Interruptible foldback
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IFB (interruptible foldback often misidentified as interruptible Feedback) systems are used in broadcast and motion picture production for crew communication, audio monitoring and cueing.
The IFB is a special intercom circuit that consists of a program feed sent to an earpiece worn by talent via a wire, telephone, or radio receiver that can be interrupted and replaced by a producer's or director's intercom microphone. On a television news program for example, a producer can talk to the anchors, to tell them when they are live on the air and when to begin reading off the script on the teleprompter or cue cards.
In electronic news gathering (ENG), the IFB can be set up through the station's private audio channel during live remote broadcasts. Program audio sent through the IFB, in this configuration, is in most cases a mix-minus; the complete mix of program audio minus certain elements. Because of the physics of electronics, all signals will suffer time delay as they travel through a wire, or during the conversion to radio frequency and then back to electronic audio, or in the conversion from analogue to digital signal and back again to the analogue realm. Certain audio elements that originate remotely from the mix point will be eliminated (minused) from the mix sent back to the remote site to avoid an undesirable echo. For example, a correspondent in Baghdad connects to the bureau in New York via satellite. The bureau in New York broadcasts the program including the Baghdad element. The Baghdad signal is subtracted from the IFB feed to Baghdad to eliminate an echo that would occur because the signal travels so great a distance first up to the satellite then down to earth (Baghdad to New York) then back up the satellite and then down to earth (New York to Baghdad).
IEM (in-ear monitor) refers to a system of small, usually custom fitted, earpieces and transmitters used to provide foldback that in the past was fed to speakers in front of a performer on a stage. Wireless in-ear monitor systems are increasingly popular with musicians, since they allow more precise monitoring and freer movement on stage than fixed monitor speakers, and can incorporate sound-insulated earbuds to provide hearing protection. For performances that include pre-recorded backing tracks, IEM let the entire band synchronize with a click track without wearing cumbersome headphones.
In-ear monitor can also refer to any headphone with sound-insulating earbuds which are placed inside the ear canal.
