Digital
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Wikipedia-Article "Digital"
- For other uses, see Digital (disambiguation)
A digital system is one that uses numbers, especially binary 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 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.
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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. Care must be taken when connected digital and analog systems; tolerable variances for the digital part can leak into the analog part and become intolerable.
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.
Analog to digital conversion
- Main article: Analog-to-digital converter
Converting an analog source to digital data is done with two steps: sampling, which changes the source to a series of discrete values (called samples), and quantization, which converts each sample to a number. For example, the sensor of a digital camera contains millions of sensing elements (one for each pixel). When an exposure is made, the light focused on the array is converted into millions of electric charges (sampled). These charges are then amplified and converted to numbers (quantized). The resulting digital image is then processed and stored in the camera's memory card. The samples in this case are spatial. In contrast, converting an audio source to digital requires temporal samples: it is converted to an electrical signal using a microphone, and the voltage of this signal is sampled thousands of times per second (the sampling frequency). Each sample is then quantized to form the digital audio data.
Both sampling and quantization will result in a loss of data. Changes in the original data that occur between the samples will not appear in the digital data (or worse, will cause aliasing, the appearance of data not present in the original source). And while a voltage can be any of a seemingly unlimited number of values between its minimum and maximum (limited only by quantum mechanics), 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.
The amount of information that can be stored in a digital representation is called its resolution. And since the conversion to digital is a two step process, there are two types of resolution: sampling resolution and quantization resolution. Sampling resolution can be either spatial (expressed in pixels per inch) or temporal (expressed as samples per second) or both (for example, a video). Quantization resolution is usually expressed as the number of bits used to represent each sample and is thus often called the bit depth or (for pictures) the color depth.
The best resolution for a given set of digital data depends on the processing it will undergo and its ultimate purpose. For example, compact discs use a sampling resolution of 44,100 samples/second, which is sufficient for audio in the range of human hearing. Most digital photographs use a bit depth of 8 bits/color, which produces more colors than the human eye can discern. However many photographers use camera raw with 12 bits/color to allow for more accuracy during processing before producing a final photograph at 8 bits/color for display or printing. Scientific photography may also require greater bit depth.
If sufficient resolution is used, the data loss caused by the conversion to digital is offset by the accuracy of digital processing. When analog signals are transmitted and stored, accuracy is lost due to noise and distortion. So neither digital nor analog offer perfect fidelity; resolution is sacrificed for accuracy with digital and vice versa for analog. When both high resolution and high accuracy are needed, either a high resolution digital system or a high accuracy analog system must be used (with a correspondingly high cost).
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. (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.
- 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.