Advanced Video Camera and Editing

Compressor/Limiter Filter

Using this filter requires an understanding of an audio track's dynamic range.

Dynamic range is the difference between a clip’s softest audio level and its loudest.

Wide Dynamic Range – an audio clip that has both very loud and very quiet parts.

Narrow Dynamic Range – an audio clip that plays at largely uniform volume.

The audio levels for a clip with wide dynamic range can be difficult to set, but not impossible. First, you would need to narrow the range, or compress it, bringing the loud and quiet parts of the audio closer to the average level of the clip. To compress the audio’s dynamic range, use the Compressor/Limiter filter.

The Compressor filter reduces the dynamic range so that the quietest and loudest parts of the audio all occur at roughly the same level. This is done by attenuating the loud parts until they are nearly the same volume as the quiet parts. Once that’s done, you can raise the overall level to whatever volume you prefer.

  • Double-click the audio clip so that it shows up in the Viewer
  • Choose Effects > Audio Filters > Final Cut Pro > Compressor/Limiter
  • In the Viewer click on the Filters tab

The Compressor/Limiter filter defines the target maximum level using the Threshold (db) slider. The Ratio slider defines how aggressively the area above that level is reduced. The other two sliders – Attack Time and Release Time – affect how quickly the effect is applied and removed. These you can ignore for now.

When you apply audio filters the best way to hear the changes in the audio is to play the clip in loop playback.

Playing Clips in Loop Playback in the Timeline

  • park the playhead over the clip that you wish to modify
  • Press X to mark the clip (setting an In Point at the beginning and an Out Point at the end
  • Choose View > Loop Playback, or Press Control-L
  • Press Shift \ (backslash) to Play In to Out

Setting the Compressor/Limiter Filter

Go back to the Filters tab so you can adjust the sliders:

First, play the clip in loop playback. Then raise the Threshold to 0db and the Ratio to 100. Setting the ratio at its highest value will help you hear the effect of the filter. 

  • Lower the Threshold (dB) slider slowly and listen carefully. You should begin to hear an effect near -30 dB. The further you go, the quieter the clip seems to get. If you watch the audio meters, you can see the dynamic range being reduced.
  • Set the Threshold to approximately -42 dB.
  • Lower Ratio to 1. At a ratio of 1 the filter has no effect, but you can raise it slowly until you achieve your desired effect.
  • Select the Preserve Volume checkbox. When this box is enabled, the clip level is gained up to compensate for the amount of applied compression. The result is that the more compression you apply, the louder the clip.
  • Drag the Ratio slider slowly from 1 to 4. You can now hear significantly the compression affecting the clip.
Now that you've compressed the dynamic range, apply normalisation gain.