BSS Audio
DPR-402 User Manual v3.0
Page 15
11 Compression meters
11
.1 Below Threshold and Gain Reduction Meter
Referring back to
section 10.1; Threshold
, where we discussed the VCA window of
operation, the five LEDs of the BELOW THRESHOLD meter will give you an
indication of the input signal in relation to this window.
You will notice that the LED marked 'TH' is half-on all the time. This point
represents the threshold point as set by the THRESHOLD control, and any signal
level that exceeds this LED will start the compressor operating. Rotating the
threshold control anticlockwise effectively lowers the window of the VCA,
allowing it to sit at the right point on the input signal. Observation of this meter
during program will give an instant picture of how much of the input signal is
being processed, or how near the peaks are to being processed. This will provide
valuable information at all times during use, especially for live concert work,
where signal levels tend to increase during the course of the show.
Once the input signal has exceeded the threshold point on scale, the compressor
starts to operate, and gain reduction will occur. The amount of gain reduction
being used is displayed on the GAIN REDUCTION meter.
Gain reduction is a useful way of expressing compressor action. We have seen that
the output level of a compressor is less than the input level by some amount that
depends on the threshold, ratio, attack and release time settings. If, for example, a
particular signal transient exceeds the threshold point by 10dB and the ratio knob
is set to 2:1, then we would expect the output to have only increased by 5dB
(providing the time controls are set accordingly). Assuming the gain control is at
0dB, the difference between the input and output levels of 5dB then represent the
amount of gain reduction which has occurred and will be displayed on the gain
reduction meter as 5dB.
The range of the gain reduction meter is set to display the 27dB operating window.
In practice, over 30dB of range is available before noticeable distortion occurs.
Another piece of information also displayed by these meters is the amount of gain
reduction still available from the VCAs window of operation. If you consider that
the input signal is of such a level that 15dB of gain reduction is occurring (i.e.: the
15dB LED on the gain reduction meter is on), then the length of GAIN
REDUCTION meter display remaining unlit is an accurate visual indication of how
much headroom still exists within the circuitry. Observing this information will
ensure that the output signal is unlikely to incur distortion as a consequence of
overloading the VCA.
A further operational feature of this meter is its ability to intensify brightness when
the VCA window is in danger of being exceeded and distortion is likely to occur.
Input signals causing an excess of 30dB of gain reduction will initiate the 'bright-
up' and the display will appear to flash to alert attention. This feature is especially
