English is not my native language and I do not understand why the words "shift keying" were used to describe some types of digital modulation.

What does "keying" mean in the context of signals and modulation? I know what a key is: something that unlocks a door or a password but I have no idea what does the verb "keying" mean in this specific context of modulation.

What do those words mean individually in this context and what do they mean together in this context?

My question is one about terminology more than about electronics. I understand what Amplitude Shift Keying (ASK) is now. What I do not understand is what is the meaning of the words "shift keying" and why this type of modulation was not named simply "amplitude modulation"?

In none of the Wikipedia articles that contain the phrase 'shift keying' and from the context I do not understand why were those words used. What I understand is 'moving a key' I do not know how is this related to modulation or to electronics in general.

  • \$\begingroup\$ As davidmneedham commented in your previous question: "Key" comes from telegraph key: en.wikipedia.org/wiki/Telegraph_key. These were used to send messages using Morse code. This uses long and short signals, a bit like ones and zeros for digital. So you could see Morse code as the (grand)parent of digital communication. You should not pay too much attention to the name of ASK modulation, the name is very likely a result of some history. \$\endgroup\$ Commented Sep 6, 2017 at 15:08
  • \$\begingroup\$ BPSK binary phase shift keying, usually the simple reversal of the carrier phase by selecting opposite outputs of a center-tapped secondary of transformer; QPSK quadrature phase shift keying: switching among the 4 90 degree points of a sinusoid. \$\endgroup\$ Commented Sep 6, 2017 at 16:10

5 Answers 5


It's not (phase) (shift keying), for example. It's (phase shift) (keying).

Another answer has explained why the word keying is used --- it dates to the days when modulation was controlled by a human operator using a telegraph key.

The phase shift (or amplitude shift or whatever) is because something is changing (shifting) when keyed.

If the system were manually operated, you could say that each time the operator pressed the key, the phase is shifted.


It's called "shift keying" because during the course of transmission of digital data the values of the quantity that is used for coding the data (e.g. amplitude, frequency, phase) shift between two (or more) discrete switching (=keying) values.

  • \$\begingroup\$ so keying means switching? Why was this word chosen for this? Why not shift switching? \$\endgroup\$
    – yoyo_fun
    Commented Sep 6, 2017 at 15:03
  • \$\begingroup\$ "key" is not an unusual word for switch: think e.g. about the keys of a keyboard or Morse key. \$\endgroup\$
    – Curd
    Commented Sep 6, 2017 at 15:04
  • 1
    \$\begingroup\$ @yoyo_fun "Keying" possibly has a historic origin. On-off modulation was originally done with a hand-operated switch called a key. That was more than a century ago. \$\endgroup\$
    – glen_geek
    Commented Sep 6, 2017 at 15:07
  • 1
    \$\begingroup\$ @glen_geek actually one needs to go back further than that still. I believe the telegraph-key was named a key due to it's similarity to a piano "key". As to why piano keys are called "keys" ... "A musical keyboard calls it a key because pressing it plays a key, in the sense of a note or tone. The "note" sense of keys, like on a piano, date back to the mid 15th century." \$\endgroup\$
    – Trevor_G
    Commented Sep 6, 2017 at 16:01
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    \$\begingroup\$ @Curd The key that you are referring is part of the symbol that appears at the start of a musical score (the sharps and flats), not the notes shorter than 1/2. These are called crotches (1/4th), quavers (1/8th), semiquavers (1/16th) and so on. So I think the similar shape is accidental. \$\endgroup\$
    – Bart
    Commented Sep 22, 2017 at 12:21

"Keying" is synonymous with modulation, in the context of referencing how a signal is constructed.

Think of it as saying a signal is 'Phase Shift, Keyed', or that the phase of the signal is modified.

I hope that helps with the language/translation.

  • 2
    \$\begingroup\$ Keying is synonymous with discrete modulation. \$\endgroup\$ Commented Sep 22, 2017 at 16:21
  • 2
    \$\begingroup\$ As @SolomonSlow said, keying means digital modulation. It is explained in this Wikipedia article. \$\endgroup\$
    – solitone
    Commented Mar 29, 2019 at 19:50

Short answer

  • Modulation is used when the carrier property used (e.g. amplitude) is changed progressively, because the signal to transmit itself varies progressively. Modulation is associated mostly with analog signals modulating a sinusoidal carrier.

  • Keying is used when the carrier property used is changed abruptly, because the signal itself changes abruptly. Keying is associated mostly with digital signals modulating a sinusoidal carrier.

For keying the carrier property used is changed between discrete values only, e.g. for 2 values this can be on/off for amplitude, or 0° and 180° for phase. The change itself is named the shift, it can be an amplitude shift, a frequency shift, etc.

I'm not going into the details, but a continuous wave is not the only type of carrier which can be used. Sometimes, for digital communication, a train of pulses is used. We talked about pulse keying, e.g. pulse width keying, pulse position keying, etc.

Keying just comes for Morse telegraphy where a key was used to switch the carrier on/off. Shift denotes the sudden change between two states.


Communication is a domain which has experienced and still experiences spectacular changes thanks to e.g. DSP and software-defined radio (SDR). The consequence is the new wording constantly entering in use and the difficulty to match it with what was up to date just a few years ago.

