The term digital is derived from the way in which computers perform operations using digits. Initially, applications of digital electronics were confined to computer systems.

**Digital electronics**

The term
digital is derived from the way in which computers perform operations using
digits. Initially, applications of digital electronics were confined to computer
systems. Nowadays, digital techniques are applied in many areas, such as
telephony, radar, medical instruments, navigation and military systems etc.
Digital Electronics involves circuits and systems in which there are only two
possible states which are represented by voltage levels. Other circuit
conditions such as current levels, open or closed switches can also represent
the two states.

**Analog signal**

The signal
current or voltage is in the form of continuous, time varying voltage or
current (sinusoidal). Such signals are called continuous or analog signals. A
typical analog signal is shown in Fig.

**Digital signal and logic levels**

A digital signal (pulse) is shown
in Fig. It has two discrete levels, 'High' and 'Low'. In most cases, the more
positive of the two levels is called HIGH and is also referred to as logic 1.
The other other level becomes low and
also called logic 0. This method of using more positive voltage level as logic
1 is called a positive logic system. A voltage 5V refers to logic 1 and 0 V
refers to logic 0. On the other hand, in a negative logic system, the more
negative of the two discrete levels is taken as logic 1 and the other level as
logic 0. Both positive and negative logic are used in digital systems. But,
positive logic is more common of logic gates. Hence we consider only positive
logic for studying the operation of logic gates.

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