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11
GE N E R AL IN TERFACES
11 RB-MM1 Mix Minus Generator
Introduction
Fig 11-1: RB-MM1 Front Panel.
Whenever programming originates from outside of the studio, or if listeners/viewers are
calling up a phone-in using telephone hybrids or codecs, mix-minus feeds are required.
Most telephone lines incur delays which prohibit off-air monitoring, because the caller, or
remote talent, would hear their own voice in delay which is very disconcerting. The solution
is to feed a mix back to the caller minus his or her own voice. Some mixing desks do not
have a dedicated telco channel to generate a clean-feed, or mix minus, so the RB-MM1 can
be used.
The RB-MM1 is a unit for generating a suitable mix to send to a telephone hybrid or codec.
A stereo output is taken from a mixer, together with a post fader output from the mono
telephone fader on the mixer. The caller audio is removed from the station output so that it
can be sent to the telephone line via the hybrid.
Analogue audio inputs and outputs are via Neutrik XLR connectors. The output level to the
TBU can be adjusted using a rear panel pre-set potentiometer. To control the cancellation
null, 2 multi-turn potentiometers are provided, one for the LF null and the other for the full-
band null. Additionally, a band pass filter can be switched in and out, via a rear panel switch,
to condition the signal for the telephone hybrid. To use the RB-MM1 unit with full-band
ISDN codecs, the band pass filter can be switched out. For stereo codecs, or conference calls,
multiple RB-MM1 units can be used.
A red LED indicates when power to the RB-MM1 is on.
System Block Diagram
Inputs
Balanced Stereo
Input From Mixer
Balanced Mono
Input From
Telephone Fader
Fig 11-2: RB-MM1 System Block Diagram.
64
L
R
Null
Filter
LF Null
Switch
Bandpass
Filter
Output
Output
Level
Balanced Output
To TBU
Redbox User Handbook No 3
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