Some definitions

Signal (the information to be transmitted):

  • Analog: At any time the signal can have any value in between some chosen limits (e.g. any voltage between -5V and +18V).

  • Digital: At any time the signal can have only one of the discrete values included in the digitization scheme (sampling depth). For example if the digitization scheme uses 2 bits, only four discrete integral values are possible, 0 to 3 and a coding procedure might be required to establish a correspondence between the actual signal values and these 4 numbers.

Carrier (the electromagnetic wave used to transmit the information signal):

A wave is continuously generated, i.e. a sinusoidal continuous wave (CW).
The signal will change some property of the carrier: Its amplitude, its frequency or it's phase. The term continuous wave was adopted to contrast with the first radio technology it replaced: Damped waves created by sparks.

The principle of a transmission by carrier, e.g. optical or radio, is to adjust one or more carrier properties in correlation with the signal, so that the signal can be sent on a long distance using the carrier altered by the signal, and recovered by the receiver from the altered carrier.


Modulation is used to transmit an analog signal onto an analog/continuous carrier. The legacy (analog) radio and TV broadcast fall in this category.

In this case the carrier is continuously altered by the signal, and the adjustment can be of any magnitude (within permitted limits). Usually the property is correlated to the signal, therefore if the signal is progressive, the carrier property change is also progressive. Depending on the carrier property altered, we talk about amplitude modulation (AM), frequency modulation (FM) or phase modulation (PM).

  • AM includes variants: Double sideband, vestigial sideband, single sideband, etc.

  • For FM, the spectrum obtained is infinite, but depending on the modulation index used, the useful spectrum is narrow (NBFM or wider though not ilimited (WBFM).

Modulation is mostly used for analog signals, but also for digital signals (see continuous phase modulation further down). The key element is the carrier property can take all technically permitted values.


Keying is used to transmit a digital signal onto an analog/continuous carrier. Morse radiotelegraphy falls into this category as well as GPS and HDTV (and likely everything modern which comes to your mind).

In contrast to modulation, a keyed carrier property takes only discrete values.

The most simple digital signal has only two values (0/1 or on/off). Marconi radiotelegraphy using Morse code was dealing with digital signals without knowing. In that case the modulation was done manually using a switch (key) to turn on/off a continuous wave (CW) carrier.

enter image description here
Morse telegraphy switch, aka "key", circa 1945. Source

This example of radiotelegraphy is representative of the difficulty to talk about signal transmission:

  • For most persons it's Morse, emphasizing the code used to represent characters, rather than the transmission channel.

  • Hams refer to it just as "CW", because the new CW was so associated with the also new telegraphy. This is the channel which is emphasized, not the code or the keying method, even is a CW carrier is used for almost any radio channel and is in no way particular to radiotelegraphy.

In the engineering field, keying is used to refer to continuous wave modulation by signals with discrete values, whatever the code used (telegraphy is technically binary amplitude shift keying with Morse code).

A carrier property will be adjusted to imprint the signal into the wave. The adjustment takes the name of shift, giving amplitude shift keying (ASK), frequency shift keying (FSK) and phase shift keying (PSK).

Depending one the number of discrete values for the signal (hence the number of shifts allowed for the property), the name is refined, e.g. for a signal with two values we talk about binary phase shift keying (BPSK) and quadrature phase shift keying (QPSK) for four values.

For the simple case a binary amplitude shift keying, the term on-off keying is also used, as the amplitude can only be maximal or null.

Sometimes the carrier change between successive signal values is not abrupt, but progressive, as in the continuous phase modulation (CPM) where the carrier adjustment and the digital signal are not exactly coherent. Note it's a modulation, not a keying.


The use of Morse code is not as old as you might think for radio telegraphy. FCC licensed amateur radio operators still use radio telegraphy that they call CW (continuous wave). Morse code is a 2 symbol code of long and short. For example A-di dah. Passenger jets use VOR that used a Morse code airport identifier for radio navigation. For example LAX for Los Angeles International Airport.

In reductionism in philosophy and science one attempts to reduce to the simplest "atom" of an idea. I think these ideas may have affected nomenclature for electronics. It may be more philosophical that the word keying was chosen. It is true a Morse code key can be considered a simple switch- on off- 1 or 0. A key and Morse code transmitter are very simple. Simpler than a receiver. It was considered the simplest modulation technique at one time, requiring operator skill and FCC license.

I suppose the word shift is more questionable, but it seems to be common jargon now.

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    \$\begingroup\$ Welcome to EE.SE, Chuck, but it's difficult to see an answer to "What is 'shift-keying'?" in this post - if there is an answer. Can you improve your answer by removing the irrelevant stuff and giving some angle on the meaning of "shift-keying" that hasn't been given in another answer already? Signatures are not allowed in posts by site rule as they are automatically appended to your post. \$\endgroup\$
    – Transistor
    Commented Apr 26, 2019 at 15:53
  • \$\begingroup\$ This is all very interesting historically, but it would be good to tie Morse in with ASK, i.e. The simplest and most common form of ASK operates as a switch, using the presence of a carrier wave to indicate a binary one and its absence to indicate a binary zero. This type of modulation is called on-off keying (OOK), and is used at radio frequencies to transmit Morse code (referred to as continuous wave operation), - From Wikipedia \$\endgroup\$ Commented Apr 26, 2019 at 16:31

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