in between two LED readings, position the control by interpolating, bearing in
mind that consecutive LEDs illuminate at 2 dB intervals. Having achieved
this setting of the LOOP CURRENT, check with headphones plugged into
the loop monitor output socket that a satisfactory sound quality is obtained
from the loop current. If a standard field–strength measuring unit is available,
then check that the field has the correct strength. From this point onwards,
the LOOP CURRENT control will not need re–adjusting, as this only affects
the peak field strength.
Before connecting to the sound system, check the entire sound system for
crosstalk from the loop into the audio inputs. While still playing a music tape
or CD, check every input circuit of the sound system, and if a significant
amount of signal is picked up, identify the reason and correct. This ensures
adequate stability for the complete system.
Connect the cable from the audio system to the line input, and readjust the
input gain control for optimum compression. If only the microphone input is
used directly, without another sound system, then set up the entire system
from a normal sound source driving the microphone. The level is set with the
“MIC” control. When using separate pre-amps, then adjust the gain controls
on these pre-amps, and the “LINE” gain control to achieve a satisfactory
balance.
6.2 Adjusting Metal Loss Correction.
6.3 Optimising setting of MIC / LINE controls.
In order to obtain the greatest possible dynamic control range from the
compressor, it is now necessary to establish the highest level of input signal
which the equipment may receive in the operational installation. This will
often be loud, close talking into a microphone. If the MIC / LINE gain is
adjusted so that the amber 36dB LED does not illuminate (just), then the
compressor will maintain the highest possible level into the loop for faint
speech, etc. Monitoring with the output headphones will indicate the clarity of
the signal under all levels of compression. It should be pointed out here that
Correct adjustment of the frequency response requires the use of
specialised test equipment to measure the frequency response of the
system, as specified by international standards (such as EN60118-4)
A simple method is to listen to the sound with a good quality receiver such as
the ILR2. Using the same headphones, listen first to the loop current signal
obtainable from the Loop Monitor outlet on the ILD252 with metal loss
correction set to minimum.. Then listen to the loop signal using the ILR2 and
adjust the “Loss Correction” to obtain a similar sound quality.
A full plot of the actual response can be made with suitable equipment, using
the CMR2 calibrated receiver. This can be done with Pink Noise, or a
frequency sweep (which must be done at 12 dB below normal maximum loop
current). Please contact Ampetronic Ltd for further advice.
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