Multi-frequency receiver

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."

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.

Digital filters 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).