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Operating Manual - DPX-100 Graphic Equalizer - Compressor/Limiter
6. COMPRESSOR/LIMITER CONTROLS
6.1 GAIN
The Gain control is used to adjust incoming sig-
nal level to the VCA circuit. It is always active, so
switching out the limiter function has no effect on this
control. Used in conjunction with the input/output level
meter display, this control is useful for setting up optimal
system levels. This control should normally be left at
"0" to achieve accurate threshold calibration.
6.2 THRESHOLD
The threshold control has a range of -40dB to
+22 dB, allowing applications from low level compres-
sion to high level limiting. The threshold control de-
termines the audio level above which gain reduction
occurs. When the threshold LED comes on, that means
that gain reduction is beginning to occur, due to input
signal peaks exceeding the selected threshold in dB.
6.3 RATIO
This control determines the resultant change in
output level to changes in input level for all signals above
threshold. The numbers printed around the ratio con-
trol are calibrated in db and indicate the increase in
input (above threshold) required to produce a 1db in-
crease in output. This can be expressed conveniently as
a ratio. If the output remains constant no matter how high
the input level, we have an infinite (∞) input/output ra-
tio. It should be remembered that the ratio control has no
effect on signals which are below threshold.
There is a common but incorrect notion that lim-
iting always implies the use of an infinite ratio. Although
there are times when an infinite ratio is desirable, there
will be situations where infinite, or “hard”, limiting ac-
tion is neither appropriate nor necessary. In fact, it should
be noted that an infinite ratio setting is likely to cause
noticeable side effects in the sound, and may not be us-
able on programs where subtle control is desired.
6.4 ATTACK TIME
The response of the compressor/limiter to signal
levels above threshold is further defined by the attack time
control. Attack time is the amount of time it takes to
attenuate the output level after threshold has been
reached. For very fast transients, such as hand claps, snare
drums, or other percussive sounds, a fast attack time is
usually desirable so that the limiter can respond in time
to control the peak level. On other types of program ma-
terial, a slower attack time may be preferred. An abrupt
attack may, on some material, “square off” the top of a
waveform, producing a distorted sound. The DPX-100 pro-
vides continuously variable attack times from 200 micro-
seconds to 20 milliseconds.
6.5 RELEASE TIME
Another parameter which affects compressor/lim-
iter performance is release time, or the time required to
restore system gain to normal after the input signal
has fallen below threshold level. Again, proper release
time will depend on the type of program material being
processed and the way in which the limiter is being used.
When subtle limiting is desired, slow release
times are often chosen to avoid condition referred to as
“pumping” or “breathing”. This occurs when overall gain
is modulated up and down by repeated peaks which are
followed by quieter intervals. If the release time is set
too fast, then the overall level will jump up and down,
producing an objectionable and unsettling effect. Note
that, in some cases, an individual track or channel which
seems to be pumping may sound acceptable when heard
in context of a complete mix.
A unique feature of Ashly Compressor/Limiters
is the incorporation of a double release-time constant.
When a conventional compressor/limiter is adjusted for
slow release times, transients such as mic “pops” may
cause a severe reduction in gain followed by a slow fade-
up, making the action of the limiter very obvious. With
dB
∞
+20
+15
+10
+6
+3
0
-3
-6
-10
-20
-
.1
.2
.5
1
2
3
Sec
20
.5
.2 20
1
1.5
2
3
5
10
15
mS
2
∞
2.5
3
5
7
10
30
dB
+22-40
+20
+10
+6
+3
0
-3
-6
-10
-20
2Th. 4 6 8 10 12 14 16 18 20 -15-18 -12 -9 -6 -3 0 +3 +6 +9 +20
Gain Reduction (dB) Input/Output Level (dB)
Clip
Gain
Threshold Ratio Attack Release Output Level In
Power
Output
Input
dB
+15-15
+6
+3
0
-3
-6
-10
+10