Philips LBB 1330, Инструкция по эксплуатации

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5. SM40 MODULES
5-33
5
5.9.1 Automatic Noise Dependent Volume Control (AVC)
Automatic Volume Control Card LBB 1359/09 or LBB 1359/21
Adjustments: ‘Max Call Level’ potentiometer:
1. Set SENSE GAIN potentiometer to maximum (anticlockwise). Set BLOCKING potentiometer to short
(clockwise). Produce a constant audio signal at the sense input. Any level above 10 mV will produce a 5
Volt DC voltage on jumper J3 resp. J7. Remove jumpers J1 (CH.A) and J5 (CH.B) to interrupt the 0 dB
bypass
2. Produce a constant audio signal (e.g. 300 mVolt) at the input of the AVC channel and check its output
level of that AVC channel. The level should be equal to the to the input signal. Correction can be made
by adjusting the MAX. CALL LEVEL potentiometer.
3. Check the control range of 9 dB (or 21 dB) by viewing the output level while turning the SENSE GAIN
potentiometer to min (clockwise), or by disconnecting the audio signal at the same input.
Adjustment procedure for Automatic Volume Control (actual situation):
1. Set the SENSE GAIN potentiometer and BLOCKING potentiometer to minimum (clockwise). Remove
jumpers J1 (CH.A) and J5 (CH.B). The bypass path is interrupted. The AVC channel now remains on a
reduced level, but still senses continuously (not blocked).
2. Check the loudness of a regular call in the AVC controlled loudspeaker zone. Ensure the listening area is
quiet i.e the ambient noise level is lower than 60 dB (SPL). The 9 dB (or 21 dB) reduced call level should
be at a comfortable listening level (CLL) of 80 dB (SPL) peak-level or according to the specified level.
If this level cannot be achieved, the selected AVC control range is incorrect.
3. Check the ambient noise sensing microphone connection from the relevant loudspeaker zone to the sense
input of the AVC channel. Set the SENSE GAIN potentiometer to maximum (anti-clockwise). Produce a
pink noise signal at the input of the AVC channel. To produce a pink noise signal activate test signal 27
from the SM40 Signal Generator Card (SGC)
Measure the pink noise signal in the area of the relevant loudspeaker zone. Adjust the disturbing LS-noise
level to the level specified as the highest ambient noise level - for example 75 dB (SPL).
Adjust the SENSE GAIN to the point where channel gain reduction begins. (See VU meter of the
channel amplifier or monitor the DC voltage on Jumpers J3 (CH.A and J7 (CH.B).
4. Put the sample BLOCKING potentiometer to its mid position. This allows approximately two seconds of
silence before the AVC sensing alter the call output level. Turning the BLOCKING potentiometer further
in a clockwise direction, shortens this time, thus allowing readjustment of the level during shorter call
pauses. If the re-verberation time in the loudspeaker zone is too long,the sensing microphone could sense
the reverberation of the call as ambient noise. This will cause the AVC to compensate for its own calls,
therefore creating instability.
5. Carry out a listening test in order to check that it functions correctly.
Note: Jumper J1 (CH.A) and J5 (CH.B) should remain removed.