in a series of occasional articles about basic broadcast concepts, for new LPFM
broadcasters and others who may be unfamiliar with industry terminology.
A “mix-minus” is a
mix of audio channels minus one (or
more) of them — subtracted for reasons that will become apparent. It
is an important component of broadcast audio. Personnel involved in engineering
for on-air broadcast or certain recorded radio program production must
understand the practice.
Telephone audio plays an important part
in radio. But using a telephone with a nontraditional audio board —
the typical inexpensive multipurpose audio mixer — can be a little challenging,
and confusing to a novice. Many people who are newer to radio operations or
engineering may not understand this concept of mix-minus and how it’s
used with a phone coupler, also called a hybrid.
|This setup allows a
standard mixer to be used with phone couplers to provide a mix-minus to
callers. Note Aux settings.
The coupler is a device that takes
sound from the caller and returns audio to the caller. It grabs or
“seizes” the phone call when selected and acts as the
interface between the actual telephone line and the console or mixer.
Traditional broadcast (standard radio)
consoles often have built-in, dedicated mix-minus outputs. These can be
assigned so the caller only hears the sound from certain mics or audio sources
(though never their own voice) via the coupler.
When a coupler is connected to an audio
mixer, you’ll need to provide a console output to a caller so they
can follow what is being said and respond. But you do not want to loop back (or
“return”) the caller’s own voice back to them;
doing so would create an audio feedback loop.
Most phone couplers provide a short
burst of noise when connected to the phone line that acts as a reference for
equalizing the audio from the caller for the best quality sound.
Couplers offer a control labeled
“null” that helps keep the return audio (to the caller)
from remixing back in with the caller’s own voice on the coupler
output. If the null didn’t occur, you’d hear the same
program material and mics going to the caller “bouncing
back” into the console/mixer but sounding tinny (“phone-like”).
The null control helps keep this minimized.
So now we know the phone coupler
returns sound from the console’s mix-minus to the caller, equalizes
the callers audio to the console and provides a null control of audio to/from
Some stations find themselves using
standard audio mixers as opposed to radio consoles. These mixers are designed
for use in a small recording studio, mixing a band or use in a radio program
production studio; they were not built for use in a live radio studio environment.
They lack features like speaker muting
and monitor, and they also don’t provide automatic mix-minus returns
for phone couplers.
|The Telos Hx1 hybrid
provides the phone interface for the mixer at
But this does not mean you
can’t use a mixer with a phone coupler; it’s just a matter
of figuring out how. Keeping in mind that a mix-minus is the mix without the
caller’s own audio, it’s easy to use a mixer’s
“aux sends” to make your own mix-minus.
Here’s how. Connect your Aux
1 output to send that audio to your caller. Then turn up all the Aux 1 pots
that you want to send to the caller other than the one from the phone
coupler’s channel (which would send that the caller’s own
voice back to them — defeating the purpose of the mix-minus). If you
add a second phone coupler, you can feed that phone coupler from your Aux 2
send and send that output (less their own Aux 2) to that
If you want the callers to hear each
other, just be sure the Aux 2 pot for Caller 1 is turned up and turn up Aux 1
for Caller 2. This allows each caller to hear the other without getting their
own voice returned.
The picture of a mixer surface shows
how Aux 1 and Aux 2 would be adjusted to allow each caller to hear the other
but without returning their own audio (assuming that the phone coupler for
Caller 1 is wired to Aux 1’s output and the phone coupler for Caller
2 is wired to Aux 2’s output). As phone sources are mono, a single
mono pot can be used.
The author is a longtime
radio/TV engineer. He helped found WDPE(LP) in Dover/New Philadelphia,