Stereo and Mono Sidechain Compressors for Tracktion

Compression Ratio

The compression ratio compares the change in input level with the change in output level. From the graph on the previous page, the input range from 0 dB to the threshold is –30 dB. From the graph, this input range is compressed into an output range of –12 dB, and the ratio is 30/12 (= 2.5/1).

If we select the E node, the ratio in our example is displayed as 2.50:1. (For signals above the threshold, a 2.5 dB change in input level will yield 1 dB change in the output level.)

The displayed ratio is the ratio of the line segment to the left of the selected node.

Negative compression ratios can also be shown. This is useful mainly for special effects or repair work. In this case, a node appears lower in the graph than the one to its immediate left so its line segment is going downwards. For negative compression, the ratio is negative, for example, –2.5:1.

The ratio can display compression or expansion. When a line segment represents expansion, the ratio is displayed as, for example, 1:2.5. (See the page 21 for more about expansion.)

Gain Makeup and Auto Makeup

Gain Makeup is used to make up for volume that has been decreased due to compression. The Gain Makeup level ranges from –15 dB of attenuation, to +15 dB of gain, and can be adjusted by moving the slider to the left or right .

When AUTO MAKEUP is IN, the compressor automatically chooses an appropriate makeup gain for you. This is based on your dynamics contour. You can still use the ‘manual’ gain makeup control in this case. It merely adds to the makeup gain automatically applied. When you first set the AUTO MAKEUP to IN, the compressor adjusts your makeup gain parameter so that the volume does not suddenly jump up. This is hearing (and speaker) protection!

Attack and Release

Attack specifies how quickly gain reduction (or expansion) occurs at the attack, or beginning, of a signal crossing the threshold.

Release controls how quickly gain reduction backs off when the signal drops back down. The time is measured in milliseconds, and corresponds to the length of time it takes to achieve a fixed amount of compression (or expansion).

In general, the attack time should be much faster than the release time to get pleasing re- sults. If the attack time is set too quickly, you hear something resembling clipping on attacks, especially with low-frequency signals. Leaving the attack a little bit longer also allows more of the ‘snap’ of the attacks to pass through the compressor. Setting a release time too fast can result in ‘buzzing’, especially on low notes. This occurs because the compressor gain is actually following individual cycles of the incoming waveform.

If you set the attack time to 0 ms, the dynamics section will compress everything over the threshold, allowing no transients through un- processed. The amount of compression depends on the ratio.

Generally, lower frequencies require longer release times than higher frequencies. For high- or mid-frequencies, if the release time is too long, you may hear the compressor ‘breathing’.

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User’s Guide