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Chapter 6: Compressor Control Functions
DN540
22 Operator Manual
Soft knee and hard knee
The point where the slope of the compressor curve changes is known as the “knee”.
The DN540 can be set to operate as either a soft knee or hard knee compressor; the
effects of both can be seen in Figure 5 “Soft knee and hard knee graphs”.
Figure 5: Soft knee and hard knee graphs
Soft knee compression gives a more gradual transition as the signal passes the
threshold (ratio changeover point). The ratio value of signals approaching threshold
increase exponentially in proportion to the input signal level. At threshold the ratio
level becomes that selected via the RATIO control knob. Soft knee is useful on
high-ratio compression or limiting, as it is less obtrusive than hard knee.
Most compression sounds more natural in soft knee mode and thus this is the default
setting. Soft knee compression blurs the distinction between over threshold and under
threshold signals, such that signals that are a long way below threshold remain
unaffected by compression and signals that near the threshold get compressed, but at
greatly reduced ratios. When signals are just over threshold the compressor ratios are
still somewhat reduced and it is only when signals go well over threshold that full ratio
compression is applied.
The soft knee character is produced after the envelope generation such that it also
modifies the envelope shapes and typically slows attacks on the programme when it is
in the knee area. This effect is similar to many vintage compressors (where they
exhibit soft knee characteristics due to non-linearities in the gain reduction element)
and provides a very natural sounding compression, generating more full bodied punch
and definition when attack times are deliberately set very slow.
In hard knee mode the compressor operates as soon as the input signal reaches
threshold level and is better suited to a limiting style of compression. This gives a more
defined transition between under threshold and over threshold, imposing the new ratio
— selected via the RATIO control knob — on the signal. This mode is useful as a brick
wall limiter, which stops transients without affecting the lower level signals. In this
mode a small amount of soft knee is still retained — to keep the sound reasonably
natural — but it does not modify or slow down the envelope, allowing fast acting
limiting if desired.
The difference in the effect produced by soft knee and hard knee compression is more
noticeable at higher ratios.
Increasing input signal level
1:1
10:1
Hard knee
Threshold
Output signal
Soft knee
10:1
Increasing input signal level
1:1
Threshold
Output signal