Mix Minus describes the arrangements used when a broadcast studio is connected to an external site such as an Outside Broadcast as a contribution or another studio which is simultaneously broadcasting the same programme in another region, two way working. In both cases the remote source needs a feed of the programme but not of its own contribution, otherwise feedback or howlround would occur, and with distant contributions, delays in headphone feeds would be disruptive for the presenters. The BBC use the term Clean Feed for mix minus, while many console manufacturers use the term N-1.
The mix minus signal can be derived in several ways. Some mixing desks have dedicated channels for outside sources, others have the ability for any channel to be used as one. There can be a Matrix, either computer controlled and electronic or a physical wired matrix with an associated Jackfield which requires plugging with appropriate sources from the desk and the outputs passed to the correct destination. The use of a jackfield matrix is very flexible but does require careful thought. Many matrices have a “programme” input that is fed to all outputs. Obviously this must not be fed with the main mix as that will contain all outside sources. The feed to the outside source is called the cue feed, and may be of programme quality (for two way working) or more usually of reduced quality via a telephone line. Dedicated switches can provide individual Talkback to each cue feed.
If more than one remote source is required, as is often the case with phone-in discussion programmes, a phase cancellation system may be used. This uses a single bus to which all the channels required in the clean feed mix are selected, including the remote (phone-in) channels – as each caller may need to hear all the other callers as well as the studio contributors. The required clean feeds are manufactured in each channel by mixing output from the clean feed bus in anti-phase with that channel’s output, thus canceling out the signal from that channel.
Clean feed and mix minus normally refer to high quality programme paths but the Telephone Balance Unit performs essentially the same function – separating local and remote contributions.