Use Compressor in Logic Pro for iPad

This section explains how to effectively use the main Compressor parameters.

Compressor Threshold and Ratio

The most important Compressor parameters are Threshold and Ratio. The Threshold parameter sets the floor level in decibels. Signals that exceed this level are reduced by the amount set per the Ratio.

The Ratio parameter is a percentage of the overall level; the more the signal exceeds the threshold, the more it is reduced. A ratio of 4:1 means that increasing the input by 4 dB results in an increase of the output by 1 dB, if above the threshold.

For example, with the Threshold set at −20 dB and the Ratio set to 4:1, a −16 dB peak in the signal (4 dB louder than the threshold) is reduced by 3 dB, resulting in an output level of −19 dB.

Compressor envelope times

The Attack and Release parameters shape the dynamic response of Compressor. The Attack parameter determines the time it takes after the signal exceeds the threshold level before Compressor starts reducing the signal.

Many sounds, including voices and musical instruments, rely on the initial attack phase to define the core timbre and characteristic of the sound. When compressing these types of sounds, set higher Attack values to make sure that the attack transients of the source signal aren’t lost or altered.

When attempting to maximize the level of an overall mix, it is best to set the Attack parameter to a lower value, because higher values often result in no, or minimal, compression.

The Release parameter determines how quickly the signal is restored to its original level after it falls below the threshold level. Set a higher Release value to smooth out dynamic differences in the signal. Set lower Release values if you want to emphasize dynamic differences.

Important: The results of your settings for the Attack and Release parameters depend not only on the type of source material but on the compression ratio and threshold settings.

Compressor Knee

The Knee parameter determines whether the signal is slightly, or severely, compressed as it approaches the threshold level.

Setting a Knee value close to 0 (zero) results in no compression of signal levels that fall just below the threshold, while levels at the threshold are compressed by the full Ratio amount. This is known as hard knee compression, which can cause abrupt and often unwanted transitions as the signal reaches the threshold.

Increasing the Knee parameter value increases the amount of compression as the signal approaches the threshold, creating a smoother transition. This is called soft knee compression.

Other Compressor parameters

As Compressor reduces levels, the overall volume at its output is typically lower than the input signal. Beyond the Output Gain control, you can adjust the output level with the Make Up slider.

You can also use the Auto Gain buttons to compensate for the level reduction caused by compression (choose either 0 dB or −12 dB).

When the Platinum Digital model is chosen, Compressor can analyze the signal using one of two methods: Peak or root mean square (RMS). While Peak is technically more accurate, RMS provides a better indication of how people perceive the signal loudness.

Note: If you turn on Auto Gain and RMS simultaneously, the signal may become oversaturated. If you hear any distortion, turn off Auto Gain and adjust the Make Up slider until the distortion is inaudible.

Use a side chain with Compressor

Use of a side chain with a compressor is common. The dynamics (level changes) of another channel strip is used as a control source for compression. For example, the dynamics of a drum groove can be used to rhythmically change the compression, and therefore dynamics, of a guitar, synthesizer, or bass part.

The side chain signal is used only as a detector or trigger in this situation. The side chain source is used to control the compressor, but the audio of the side chain signal is not actually routed through the compressor.

In Logic Pro, insert Compressor into a channel strip.
In the plug-in menu bar, choose the channel strip that carries the signal you want to use as the side chain source from the Side Chain pop-up menu.
Choose the Max or Sum analysis method with the Side Chain Detection buttons.
Adjust Compressor parameters.

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Logic Pro User Guide for iPad