Soft knee and hard knee

Chapter 6: Compressor Control Functions

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”.

Threshold	10:1

1:1	Soft knee	signal

		Output

Increasing input signal level

Figure 5: Soft knee and hard knee graphs

Threshold		10:1

1:1	Hard knee	signal

		Output

Increasing input signal level

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.

	DN540
22	Operator Manual