BSS Audio DPR-402 Owner's Manual - Manual (Page 18)

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10.0 Compression Controls
In the DPR-402, the gain reduction is achieved using a Voltage Controlled
Attenuator with a range or operational 'window' of 30dB. This means that it is
capable of reducing the input signal by up to 30dB or 30 times (the GAIN
REDUCTION METER displays this window showing how much of it is being
used). The voltage level corresponding to the lower boundary of this window
is called the threshold, and input signals below this level cannot cause gain
reduction. When the input level enters the window by exceeding the
threshold, gain reduction can occur (the amount of gain reduction or
'compression' produced being directly proportional to the amount by which
input signal level exceeds the threshold). The upper boundary of the VCA
window is set by the circuitry at a level 30dB above the threshold. The
THRESHOLD control (See figure 10.1) adjusts the threshold relative to the
input signal in order to set the compressor to the required amount of gain
reduction. For example, if the average input is +12dBv and the THRESHOLD
control is set to -10dBv, then the top 22dB [12 - (-10) = 22dB] of the signal can
be compressed. The THRESHOLD control is adjustable from -30dBv to over
+20dBv.
It should be noted that when the THRESHOLD control is set fully clockwise to
the position marked OUT, this corresponds to a threshold level above +20dBv,
so that no matter how high the input level stage will clip before the threshold
is reached, the signal will reach the output uncompressed.
How the compressor performs on the signal once inside the window is
controlled by the RATIO, ATTACK, and RELEASE controls.
10.1 Threshold
Fig 10.1 Varying the
THRESHOLD controls
the VCA window on
the input signal
10.2 Ratio The effect of the ratio control (See figure 10.2) can be shown on a graph
which plots input level, and clearly shows that below threshold the DPR-402
acts purely as a linear amplifier (See figure 10.3).
In applications where gentle compression is required, it is advantageous to
change from the linear to compression region in a very gradual manner, rather
than the more conventional abrupt manner, as shown in the diagram. The
DPR-402 has been configured so that for low settings of the ratio control and
low levels of compression, the transfer is soft, and for increasing ratio settings
and high levels of compression the transfer becomes harder. This 'progressive
knee' gives inaudible compression for low levels of ratio and gain reduction,
whilst allowing harder compression for extreme control when required.
Fig 10.2 RATIO control
Compression Controls