Handbook for Sound Engineers

746 Chapter 21

Permanent Activation of an Input Channel. In a
house of worship, a microphone over the congregation
could be kept on at all times to provide constant room
ambience for a hearing assistance system.

Routing of Input Channels to Different Outputs. In a
hotel meeting facility with movable dividing walls,
input channels could be sent to different banks of loud-
speakers depending on the room configuration.

Status Terminal. As input channels are activated and
attenuated by the automatic mixing process, it is valu-
able to have a status terminal that indicates if a particu-
lar input channel is activated or not. This status
terminal, also known as a gate terminal, can be thought
of as an electronic switch that changes from open to
closed based on the activity of the input channel. Exam-
ples of use for a status terminal:

• Control of an LED or lamp to indicate input
  channel activity. In a city council chamber, a council
  member could have a tally light located near the
  microphone indicating when the microphone’s input
  channel is activated by the automatic microphone
  mixer.


• Control of a relay used to attenuate the nearest
  loudspeaker. As the typical feedback path in a sound
  system is between a microphone and the nearest
  loudspeaker, attenuating the closest loudspeaker
  when a microphone is active could improve gain
  before feedback.


• Control of a video switcher connected to multiple
  cameras. In a courtroom, the proceedings could be
  videotaped by using cameras that follow the activa-
  tion of input channels by the automatic microphone
  mixer.


• Mute other input channels. In a hotel meeting
  facility, one input channel could override all others in
  case of an emergency announcement.
  Combining the externally controlled functions with
  the status terminals provides hundreds of unique system
  configurations. Most manufacturers of automatic micro-
  phone mixers have documentation of such configura-
  tions, often printed in product installation manuals and
  available on the manufacturer’s web site. As previously
  noted, the communication protocol used to interpret the
  status terminals and control the mixer functions depends
  upon the manufacturer and model of the automatic
  microphone mixer.

21.3.3.3 Examples of Communication Protocols Used

in Automatic Microphone Mixers

Contact Closure Protocol. The most basic of commu-

nication protocols, contact closure is provided by a sim-

ple single pole/single throw (SPST) switch or relay. The

switch is connected to two terminals on the mixer that

control a certain function —e.g., mute of an input chan-

nel. When the switch is closed, the input channel is

muted. When the switch is open, the input channel is

unmuted and can be activated.

Resistance Change or Voltage Change Protocol. Used

primarily to control signal levels via a VCA (voltage-

controlled amplifier), this protocol requires that defined

changes in resistance or voltage be applied to the

mixer’s control terminals. In response, the VCA in the

automatic microphone mixer will change the level of

the audio signal.

TTL (Transistor-Transistor Logic). An electronic

protocol established in the 1960s, TTL is simple to use.

A control terminal on the automatic microphone mixer

has one of two states: logic high (+5 Vdc) or logic low

(0 Vdc). A status terminal could be logic high when a

mixer input channel is attenuated and logic low when

the mixer input channel is activated. This change of

voltage informs an external control device that there is a

change in the input channel status and some predeter-

mined action should take place —e.g., illuminate an

LED or switch on a camera.

RS-232. Used for communication with a computer,

RS-232 is another common electronic protocol. RS-232

is most often used when proprietary control software is

supplied with the automatic microphone mixer or when

the mixer is connected to a control system such as those

manufactured by Crestron or AMX.

RS-422. Basically a balanced line version of RS-232,

RS-422 is designed for situations where an extremely

long cable run must be used to connect the automatic

microphone mixer to the external control device.

21.3.3.4 Number of Open Microphones Attenuation

(NOMA)

NOMA is a function shared by all well-designed auto-

matic microphone mixers. It is a simple method of

ensuring system stability by automatically reducing the

mixer output gain in proportion to the number of acti-

vated input channels. NOMA offsets the increase of

gain that occurs as more microphones are activated. The

attenuation in decibels should vary as: