a hybrid’s ability to isolate the transmit signal from the receive signal in the 2-wire-to-4-wire interface is critical. The quality of this isolation is technically known as return-loss. A side tone nulling control fine tunes the circuitry to best match the devices to the acoustic conditions near the intercom, as well as to the electronic conditions on the intercom line. They should be set at the time of system installation and adjusted as is comfortable for the user. This hybrid circuit connects the four-wire audio to the single wire in such a way as to variably restrict the user’s reception of his own voice on the intercom line, which is often referred to as side tone.