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rotary dial

A rotating number selector on early telephones (and TVs). Introduced in the early 1900s, rotary phones were commonly used from the 1920s until the 1960s, when dial phones were gradually replaced with push button phones (see Touch-Tone).




In the Antique Shop
As the caller manually rotated the spring-loaded dial clockwise, it was halted at the finger stop (red arrow). Letting go, the spring automatically reversed the rotation sending a stream of pulses to the telephone company central office (CO). The larger the digit, the more pulses, with 0 generating 10 pulses.






Touch-Tone

The dialing technology in an analog telephone. Each digit has a physical button that transmits a different audio tone when depressed. Known as the dual-tone multifrequency (DTMF) system, the signal is a combination of the row and column frequencies of the digit key.

The first touch-tone phones were installed in two Pennsylvania towns in the early 1960s. Digital phones do not use the Touch-Tone system. Contrast with rotary dial. See analog phone and digital phone.




Four Rows - Three Columns
Each row and each column in the keypad matrix is assigned a different frequency, and the audio signal transmitted is a superimposition of the two frequencies of the depressed key.






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