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In audio, a mix-minus is a particular setup of a mixing console or matrix mixer, such that the output to a certain device contains everything except the input from that device. This prevents echoes or feedback from reverberating or howling and squealing through the broadcast or sound reinforcement system.
A mix-minus signal is sometimes also known as a 'clean feed' or a 'Select Audio Return' (SAR), it can be written as N-1. It's worth noting that some broadcast mixing desks, notably those designed in house by the BBC maintain a separate mix bus for 'clean feeds'. This is technically different, and arguably superior to most implementations of mix-minus, but the end result is the same.
The most common example of mix-minus is when hooking up a telephone hybrid to a console, at a radio station. The person on the telephone hears everything playing, including the DJ, except that the caller does not hear his own voice. Mix-minus is also often used together with IFB systems in electronic news gathering (ENG) for television news reporters and interview subjects speaking to a host from a remote outside broadcast (OB) location. Because of the delay that is introduced in most means of transmission (including satellite feeds and digital fiber optic connections), the remote subject's voice has to be removed from his earpiece. Otherwise, the subject would hear himself with a slight (but very distracting) delay.
Another common example is in the field of sound reinforcement. Consider a room with sound stations for multiple users, each station containing a microphone and a loudspeaker. Such a room might be used in a government house of parliament. The microphone in station #1 would feed the loudspeakers in every other station except station #1. In other words, station #1 receives a mix of all microphones minus the station #1 microphone. This enables all participants to hear each other clearly but minimizes problems with acoustic feedback.
Mix minus is also used with VOIP calls such as Skype when recording for podcasts, the mix minus removes the caller's voice from the VOIP call, but allows them to hear all other channels available at the mixing desk (mixer).