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Autovon keypads were one of the few production units to include all 16 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 by users with a loop-disconnect (LD) signaling, more commonly known as pulse dialing (dial pulse, DP) in the United States.
Multifrequency signaling is a technological precursor of dual-tone multi-frequency signaling (DTMF, Touch-Tone), which uses the same fundamental principle, but was used primarily for signaling address information and control signals from a user's telephone to the wire-center's Class-5 switch. DTMF uses a total of eight frequencies.
Pressing a single key of a traditional analog telephone keypad produces a telephony signaling event to the remote switching system. For touchtone service, the signal is a dual-tone multi-frequency signaling tone consisting of two simultaneous pure tone sinusoidal frequencies. The row in which the key appears determines the low-frequency ...
Symbols in the DTMF and MF alphabets are sent as tone pairs; DTMF selects one tone from a "high" group and one from a "low" group, while MF selects its two tones from a common set. DTMF and MF use different tone frequencies largely to keep end users from interfering with inter-office signaling.
Some central office switches in the United States, notably older GTD-5 EAX systems, utilize a single frequency tone, 480 Hz, known as High Tone for this purpose. In either case, the tone is substantially louder than any other signal transmitted over a copper POTS circuit; loud enough to be heard across a room from an unused off-hook telephone.
Some decoders may require much longer-duration digits. DTMF digits consist of paired tones: a row tone and a column tone. The levels of row and column tones must be similar in order for a decoder to interpret them reliably. Radios with DTMF decoders may monitor all system traffic or remain muted until called, depending on the system design.
Because its six tones are used only in pairs, this signaling format is sometimes referred to as "two-out-of-five code" or "two of six." DTMF receiver, late 1960s, in a crossbar exchange Multi-Frequency receivers have been present in US telephony at least since the late 1940s.
By the 1940s they had developed a system that used audible tones played over the long-distance lines to control network connections. Tone pairs, referred to as multi-frequency (MF) signals, were assigned to the digits used for telephone numbers. A different, single tone, referred to as single frequency (SF), was used as a line status signal.