- For other uses, see Digital (disambiguation)
A digital system is one that uses numbers for input, processing, transmission, storage, or display, rather than a continuous spectrum of values (an analog system) or non-numeric symbols such as letters or icons.
The distinction of 'digital' versus 'analog' or 'symbolic' 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: the Latin word for finger (counting on the fingers) as these are used for discrete counting.
The word 'digital' is commonly used in computing, especially where real-world information is converted to numeric form as in digital audio and digital photography. The term is often meant by the prefix 'e-', as in e-mail and ebook, even though not all electronics is digital.
Digital vs analog
Digital noise
When data is transmitted using analog methods, a certain amount of noise enters into the signal. This can have myriad causes: data transmitted by radio may be received badly, suffer interference from other radio sources, or pick up background radio noise from the rest of the universe. Electric pulses being sent down wires are attenuated by the resistance of the wire, and dispersed by its capacitance, and heat variations can increase or reduce these effects. While digital transmissions are also degraded, any slight variations can be safely ignored. Any variance could provide a great amount of distortion in an analog signal. In a digital signal, these variances can be overcome, as any signal close to a particular value will be interpreted as that value.
Analog, symbolic, and digital displays; ease of reading
For human readable information, digital, analog, and symbol display methods can all be useful. Should an instant impression be required, analog meters and indicator lights often give information quickly. Many people glance quickly at their analog watch and know roughly what the time is or at an automobile dashboard and know that a door is ajar. When accuracy is required, however, digital displays are preferred. Reading analog meters requires time and a little bit of skill, whereas writing down the value on a digital display is merely a case of copying down the numbers. In cases where both accuracy and quick reckoning are both required, dual displays are often used.
A needle (analog) just touching onto the bottom of an orange shaded area is much different to a needle almost touching into the red area, but an indicator lamp (symbol) would just glow orange and a numeric (digital) display, although it could be colored orange, would not indicate the relative level of danger to an untrained operator.
Systematic loss of data
When an analog source is converted to digital, some data will be lost. The analog-to-digital converter only has a certain resolution. For example, a voltage on a copper wire can be any of a seemingly unlimited number of values between its minimum and maximum (limited only by quantum mechanics). But a digital representation using n bits can have only 2n possible values. While this information will be preserved in future transmission, the data has been lost.
It should be noted that photographic film is not perfect, being subject to aberrations. Losses in analog systems are often modelled as a noise spectrum and modulation transfer function (MTF). The MTF of many analog systems, including film, typically "rolls off" with increasing frequency.
Converting symbols to digital and back does not have this problem since both are discrete. And 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. (In fact, in the double helix structure, there are two strands, but one of them is never read.) 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, and evolution has developed its digital properties into a robust method of communication.
- 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.
- Semaphore signalling uses rods or flags held in particular positions to send messages to the receiver watching them some distance away.
- 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.
See also
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