Digital data, in information theory and information systems, are
discrete, discontinuous representations of information or works, as
contrasted with continuous, or analog signals which behave in a
continuous manner, or represent information using a continuous
Although digital representations are the subject matter of discrete
mathematics, the information represented can be either discrete, such
as numbers and letters, or it can be continuous, such as sounds,
images, and other measurements.
The word digital comes from the same source as the words digit and
1 Symbol to digital conversion 2 States 3 Properties of digital information 4 Historical digital systems 5 See also 6 References 7 Further reading
Symbol to digital conversion
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Since symbols (for example, alphanumeric characters) are not
continuous, representing symbols digitally is rather simpler than
conversion of continuous or analog information to digital. Instead of
sampling and quantization as in analog-to-digital conversion, such
techniques as polling and encoding are used.
A symbol input device usually consists of a group of switches that are
polled at regular intervals to see which switches are switched. 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 typically sends an interrupt, in a specialized format, so
that the CPU can read it.
For devices with only 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 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
A custom encoding can be used for a specific application with no loss
of data. However, using a standard encoding such as
Synchronization: Since digital information is conveyed by the sequence in which symbols are ordered, all digital schemes have some method for determining the beginning of a sequence. In written or spoken human languages, synchronization is typically provided by pauses (spaces), capitalization, and punctuation. Machine communications typically use special synchronization sequences. Language: All digital communications require a formal language, which in this context consists of all the information that the sender and receiver of the digital communication must both possess, in advance, in order for the communication to be successful. Languages are generally arbitrary and specify the meaning to be assigned to particular symbol sequences, the allowed range of values, methods to be used for synchronization, etc. Errors: Disturbances (noise) in analog communications invariably introduce some, generally small deviation or error between the intended and actual communication. Disturbances in a digital communication do not result in errors unless the disturbance is so large as to result in a symbol being misinterpreted as another symbol or disturb the sequence of symbols. It is therefore generally possible to have an entirely error-free digital communication. Further, techniques such as check codes may be used to detect errors and guarantee error-free communications through redundancy or retransmission. Errors in digital communications can take the form of substitution errors in which a symbol is replaced by another symbol, or insertion/deletion errors in which an extra incorrect symbol is inserted into or deleted from a digital message. Uncorrected errors in digital communications have unpredictable and generally large impact on the information content of the communication. Copying: Because of the inevitable presence of noise, making many successive copies of an analog communication is infeasible because each generation increases the noise. Because digital communications are generally error-free, copies of copies can be made indefinitely. Granularity: The digital representation of a continuously variable analog value typically involves a selection of the number of symbols to be assigned to that value. The number of symbols determines the precision or resolution of the resulting datum. The difference between the actual analog value and the digital representation is known as quantization error. For example, if the actual temperature is 23.234456544453 degrees, but if only two digits (23) are assigned to this parameter in a particular digital representation, the quantizing error is: 0.234456544453. This property of digital communication is known as granularity. Compressible: According to Miller, "Uncompressed digital data is very large, and in its raw form, it would actually produce a larger signal (therefore be more difficult to transfer) than analog data. However, digital data can be compressed. Compression reduces the amount of bandwidth space needed to send information. Data can be compressed, sent and then decompressed at the site of consumption. This makes it possible to send much more information and result in, for example, digital television signals offering more room on the airwave spectrum for more television channels."
Historical digital systems Even though 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 or electronic.
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^ Ceruzzi, Paul E (June 29, 2012).
Tocci, R. 2006. Digital Systems: Principles and Applications (10th Edition). Prentice Hall. ISBN 0-13-172579-3
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