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 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.