate=Bit Rate X Number of bits per signal element
3000 X 6 =500 Baud per secound
