Multi-Frequency signalling, (MF), is similar to the European version, CCITT
Signaling System 5, (SS5). The original format was five tones used in pairs. This later evolved to six tones. Because its six tones are used only in pairs, this signaling format is sometimes referred to as "
two-out-of-five code
A two-out-of-five code is a constant-weight code that provides exactly ten possible combinations of two bits, and is thus used for representing the decimal digits using five bits. Each bit is assigned a weight, such that the set bits sum to the ...
" or "two of six."
Multi-Frequency receivers have been present in US telephony at least since the late 1940s. In 1940s technology, receivers in 4XB and similar equipment used
vacuum tube
A vacuum tube, electron tube, thermionic valve (British usage), or tube (North America) is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. It ...
s. Later ones used RC filters and transistors.
Digital filter
In signal processing, a digital filter is a system that performs mathematical operations on a Sampling (signal processing), sampled, discrete-time signal to reduce or enhance certain aspects of that signal. This is in contrast to the other ma ...
s became commonplace in
electronic switching system In telecommunications, an electronic switching system (ESS) is a telephone switch that uses solid-state electronics, such as digital electronics and computerized common control, to interconnect telephone circuits for the purpose of establishing te ...
s of the 1980s. For example, in
5ESS switch
The 5ESS Switching System is a Class 5 telephone electronic switching system developed by Western Electric for the American Telephone and Telegraph Company (AT&T) and the Bell System in the United States. It came into service in 1982 and th ...
such jobs are done by DSPs in the Global Digital Services Unit (GDSU).
References
* The 5ESS Switching System (The AT&T Technical Journal, July–August 1985, Vol. 64, No. 6, Part 2)
*Multi-Frequency Pulsing in Switching (AIEE Transactions, Volume 68, Part 1, 1949, pp. 392–396)
* Multi-Frequency Pulsing in Switching (Electrical Engineering, June 1949, pp. 505–510)
Telephony signals
Telephony equipment