Digital data | Wikipedia audio article


Digital data, in information theory and information
systems, is the discrete, discontinuous representation of information or works. Numbers and letters are commonly used representations. Digital data can be contrasted with analog
signals which behave in a continuous manner, and with continuous functions such as sounds,
images, and other measurements. The word digital comes from the same source
as the words digit and digitus (the Latin word for finger), as fingers are often used
for discrete counting. Mathematician George Stibitz of Bell Telephone
Laboratories used the word digital in reference to the fast electric pulses emitted by a device
designed to aim and fire anti-aircraft guns in 1942. The term 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.==Symbol to digital conversion==
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 character encoding. A custom encoding can be used for a specific
application 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. It is estimated that in the year 1986 less
than 1% of the world’s technological capacity to store information was digital and in 2007
it was already 94%. The year 2002 is assumed to be the year when
human kind was able to store more information in digital than in analog format (the “beginning
of the digital age”).==States==
Digital data come in these three states: data at rest, data in transit and data in use. The confidentiality, integrity and availability
have to be managed during the entire lifecycle from ‘birth’ to the destruction of the data.==Properties of digital information==
All digital information possesses common properties that distinguish it from analog data with
respect to communications: 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 re-transmission. 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. DNA genetic code is a naturally occurring
form of digital data storage. Written text (due to the limited character
set and the use of discrete symbols – the alphabet in most cases)
The abacus was created sometime between 1000 BC and 500 BC, it later became a form of calculation
frequency. Nowadays it can be used as a very advanced,
yet basic digital calculator that uses beads on rows to represent numbers. Beads only have meaning in discrete up and
down states, not in analog in-between states. 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. Morse code uses six digital states—dot,
dash, intra-character gap (between each dot or 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. Flag semaphore 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.==See also

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