A 'Mix-minus' is, as the term implies, a full programme mix but with one contributing source missing from the mix. Most commonly used in broadcast applications, the mix-minus feed is used as the return (foldback) monitoring signal to a remote contributor. The idea is that the remote contributor needs to hear contributions from all other sources being mixed into the programme, but must not receive their own contribution coming back as that could result in howlrounds and confusion due to transmission delays. In programmes involving multiple remote contributors, each would need to receive its own independently generated Mix-Minus signal.
A Mix-Minus signal can be generated in several different ways, but this term normally refers specifically to a technique whereby the remote contributor is sent the full programme mix with a second version of the remote contributor's signal added in opposite polarity, thus cancelling out the remote contribution. Another technique is to use the equivalent of an Aux bus which receives contributions from every source except the remote contributor. This approach is normally described as 'N-1' or 'Clean-feed' — but the results are exactly the same.