![]() Various MF signaling protocols were devised by the Bell System and CCITT. Multi-frequency signaling ( MF) is a group of signaling methods that use a mixture of two pure tone (pure sine wave) sounds. Operators used an earlier type of multi-frequency signaling. Placing calls over longer distances required either operator assistance or provision of special subscriber trunk dialing equipment. The physical distance for which this type of dialing was possible was restricted by electrical distortions and was possible only on direct metallic links between end points of a line. The exchange equipment responds to the dial pulses either directly by operating relays or by storing the number in a digit register that records the dialed number. ![]() 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 is operated by the rotary dial as it spins back to its rest position after having been rotated to each desired number. Prior to 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. ![]()
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