The Compressor reduces the dynamic range of the signal by lowering its gain when the input power exceeds a threshold. It is best placed at the start of an effects chain and is most effective when used with clean settings. Using a Compressor with high-gain distortion can lead to excessive noise or squealing. The Compressor can also be placed near the output of an effects chain if desired to even out the response if using, say, two different distortion types. The low-gain distortion can be followed with a Compressor to even out its response compared to a high-gain distortion. Otherwise the low-gain distortion will be too dynamic.
- THRSH - Threshold control. The Compressor reduces its gain when the input power exceeds this value.
- COMP - Sets the gain reduction ratio. This is the ratio of input power to output power beyond the threshold. For example, if the threshold is -40 dB, the input signal is - 30 dB and the ratio is 2.0 the output will be -35 dB.
- ATT - Attack rate. Sets how fast the Compressor reduces its gain once the threshold is exceeded. Higher values equal faster response.
- REL - Release rate. Sets how fast the Compressor restores the gain once the signal has fallen below the threshold.
- LEVEL - Sets the output level of the block. This is the same control as on the MIX page.
- KNEE - Sets the "knee" type. A soft knee gradually varies the compression ratio around the threshold whereas a hard knee switches from no compression to the selected compression ratio abruptly at the threshold. With high compression ratios a hard knee may produce undesirable artifacts. In this case set the knee type to SOFT.
- DET - Selects the type of detector to use, RMS or Peak.
- MAKEUP - With this set to on compressor will automatically compensate for the gain reduction caused by the compressor. As you vary the amount of compression applied to a signal the output level will stay roughly constant.
- SCSEL - To use the sidechain you simply feed two (or more) rows to the compressor input, say rows 1 and 2. The row you desire to be the sidechain is set with SRCSEL. The remaining rows are summed and used as the input.
- FILTER - Filter is a high-pass on the detector input. This can be used to eliminate "pumping" and/or "breathing" if the source has a lot of low-frequency content.
Try putting the compressor as the the first block in the chain. Compression works better on clean tones than high-gain tones. The following settings are some very basic starting points to achieve some specific sounds.
The Axe-fx use from 1-10 rather than ms for the attack and release settings. They are scaled in an inverse log relationship to 1 to 1000ms. This was confusing at first because a setting of 10 indicates a faster attack rate which means 1 millisecond. Same thing with release - just remember they are rates and not times. Faster rates mean lower values.
Axe-Fx Setting : Actual Time (attack or release)
1 : 1000ms
2 : 954ms
3 : 903ms
5 : 778ms
7 : 602ms
8 : 477ms
I tend to stay the 9.X range for most things:
9.0 : 301ms
9.2 : 255ms
9.5 : 176ms
9.7 : 114ms
9.9 : 41ms
9.92 : 33ms
9.95 : 21ms
9.97 : 13ms
9.98 : 9ms
9.99 : 4ms
10.0 : 1ms
The compressor will cause some distortion if the release rate is set too high along with a high attack rate. The compressor has been optimized for attack speed as this is what guitar players like. If you set the release rate too high, though, you can get distortion. The proper way to set the release rate is to observe the natural decay of your instrument and set the release rate just slightly faster than that. For the typical electric guitar this is around 3.0.
- THRSH: -17.8dB
- RATIO: 10:1
- ATT: 150ms
- REL: 987ms
This setting is heavily compressed, making chords and single notes thick and even, and sustain like, forever. The medium attack allows the guitar strings to ring, and the long release holds on to produce a massive, Neil Young-like sustain.
- THRSH: -15.5dB
- RATIO: 5.1
- ATT: 11ms
- REL: 102ms
This is the fat, ballsy sound that drives many of the guitar-heavy bands of the late ’70s and early ’80s. It’s mildly compressed with a fast attack and a medium release, which allows all the strings - from low to high - to sound evenly for a massive punch. Think "Barracuda" by Heart.
- THRSH: -31.4dB
- RATIO: 6:1
- ATT: 1ms
- REL: 25ms
Play this sucker on a Telecaster and ride it all the way to Nashville. The fast attack nails the initial string impact, and the fast release lets it go pronto to minimize sustain. The result is a clean, snappy pluck that works great for those blazing cowboy cascades.
- THRSH: -27.5dB
- RATIO: 10:1
- ATT: 50ms
- REL: 38ms
This setting is squashed pretty hard with a high threshold that pretty much grabs the entire signal to produce a sound with no sharp edges. The resultant velvety texture is wonderful for smooth jazz tracks and other sophisticated guitar tones.
- THRSH: -30dB
- RATIO: 7:1
- ATT: 250ms
- REL: 250ms
Clean notes that float in languid, ethereal sustain - think Ry Cooder’s brilliantly sparse lines on the Thelma & Louise soundtrack - are provided here. The slow attack allows the note attack to retain punch and impact, and the long release keeps the sound ringing. This expansive and evocative setting works great for slide parts, as well.
- THRSH: -21.6dB
- RATIO: 7:1
- ATT: 61ms
- REL: 84ms
To accentuate those funky, biting pick attacks - as heard on classic Sly Stone and Earth, Wind & Fire tracks - this medium attack and fast release does the trick.
Adapted from Guitar Player magazine (Aug 2006)