The term mix-minus is often encountered in radio. Its most common application is in conjunction with on-air telephone systems. With the advent of ISDN codecs, mix-minus has filled a useful and necessary function as well.
To explain mix-minus, we'll use the telephone example. The analog telephone uses a single pair of wires to transmit and receive an audio signal. The telephone set itself separates this multiplexed signal to two signals, the send and receive audio. The send audio is your telephone mouthpiece; the receive audio is your telephone earpiece.
When a telephone call is used as an on-air source, the telephone interface -- called a hybrid -- handles the splitting and multiplexing of the two audio signals. The caller's audio is routed to the on-air console or program feed. That's the easy part. To interact with the caller, audio must be fed back into the phone line (send audio). It's often desired to send more than just the host's microphone back to the caller. It sounds simple to just send the program mix back to the caller, but doing so will create a feedback loop. Instead of sending the entire program audio mix, it is better to send the program audio mix without the caller's audio. In other words, the program mix minus the caller.
When a two-way communications link is established, such as an ISDN or IP codec, there is an inherent amount of delay through the link. Like the on-air telephone, it is necessary to send audio back and forth. While these links do no multiplex the audio into a single path like a telephone, the link introduce a small amount of time delay in the signal path. The audio from the field to the studio arrives at the studio a few milliseconds later. The same is true in the other direction. If the full program mix from the studio is sent back to the remote location with the remote audio included, the person at the remote site will hear his own audio delayed by several milliseconds or more. It is very distracting to talk while hearing yourself delayed.
Again, a mix minus -- the program mix minus the remote audio -- is useful.
A common way to create a mix-minus is to establish two mixes: the program feed and the mix-minus feed. They are identical except for the one audio source that is not wanted at the remote end. On consoles with bus assign switches, everything would feed program for the program feed, and everything except the remote audio (telephone call or codec) feeds the audition bus. The audition bus is then sent as the mix-minus feed.
On the remote end, the received mix-minus feed is mixed with the local audio feed and then fed to the talent's headphones to replicate the program feed.
The special mix is the cleanest way to create a mix-minus. There are products available to extract a mix minus as well. These products are fed the full program mix and the remote audio. The device then subtracts the remote audio from the program mix. This method has limitations and rarely creates a perfect mix-minus because some portion of the remote audio will be present, but in certain situations this may be a practical solution.
Mix-minus is sometimes referenced by other names. Clean feed, offline mix or select audio return (SAR) are three such names.