Editing DTMF signaling (section)

== Square, star, A, B, C, and D ==
{{Listen
|filename=DTMF dialing.ogg
|title=DTMF dialing
|description=audio output of a DTMF signal.
|format=[[Ogg]]}}
Engineers had envisioned telephones being used to access computers and automated response systems.<ref name="Keith">{{cite book |first=Keith |last=Houston |title=Shady Characters: The Secret Life of Punctuation, Symbols, and Other Typographical Marks |url=https://books.google.com/books?id=3R2SAAAAQBAJ&pg=PT45|date=24 September 2013 |publisher=W. W. Norton |isbn=978-0-393-24154-9 |page=45}}</ref> They consulted with companies to determine the requirements. This led to the addition of the ''square sign''<ref>ITU standards recommendation E.161</ref> which is typically approximated by the [[number sign]] (#), also "pound", "diamond", "hash", "gate" (UK), and "[[Number sign#Other names in English|octothorpe]]") in the fourth row of the first column of keys, and the star (*) key, or [[asterisk]] (France) in the fourth row of the third column. In addition a fourth column of keys was added for menu selection: A, B, C and D. The lettered keys were dropped from most keypads and it was many years before the two symbol keys became widely used for [[vertical service code]]s such as *67 in the United States and Canada to suppress [[caller ID]].

Public [[payphone]]s that accept credit cards use these additional codes to send the information from the [[magnetic strip]].

The [[AUTOVON]] telephone system of the [[United States Armed Forces]] used signals A, B, C, and D to assert certain privilege and priority levels when placing telephone calls.<ref>{{cite web| url = http://www.tech-faq.com/abcd.shtml| title = "What are the ABCD tones?" — Tech FAQ| date = 6 April 2019}}</ref>  Precedence is still a feature of military telephone networks, but using number combinations. For example, entering 93 before a number is a priority call.

Present-day uses of the signals A, B, C and D are rare in telephone networks, and are exclusive to network control. For example, ''A'' is used in some networks for cycling through a list of carriers.{{citation needed|date=January 2022}} The signals are used in radio phone patch and repeater operations to allow, among other uses, control of the repeater while connected to an active telephone line.{{citation needed|date=January 2022}}

The signals star, square, A, B, C, and D are still widely used worldwide by [[amateur radio]] operators and commercial two-way radio systems for equipment control, repeater control, remote-base operations and some telephone communications systems.{{citation needed|date=January 2022}}

DTMF signaling tones may also be heard at the start and/or end of some prerecorded [[VHS]] videocassettes.<ref>{{Cite book|url=https://books.google.com/books?id=Pv86AAAAMAAJ&q=DTMF+signaling+tones+VHS|title=Broadcast Engineering|date=1983|publisher=Intertec Publishing Corporation], $4c 1959|language=en}}</ref> Information on the master version of the video tape is encoded in the DTMF tones. The encoded tones provide information to automatic duplication machines, such as format, duration and volume levels in order to replicate the original video as closely as possible.

DTMF tones are used in some [[caller ID]] systems to transfer the caller ID information, a function that is performed in the United States by  [[Bell 202]] modulated [[frequency-shift keying]] (FSK) signaling.