Multi-frequency receiver

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" 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. In 1940s technology, receivers in 4XB and similar equipment used vacuum tubes. Later ones used RC filters and transistors. Digital filters became commonplace in electronic switching systems of the 1980s. For example, in 5ESS switch such jobs were done by DSPs in the Global Digital Services Unit (GDSU).

Receivers

One example of an MF receiver built 35 years after the format's original US use is a Digital Signal Processor (DSP) based printed circuit card designed at Computer Consoles Inc. (CCI) in Rochester, NY. The MFR was a service circuit module developed as part of the companies digital telephony switching system. The main function of the card was the processing of incoming digital audio to detect the presence of various voice-band tone base signaling systems present during a call connection. The CCI digital switch was deployed as part of the Digital Audio Intercept System (DAIS II), Automatic Voice Response (AVR), and Interactive Voice System (IVS) products. The initial MFR board, MFR-I, is notable for being designed in early 1983 prior to Texas Instruments officially announcing the TMS320C10 DSP April 8, 1983.

MFR-II Printed Circuit Card, circa 1985

Variants

Features

References

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