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USING THE CMA: REI
Single Line and Balanced Line Connections
The CMA provides for connections to either single line systems (typically a single line
with a ground such as coax) or a balanced pair line such as telephone or LAN wiring.
For Single Line systems:
• Connect the Green grounding connector to the Black connector.
• Connect the Black connector to wiring ground.
• Connect the Red connector to the single line to be tested.
For Balanced Pair systems:
• Connect the Green grounding connector to earth ground if available (if not
available, do not connect to anything).
• Connect the Black connect one of the wires to be tested.
• Connect the Red connector to the other wire to be tested.
Signal Power Levels
Signal power levels may vary dramatically. The CMA has two systems that allow the unit
to respond automatically to a wide range of audio levels. The automatic input attenuator
prevents gain distortion for situations when the input voltage levels exceed 0.3 volts.
But, more importantly the Automatic Gain Control Function has a dynamic range of
105dB. In basic terms, this means that that the unit can automatically adjust to audio
signals ranging from 2 microvolts to 0.3 volts.
Strong Signal Interference
If the input is attached to an AC voltage greater than 40 volts, the audio content will be
masked.
Signal Frequency Interference
Wiring in a building is very susceptible to interference from ambient electromagnetic
noise sources. The most common type of audio interference is from the AC power lines.
This noise is typically either 50 or 60Hz fundamental frequency; however, there are
harmonic frequency components that also cause interference at 100/120Hz, 150/180Hz,
and possible up to 200/240Hz. For this reason, the CMA has a built-in High Pass filter
that filters out audio noise below 320Hz.
Also, high frequency electromagnetic noise can interfere with audio signals. This type of
higher frequency noise can be generated from industrial machinery, computers, fax
machines, and digital phone systems. Hence, the CMA has a built-in High Pass filter that
filters out audio noise above 3,200Hz.
Providing DC Bias
It is a common practice to utilize electret type microphones for audio surveillance, and
electret microphones require a voltage supply in order to function. Therefore, when
evaluating suspect wiring, if the wiring does not have any voltage on the line, it is
recommended to apply a DC bias voltage to activate any potential microphones. The DC
Bias Voltage should only be used after ensuring that the line does not have an active