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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.

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  • \$\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\$ – Bimpelrekkie Sep 6 '17 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\$ – analogsystemsrf Sep 6 '17 at 16:10
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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.

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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.

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  • \$\begingroup\$ so keying means switching? Why was this word chosen for this? Why not shift switching? \$\endgroup\$ – yoyo_fun Sep 6 '17 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 Sep 6 '17 at 15:04
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    \$\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 Sep 6 '17 at 15:07
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    \$\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 Sep 6 '17 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 Sep 22 '17 at 12:21
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"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.

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    \$\begingroup\$ Keying is synonymous with discrete modulation. \$\endgroup\$ – Solomon Slow Sep 22 '17 at 16:21
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    \$\begingroup\$ As @SolomonSlow said, keying means digital modulation. It is explained in this Wikipedia article. \$\endgroup\$ – solitone Mar 29 '19 at 19:50
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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 Apr 26 '19 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\$ – Greenonline Apr 26 '19 at 16:31

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