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, 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.
The type kn ...
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 sampled, discrete-time signal to reduce or enhance certain aspects of that signal. This is in contrast to the other major type of electronic filte ...
s became commonplace in
electronic switching systems of the 1980s. For example, in
5ESS switch 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