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The telephone network employs digital echo cancellers at various nodes along the path of a phone call to avoid this
scenario. And when they malfunction or are “untrained” at the start of a call, the effect is a dramatic echo in the
caller’s ear.
Many users installing a studio-based phone system for the first time make the mistake of applying audio to the
outgoing “send’ port that contains the main program feed - the same audio used to feed the transmitter or
webstream. Since this mix contains the caller’s own audio, and there’s an inherent delay in modern digital systems,
the “slapback” effect is immediate.
The solution here is mix-minus-- a term used for a special mix of audio that explicitly excludes one source--the
audio coming from the place the mix-minus is being sent. To put it another way, mix-minus is the entire studio mix
minus one audio source.
So how do we create this special audio mix? On modern studio systems, this is usually well defined and easy to
do. Many consoles feature channels dedicated to telephone interface, and part of the channel is an automatically-
created mix-minus output.
In less full-featured consoles, a mix-minus can often be created with an auxiliary or “audition” bus function. By
selecting all relevant incoming sources on the bus except for the telephone fader, you can do this easily. The
following figure shows the block diagram of a single mix-minus feed being generated on a mixing console.
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