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In analog telephony, a telephone hybrid is the component at the ends of a subscriber line of the public switched telephone network (PSTN) that converts between two-wire and four-wire forms of bidirectional audio paths. When used in broadcast facilities to enable the airing of telephone callers, the broadcast-quality telephone hybrid is known as a broadcast telephone hybrid or telephone balance unit.
The need for hybrids comes from the nature of analog plain old telephone service (POTS) home or small business telephone lines, where the two audio directions are combined on a single two-wire pair. Within the telephone network, switching and transmission are almost always four-wire circuits with the two signals being separated. Hybrids perform the necessary conversion. In older analog networks, conversion to four-wire was required so that repeater amplifiers could be inserted in long-distance links. In today's digital systems, each speech direction must be processed and transported independently.
The line cards in a telephone central office switch that are interfaced to analog lines include hybrids that adapt the four-wire network to the two-wire circuits that connect most subscribers.
The search for better telephone hybrids and echo cancelers (a related technology) was an important motive for the development of DSP (digital signal processing) algorithms and hardware at Bell Labs, NEC, and other sites.