Digital signal

A digital signal is a way to represent data through a sequence of discrete values. In this system, a signal can only take on one value from a finite set at any given moment.

This stands in direct contrast to an analog signal. An analog signal represents continuous values within an infinite set of possibilities. Because digital signals use specific, separate steps rather than a smooth flow, they are much more organized. This discretization allows machines to process information with great precision and reliability.

In most digital electronics, we use a specific type of signal called a binary signal or logic signal. A binary signal uses only two distinguishable levels to represent information.

These two levels correspond to the values zero and one, or the Boolean terms false and true. In a circuit, these levels are often represented by voltage bands. One band is near a reference point called ground, or zero volts. The other band is near the supply voltage. At any single moment, the signal represents exactly one binary digit, also known as a bit.

To understand how these signals move, we can look at the waveform of a logic signal.

A signal switches between a low level and a high level. When the voltage moves from low to high, it is called a rising edge. When it moves from high to low, it is called a falling edge. In an ideal model, these transitions happen instantly. However, real-world circuits are not purely resistive. This means there is a finite transition time where the voltage is between the two intended levels. To manage this timing, many circuits use a clock signal. This special digital signal synchronizes the parts of a circuit so they work together.

Digital signals are incredibly useful because they possess noise immunity. Electronic noise is unwanted interference that can disrupt a signal.

In analog systems, even small amounts of noise can degrade the signal and change the data. Digital signals are much more resilient. Because the signal exists in discrete bands, small changes in voltage do not push the signal out of its intended envelope. The sensing circuitry simply ignores these small fluctuations. As long as the noise is not too great, the digital signal remains clear and accurate.

While binary is most common, we can also use multivalued logic. This refers to circuitry that uses more than two states. For example, three-valued logic uses three possible states.

There are many different physical ways to carry a digital signal. It might be an electric current or a voltage. It could also be the intensity or phase of an optical field, such as light. Other methods include using acoustic pressure or the magnetization of magnetic storage media. The method chosen depends on whether the signal is being used in a computer or a communication system.

In the field of communications, digital signals can be quite complex. They might be sent as a pulse-code modulation (PCM) signal. To send data over long distances, engineers often use modulation. This involves a carrier-modulated sine wave. One method is frequency-shift keying (FSK), which alternates between two different waveforms.

Sometimes, a digital signal must be converted into an analog signal to travel over systems like analog telephone lines. This ensures the bitstream is compatible with the existing speech-based infrastructure.

Digital signals are the foundation of modern computing and data transmission. They allow for data compression, which greatly decreases the bandwidth required for audio and video. In digital electronics, these signals are often generated by MOSFET devices. These transistors are used because they have very rapid on-off switching speeds. This speed allows for large-scale integration in complex chips. From the smallest computer component to the vast networks of the internet, digital signals make our modern technological world possible.