, a typical late 20th century American touch-tone telephone keypads were a typical application for use of all sixteen DTMF signals. The red keys in the fourth column produce the A, B, C, and D DTMF events. Before the development of DTMF, telephone numbers were dialed with
rotary dials for loop-disconnect (LD) signaling, also known as
pulse dialing. It functions by interrupting the current in the
local loop between the telephone exchange and the
calling party's telephone at a precise rate with a switch in the telephone that operates the dial which spins back to its rest position after having been rotated to each desired digit. The exchange equipment responds to the dial pulses either directly by operating relays, or by storing the digits in a register that records the dialed telephone number. Pulse dialing was possible only on direct metallic lines and was limited in physical distance by the amount of electrical distortions present. For signaling over trunks between switching systems, operators used a different type of
multi-frequency signaling.
Multi-frequency signaling (
MF) is a group of signaling methods that use a mixture of two
pure tone (pure
sine wave) sounds. Various MF signaling
protocols were devised by the
Bell System and
CCITT. The earliest of these were for
in-band signaling between switching centers, where
long-distance telephone operators used a 16-
digit keypad to input the next portion of the destination telephone number in order to contact the next downstream long-distance telephone operator. This semi-automated signaling and switching proved successful in both speed and cost effectiveness. Based on this prior success with using MF by specialists to establish long-distance telephone calls, dual-tone multi-frequency signaling was developed for end-user signaling without the assistance of operators. The DTMF system uses two sets of four frequencies in the voice frequency range transmitted in pairs to represent sixteen signals, representing the ten digits and six additional signals identified as the letters A to D, and the symbols
# and
*. As the signals are audible tones, they can be transmitted through line repeaters and amplifiers, and over radio and microwave links. AT&T described the product as "a method for pushbutton signaling from customer stations using the voice transmission path". To prevent consumer telephones from interfering with the MF-based routing and switching between telephone switching centers, DTMF frequencies differ from all of the pre-existing MF signaling protocols between switching centers: MF/R1,
R2, CCS4, CCS5, and others that were later replaced by
SS7 digital signaling. DTMF was known throughout the Bell System by the trademark
Touch-Tone. The term was first used by AT&T in commerce on July 5, 1960, and was introduced to the public on November 18, 1963, when the first
push-button telephone was made available to the public. As the parent company of Bell Systems, AT&T held the trademark from September 4, 1962, to March 13, 1984. It is standardized by
ITU-T Recommendation Q.23. Other vendors of compatible telephone equipment called the Touch-Tone feature
tone dialing or
DTMF. Automatic Electric (GTE) referred to it as "Touch-calling" in their marketing. Other trade names such as
Digitone were used by the
Northern Electric Company in Canada. As a method of
in-band signaling, DTMF signals were also used by
cable television broadcasters as
cue tones to indicate the start and stop times of local commercial insertion points during station breaks for the benefit of cable companies. Until
out-of-band signaling equipment was developed in the 1990s, fast, unacknowledged DTMF tone sequences could be heard during the commercial breaks of cable channels in the United States and elsewhere. Previously, terrestrial television stations used DTMF tones to control remote transmitters. In
IP telephony, DTMF signals can also be delivered as either in-band or out-of-band tones, or even as a part of signaling protocols, as long as both endpoints agree on a common approach to adopt. ==Keypad==