Telephone Balance Unit

From IBS_FAQ

A Telephone Balance Unit (TBU), sometimes referred to as a Hybrid (an abbreviation for Hybrid Transformer), is basically a 2-4 wire converter which provides an interface between the balanced 4-wire circuits in a broadcast studio and the 2-wire system used for telephones. A TBU has a balanced input to accept a feed from the studio mixer, a balanced output to send the telephone call to the mixer, and a connector for a telephone line.

Internally a TBU may use either analogue or digital technology; in either case the electronics is designed to cancel out the unwanted signal and provide a clean feed of the caller to present to the mixer.