A signal or a base-band signal
is the electrical waveform representing the
message which appears at the output of a transducer
and becomes input to a communication system.
Continents or Analog, and
Discrete or Digital
Continuous or analog signal v{t) is
obtained when the electrical waveform is a replica or
analogous to the input waveform. It can be represented
or specified in relation to two relatively single quantities:
The range of values within which v(t) occurs, i.e.,
difference between its maximum and minimum values.
The range can readily be changed using an amplifier
or an alternator; and
The time and frequency relation of the waveform,
i.e., how rapidly the signal changes with time. In
fact, the behavior of a communication system is specified
in terms of its frequency response.
Analog messages are characterized
by data whose value varies over a continuous range.
For example, the temperature or the atmospheric pressure
of a certain location can vary over a continuous range
and can assume infinite possible values. Similarly,
a speech waveform has amplitudes that vary over a continuous
in contrast to only a finite number of possible digital
messages.
The basic principle of transmission of digital massage
is transmitting a finite set of electrical waveforms
by which the message in the form of letters or numbers
each of which is defined by a signal level. For example,
in the Morse code, a mark can be transmitted by an electrical
pulse of amplitude A/2, and a space can be transmitted
by a pulse of amplitude - A/2. In an -ary
case, distinct electrical
pulses (or waveforms) are used ; each of the pulses
represents one of the possible symbols. The task of
the receiver is to extract a message from a distorted
and noisy signal at a channel output.
Message extraction is often easier from digitals than
analog signals. Consider a binary case ; two symbols
are encoded as rectangular pulses of amplitudes A/2
and -A/2. The only decision at the receiver is the selection
between two possible pulses received, not the details
of the pulse shape ; the decision is readily made with
reasonable certainty even if the pulses are distorted
and noisy (see below illustration). Hence a digital
communication system can transmit messages with greater
accuracy than an analog system in the presence of distortion
and noise.
Another significant advantage of digital
communication is the possibility of using regenerative
repeaters, in which a repeater station detects pulses
and transmits new clean pulses, thus combating accumulation
of distortion & noise and enabling information transmission
over longer distances with greater accuracy.
The analog message, however, demands accurate reproduction
of waveform. Even a light distortion or interference
in the waveform will cause an error in the received
signal. A further difficulty as we shall see later is
that a regenerative repeater is not viable for analog
signals because the noise and distortion, no matter
how small, cannot be cleaned up from a signal.
Therefore, in analog communications, the distortion
and the noise interference are cumulative over the entire
transmission path. In addition, the signal is attenuated
continuously over the transmission path; thus with increasing
distance, the signal becomes weaker, whereas the distortion
and noise become stronger. Ultimately the signal, overwhelmed
by the distortion and the noise, is destroyed. Any implication
of the signal is of no avail as the noise is also amplified
in the same proportion. Consequently, the distance over
which an analog message can be transmitted is limited
by the transmitter power. Yet analog communication is
being used widely and successfully despite these problems.
However,analog systems are now being replaced with digital
systems as the latter has become more economical because
of a dramatic cost reduction achieved in the fabrication
of digital circuitry.
-ary
case,