Expert Mode

If Expert mode is activated with the button under the peak level meters, the animated level display slides away to reveal controls that makes FabFilter Pro-C shine. In Expert mode, input and output controls get panning options and Pro-C is now capable of side-chain compression, both with filtered or external side-chain inputs, but also mid/side compression, mostly used in mastering studios.

Before you read on we recommend you to read Advanced compressor techniques and Mid/Side introduction.

Internal/external input

Normally, the side chain receives input from the regular plug-in input signal. In this case side-chain should be set to In. Alternatively, the side-chain can receive its signal from any other track from your DAW. In this case the side-chain should be set to Ext. (In VST hosts, the Ext option can be disabled. See VST plug-in versions.) Use the Audition button to listen to the side chain signal exclusively.

Filtering

You can filter the side chain signal with steep 48 dB/octave low-pass and high-pass filters. To adjust the filters, drag the triangular buttons at the buttom of the side chain section. The filters will be bypassed completely when they are at the far left and right sides of the filter controller. To drag both filters at once, either hold down the Alt/Option key or drag the highlighted area between the buttons. Again, use the Audition button to listen to the filtered side chain signal.

Side chain gain

The side chain can be used very precisely because of its unique gain structure. In Pro-C it is possible to use stereo compression stereo-linked or unlinked and best of all, you can have anything in between. By using the gain and panning knobs it is very easy to precisely determine the level and channel which will be sent to each level detector of each channel.

If you put both volume levels at zero dB and both panning knobs in center position, Pro-C will be stereo linked because both channel level detectors get the same audio signal. This means that if the left channel's signal rises above the threshold, the right channel's gain will react the same way and will be reduced by the same amount as the left channel, and vice versa. This keeps the stereo image from wandering from left to right when compressing a stereo mix.

If you then only turn the panning ring for the left signal to the full left position and the right panning ring to the full right position Pro-C is unlinked. Now compression on both channels will work independently. (You can hold down the Alt/Option key to link both panning rings.)

It is important to understand that all other settings will result in anything between linked and unlinked (but maybe also cross-channel-compression).

Mid/Side compression

Pro-C has a special feature that sets it apart from most other compressors: Mid/Side compression. To turn Mid/Side mode on, click the M/S button. The red indicator tries to make it clear that we're now using the two compression channels for two totally different signals.

Let's begin with a simple example: Compress the Side information and leave the Mid unprocessed. First enable Mid/Side compression with the M/S button. Now for the Mid channel level detector turn the inner level knob to the full left position. Leave the Side level knob in the center position, but turn the Side panning ring fully clockwise to the S indicator:

This means that the input for the Mid channel level detector is reduced by -36 dB, so the Mid channel will not be compressed. The Side channel will be compressed, but the Side channel level detector will only take information from the Side part of the side chain input (this is determined by the panning ring).

The gain reduction meter shows that there is no compression applied to the Mid channel. If you vary the gain of the Side channel you'll hear a change in stereo spread.

Suppose we want the opposite: compress the Mid information and leave the Side unprocessed:

To eventually hear the right mix of the Mid and Side channels you can adjust the output panning ring. Now this can act like a stereo enhancer! Play with these settings and you'll feel inspired for some great sounds or you can surgically treat your audio any way you want. There is a lot you can achieve with FabFilter Pro-C. This is not only interesting for compressing material with much Mid/Side difference, but also great to "duck" the Mid or Side with another track as its side-chain.

Here is one last example that compresses only the Side input signal using the Mid side chain signal:

Panning

Sometimes you'll find that your input signal is not balanced evenly between the right and left channel. This can make your compression work erroneously. The panning ring of the input knob makes it possible to make sure the compressor works on a evenly balanced stereo source. In addition to this you can also balance the output of the compressor and the unprocessed signal independently. When Mid/Side is used it controls the mix between Mid and Side signal.

Notes

The side chain level knob and panning ring at the left side of the side chain section control the compressor channel that works on the Left or Mid channel. Likewise, the righthand knob and ring control the compressor channel that works on the Right or Side channel.
The panning rings in the side chain section determine from which channel mix the L/M or R/S detector takes its input.
If the side chain level panning rings are in the center position, L/R and M/S processing give the same result, because the channels are always fully linked. Note that you can use the Ctrl/Command key to reset a panning ring.
You could use Pro-C solely as a mid/side processor by enabling M/S mode, setting Output to zero, and using the input and dry mix panning rings.

Next: MIDI learn