Intro to the Compressor effect in Final Cut Pro
The Compressor effect is designed to emulate the sound and response of a professional-level analog (hardware) compressor. It tightens up your audio by reducing sounds that exceed a certain threshold level, smoothing out the dynamics and increasing the overall volume—the perceived loudness. Compression helps bring the key parts of an audio clip into focus, while preventing softer parts from becoming inaudible. It is probably the most versatile and widely used sound-shaping tool in professional mixing, next to EQ.
You can use Compressor with individual clips, including voice, instrumental, and effects clips.
Setting the Compressor threshold and ratio
The most important Compressor parameters are Threshold and Ratio. The Threshold sets the floor level in decibels. Signals that exceed this level are reduced by the amount set as the Ratio.
The Ratio 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.
As an 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.
Setting suitable 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, you should set higher Attack values to ensure that the attack transients of the source signal aren’t lost or altered.
When attempting to maximize the level of an overall mix, it’s 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 a lower Release value if you want to emphasize dynamic differences.
Important: The discussion above is highly reliant on not only the type of source material, but also the compression ratio and threshold settings.
Setting the 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.
Setting other Compressor parameters
As Compressor reduces levels, the overall volume at its output is typically lower than the input signal. You can adjust the output level with the Gain slider.
You can also use the Auto Gain parameter to compensate for the level reduction caused by compression (choose either −12 dB or 0 dB).
When you use the Platinum circuit type, Compressor can analyze the signal using one of two methods: Peak or root mean square (RMS). Although Peak is more technically accurate, RMS provides a better indication of how people perceive the signal loudness.
Note: If you activate Auto Gain and RMS simultaneously, the signal may become over-saturated. If you hear any distortion, switch Auto Gain off and adjust the Gain slider until the distortion is inaudible.