Keying is a family of
modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
forms where the modulating signal takes one of a specific (predetermined) number of values at all times. The goal of keying is to transmit a digital signal over an analog channel. The name derives from the
Morse code
Morse code is a method used in telecommunication to encode text characters as standardized sequences of two different signal durations, called ''dots'' and ''dashes'', or ''dits'' and ''dahs''. Morse code is named after Samuel Morse, one of ...
key used for
telegraph
Telegraphy is the long-distance transmission of messages where the sender uses symbolic codes, known to the recipient, rather than a physical exchange of an object bearing the message. Thus flag semaphore is a method of telegraphy, whereas p ...
signaling.
Modulation is the general technique of shaping a signal to convey information. When a digital message has to be represented as an
analog waveform
An analog signal or analogue signal (see spelling differences) is any continuous signal representing some other quantity, i.e., ''analogous'' to another quantity. For example, in an analog audio signal, the instantaneous signal voltage varies ...
, the technique and term ''keying'' (or ''digital modulation'') is used. Keying is characterized by the fact that the modulating signal will have a limited number of states (or values) at all times, to represent the corresponding digital states (commonly zero and one, although this might depend on the number of
symbols used). This is in contrast to ''analogue modulation'', where an analogue signal is transmitted over an analogue channel, and where the modulated analogue signal will have an infinite number of meaningful states.
Furthermore, note that keying or digital modulation applies to transmitting a digital signal over an analogue
passband channel. When a digital signal is to be transmitted over an analogue
baseband channel, the modulation technique is termed
line coding
In telecommunication, a line code is a pattern of voltage, current, or photons used to represent digital data transmitted down a communication channel or written to a storage medium. This repertoire of signals is usually called a constrained co ...
.
Several keying techniques exist, including
phase-shift keying
Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosine inputs at a ...
,
frequency-shift keying
Frequency-shift keying (FSK) is a frequency modulation scheme in which digital information is transmitted through discrete frequency changes of a carrier signal. The technology is used for communication systems such as telemetry, weather ball ...
, and
amplitude-shift keying
Amplitude-shift keying (ASK) is a form of amplitude modulation that represents digital data as variations in the amplitude of a carrier wave.
In an ASK system, a symbol, representing one or more bits, is sent by transmitting a fixed-amplitude car ...
.
Bluetooth
Bluetooth is a short-range wireless technology standard that is used for exchanging data between fixed and mobile devices over short distances and building personal area networks (PANs). In the most widely used mode, transmission power is limi ...
, for example, uses phase-shift keying to exchange information between devices.
An overview of keying techniques is given on the
modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a periodic waveform, called the ''carrier signal'', with a separate signal called the ''modulation signal'' that typically contains informatio ...
page.
Radio modulation modes
Telecommunication theory
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