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A 'digital' system is one that uses discrete numbers, especially binary numbers, or non-numeric symbols such as letters or icons, for input, processing, transmission, storage, or display, rather than a continuous spectrum of values (an analog system).
The distinction of "digital" versus "analog" can refer to method of input, data storage and transfer, the internal working of an instrument, and the kind of display. The word comes from the same source as the word digit and digitus: the Latin word for finger (counting on the fingers) as these are used for discrete counting.
The word digital is most commonly used in computing and electronics, especially where real-world information is converted to binary numeric form as in digital audio and digital photography. Such data-carrying signals carry either one of two electronic or optical pulses, logic 1 (pulse present) or 0 (pulse absent). The term is often meant by the prefix "e-", as in e-mail and ebook, even though not all electronics systems are digital.

Digital noise

When data is transmitted, a certain amount of noise enters into the signal. This can have several causes: data transmitted by radio may be received inaccurately, suffer interference from other radio sources, or pick up background radio noise from the rest of the universe. Microphones pick up everything — signal as well as background noise — without discriminating between signal and noise, so when audio is encoded digitally, it includes noise.
Electric pulses being sent down wires are attenuated by the resistance of the wire, and dispersed by its capacitance. Heat variations can increase or reduce these effects. While digital transmissions are also degraded, any slight variations can be safely ignored. With an analog signal, any variance can provide a great amount of distortion. In a digital signal, these variances can be overcome, as any signal close to a particular value will be interpreted as that value. Care must be taken when connecting digital and analog systems; tolerable variances for the digital part can leak into the analog part and become intolerable.

Symbol to digital conversion

Since symbols are not continuous, converting symbols to digital is simpler and less prone to data loss than analog to digital conversion. Instead of sampling and quantization, similar steps are used: polling and encoding.
A symbol input device usually consists of a number of switches that are polled at regular intervals to see which switches are pressed. Data will be lost if, within a single polling interval, two switches are pressed, or a switch is pressed, released, and pressed again. This polling can be done by a specialized processor in the device to prevent burdening the main CPU. When a new symbol has been entered, the device sends an interrupt to alert the CPU to read it.
For devices with just a few switches (such as the buttons on a joystick), the status of each can be encoded as bits (usually 0 for released and 1 for pressed) in a single word. This is very useful when combinations of key presses are meaningful, and is sometimes used for passing the status of modifier keys on a keyboard (such as shift and control). But it does not scale to support more keys than the number of bits in a single byte or word.
Devices with many switches (such as a computer keyboard) usually arrange these switches in a scan matrix, with the individual switches on the intersections of x and y lines. When a switch is pressed, it connects the corresponding x and y lines together. Polling (often called scanning in this case) is done by activating each x line in sequence and detecting which y lines then have a signal, thus which keys are pressed. When the keyboard processor detects that a key has changed state, it sends a signal to the CPU indicating the scan code of the key and its new state. The symbol is then encoded, or converted into a number, based on the status of modifier keys and the desired character encoding.
Using a custom encoding for a specific application can be done with no loss of data. However, using a standard encoding such as ASCII is problematic if a symbol such as 'ß' needs to be converted but is not in the standard.

Historical digital systems

Although digital signals are generally associated with the binary electronic digital systems used in modern electronics and computing, digital systems are actually ancient, and need not be binary nor electronic.
* A beacon is perhaps the simplest non-electronic digital signal, with just two states (on and off). In particular, smoke signals are one of the oldest examples of a digital signal, where an analog "carrier" (smoke) is modulated with a blanket to generate a digital signal (puffs) that conveys information.
* DNA comprises a long sequence of four digits (denoted A, C, G, and T), effectively a base-four numeral system. Each of these digits is an organic molecule, known as a nucleotide. DNA is the major system of information transfer from one generation to another.
* Morse code uses five digital states—dot, dash, short gap (between each letter), medium gap (between words), and long gap (between sentences)—to send messages via a variety of potential carriers such as electricity or light, for example using an electrical telegraph or a flashing light.
* The Braille system was the first binary format for character encoding, using a six-bit code rendered as dot patterns.
* Semaphore signalling uses rods or flags held in particular positions to send messages to the receiver watching them some distance away.
* International maritime signal flags have distinctive markings that represent letters of the alphabet to allow ships to send messages to each other.
* More recently invented, a modem modulates an analog "carrier" signal (such as sound) to encode binary electrical digital information, as a series of binary digital sound pulses. A slightly earlier, surprisingly reliable version of the same concept was to bundle a sequence of audio digital "signal" and "no signal" information (i.e. "sound" and "silence") on magnetic cassette tape for use with early home computers.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Digital".