UNIT 1 S

Analog signal

 For a broader coverage related to this topic, see Signal (electronics).

An analog signal is any continuous signal for which the time varying feature (variable) of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. For example, in an analog audio signal, the instantaneous voltage of the signal varies continuously with the pressure of the sound waves. It differs from a digital signal, in which the continuous quantity is a representation of a sequence of discrete values which can only take on one of a finite number of values.  The term analog signal usually refers to electrical signals; however, mechanical, pneumatic, hydraulic, human speech, and other systems may also convey or be considered analog signals.

An analog signal uses some property of the medium to convey the signal’s information. For example, an aneroid barometer uses rotary position as the signal to convey pressure information. In an electrical signal, the voltage, current, or frequency of the signal may be varied to represent the information.

Advantages and disadvantages 

The primary disadvantage of analog signals is that any system has noise – i.e., unwanted variation. As the signal is copied and re-copied, or transmitted over long distances, or electronically processed, the unavoidable noise introduced by each step in the signal path is additive, progressively degrading the signal-to-noise ratio, until in extreme cases the signal can be overwhelmed. This is called generation loss. Noise can show up as “hiss” and intermodulation distortion in audio signals, or “snow” in video signals. This degradation is impossible to recover, since there is no sure way to distinguish the noise from the signal; amplifying the signal to recover attenuated parts of the signal amplifies the noise (distortion/interference) as well. Digital signals can often be transmitted, stored and processed without introducing noise. Electrically, analog noise can be diminished by shielding, good connections and several cable types such as coaxial or twisted pair.

Digital signal

A binary signal, also known as a logic signal, is a digital signal with two distinguishable levels

A digital signal is a signal that is constructed from a discrete set of waveforms of a physical quantity so as to represent a sequence of discrete values.  A logic signal is a digital signal with only two possible values,  and describes an arbitrary bit stream. Other types of digital signals can represent three-valued logic or higher valued logics.  Alternatively, the digital signal may be considered to be the sequence of discrete values represented by such a physical quantity. 

Definitions 

In digital electronics 

In digital electronics a digital signal is a pulse train (a pulse amplitude modulated signal), i.e. a sequence of fixed-width square-wave electrical pulses or light pulses, each occupying one of a discrete number of levels of amplitude.  A special case is a logic signal or a binary signal, which varies between a low and a high signal level.

In signal processing, a digital signal is an abstraction that is discrete in time and amplitude, meaning it only exists at certain time instants.

In signal processing 

Main article: digital signal (signal processing)

In digital signal processing, a digital signal is a representation of a physical signal that is a sampled and quantified. A digital signal is an abstraction which is discrete in time and amplitude. The signal’s value only exists at regular time intervals, since only the values of the corresponding physical signal at those sampled moments are significant for further digital processing. The digital signal is a sequence of codes drawn from a finite set of values.  The digital signal may be stored, processed or transmitted physically as a pulse code modulation(PCM) signal.

LOGIC GATES:

Type Distinctive shape Rectangular shape Boolean algebra between A & B Truth table
Inverter
NOT NOT symbol NOT symbol  {\overline {A}}{\text{ or }}{\sim }A
INPUT OUTPUT
A NOT A
0 1
1 0
In electronics a NOT gate is more commonly called an inverter. The circle on the symbol is called a bubble and is used in logic diagrams to indicate a logic negation between the external logic state and the internal logic state (1 to 0 or vice versa).
2-input gates
AND AND symbol AND symbol  A\cdot B{\text{ or }}A\;\&\;B
INPUT OUTPUT
A B A AND B
0 0 0
0 1 0
1 0 0
1 1 1
OR OR symbol OR symbol  A+B
INPUT OUTPUT
A B A OR B
0 0 0
0 1 1
1 0 1
1 1 1
XOR XOR symbol XOR symbol  A\oplus B
INPUT OUTPUT
A B A XOR B
0 0 0
0 1 1
1 0 1
1 1 0
The output of a two input exclusive-OR is true only when the two input values are different, and false if they are equal, regardless of the value. If there are more than two inputs, the output of the distinctive-shape symbol is undefined. The output of the rectangular-shaped symbol is true if the number of true inputs is exactly one or exactly the number following the “=” in the qualifying symbol.
Inverted 2-input gates
NAND NAND symbol NAND symbol  {\overline {A\cdot B}}{\text{ or }}A\uparrow B
INPUT OUTPUT
A B A NAND B
0 0 1
0 1 1
1 0 1
1 1 0
NOR NOR symbol NOR symbol  {\overline {A+B}}{\text{ or }}A-B
INPUT OUTPUT
A B A NOR B
0 0 1
0 1 0
1 0 0
1 1 0
XNOR XNOR symbol XNOR symbol  {\overline {A\oplus B}}{\text{ or }}{A\odot B}
INPUT OUTPUT
A B A XNOR B
0 0 1
0 1 0
1 0 0
1 1 1