"Mix-minus" is one of the terms most often used during the installation of a broadcast-to-telephone interface. Unfortunately, it's also one of the most confusing. This is a brief primer on mix-minus to help you avoid frustrations during your next installation.
"Mix-minus" refers to the signal that feeds the hybrid "send" input. The "send" is the audio that is fed back down the phone line to the caller. It's called mix-minus because it is the mix of all the sources you want to feed the phone minus the phone hybrid output. Note that the hybrid output is the caller audio.
A typical mix-minus circuit.
There are two related reasons you need this special signal. First, you want to feed the caller an audio signal so that he or she can hear what is going on in the studio. Most stations feed the entire on-air mix except, of course, the caller. This allows the caller to hear everything over-the-air listeners hear. Others set up a special mix to use as a mix-minus that may consist only of the host's mic or selected mics.
The other reason for a mix-minus is the nature of the telephone hybrid. The hybrid is designed to separate the caller audio from the signal being fed from the studio to the caller. The hybrid's output should consist of as much caller audio and as little studio audio as possible. If caller audio is fed into the hybrid send input, the hybrid is unable to properly perform this "subtraction." Sending the caller audio into the hybrid input also creates a "feedback loop" similar to the effect encountered when you send the output of a recording tape machine to its input.
Audio signals used for the mix-minus buss should be "pre-fader" so that levels remain constant. If possible, mic processors should not be included in the mix-minus buss as their automatic gain control (AGC) can "fight" the AGC performed by the hybrid. Note that Telos digital hybrids use software to set the level of the audio sent back down the phone line at the maximum permitted by the FCC. Unlike analog couplers, increasing the level of the input to the hybrid does not send more signal down the line. Therefore it is important to send proper, consistent level to the hybrid.
If you are installing more than one hybrid, you need to create a separate mix-minus feed for each hybrid. Keep in mind that you want each caller to hear all of the other callers. The best place to start is to create a mix-minus just as you would for a single hybrid. Then for each hybrid add the outputs of all the other hybrids. For your program mix, treat the hybrid outputs as simply additional audio sources. It can get a little complicated, but we have found that a quick signal flow sketch before starting installation can be a big help.
Telos ONE plus ONE
Most modern broadcast consoles make some provision for at least one mix-minus. Don't panic when confronted with a console with insufficient mix-minus capabilities! There are a number of inexpensive, off-the-shelf "splitter/mixer" products that can take care of your needs.
For example, The Telos ONE plus ONE (shown at left) has an internal mix-minus matrix that cross-couples the outputs of the two hybrids. In a dual hybrid application, you only need to send the ONE plus ONE a single mix-minus. All Telos hybrids have dual outputs to help create multiple mix-minuses.