Always consult the official Owners Manuals first!
- 1 Available on which products
- 2 Channels or X/Y switching
- 3 Purposes
- 4 Types
- 5 Factory presets
- 6 Tips and tricks
- 6.1 Use a Filter instead of a Cab block
- 6.2 Fade-in when engaged
- 6.3 Dynamic Distortion
- 6.4 Sweep
- 6.5 Envelope Filter (Auto Wah)
- 6.6 80's rock guitar sound
- 6.7 Eric Clapton's mid-boost
- 6.8 Clean boost
- 6.9 Finding the resonant frequency of a guitar cabinet
- 6.10 Alternative to Boost in the Amp block
- 6.11 Boston / Rockman sound
- 6.12 Boosting mids with FRFR amplification
- 6.13 Alternative to Crossover block
Available on which products
- Axe-Fx III: 4 blocks
- FM3: 4 blocks
- FM3: 4 blocks
- Axe-Fx II: 4 blocks
- FX8: 2 blocks
- AX8: 2 blocks
Channels or X/Y switching
- Axe-Fx III and FM9 and FM3: 4 channels
- Axe-Fx II: no
- AX8: X/Y
- FX8: X/Y
A filter is a very versatile effect. Possible applications include:
- one-band equalizer
- boost the input of an amp block
- boost the overall signal level (i.e. for leads)
- high-pass or low-pass filtering
- envelope filter / auto-wah
- find the resonant frequency of your guitar cabinet
- and more.
List of types
- Tilt EQ
- Lowshelf 2
- Highshelf 2
- Peaking 2
- FF Comb
- FB Comb
Added in firmware Ares 12.01 and later. This is a multi-stage “Phase Rotator” with feedback. With feedback set to zero it can be used to change the shape of transients. Using feedback causes constructive and destructive interference and can be used to create interesting tonalities. The Order can be set from 1 to 12 with progressively more phase rotation occurring. Attaching a modifier to the Frequency parameter can result in interesting chorus or wah effects. The red trace in the GUI is the phase response.
Many “classic” graphic equalizers use variable-Q designs which may be more familiar to some users as opposed to constant-Q filters. In the Filter block this type is selected by choosing “Peaking2 ”.
"Peaking 2 is constant Q. At +/- 12 dB they are the same. At other gains the Peaking 2 will have a wider bandwidth as the Q does not change." 
Lowshelf and Highshelf
"A lowpass with "direct" signal mixed in is a lowshelf. A highpass with direct signal mixed in is a highshelf. In fact in the analog domain that's a common way of making low/high shelf filters. It's intuitive as well. Take the direct signal and add/subtract some lowpass filtered signal. Now you have a bass boost/cut, i.e. lowshelf." 
Analog shelving EQ
“Lowshelf 2” and “Highshelf 2” recreate the analog shelving filters found on classic mixing consoles. These filters are somewhat quirky and exhibit “overshoot” which gives them a certain musical quality. Set Q between 0.5 and 0.707 to recreate those classic sounds, or experiment with Q for different amounts of overshoot. These filter types are great for getting that massive sound associated with passive equalization.
"The Lowshelf2 and Highself2 types are "analog" types and derived using "k + low/high pass"." 
Tilt EQ is a slope filter that allows broad adjustment of the tone using just two parameters: Frequency and Gain. The Gain parameter sets the maximum gain of the filter relative to the center frequency. For example, a gain of 10 dB would set the maximum gain to 10 dB. The gain at the center frequency would be 0 dB and the minimum gain would be -10 dB, therefore a total of 20 dB of EQ would be applied.
"Gain doesn't apply to a notch filter. Only frequency and Q are relevant." 
FF Comb and FB Comb
FF Comb is a feed-forward comb filter and FB Comb is a feedback comb filter. Delay Time controls the order of the comb filters, higher values result in more closely spaced notches and vice-versa. Depth controls the intensity of the filter, higher values result in deeper notches/peaks and vice-versa.
The Filter block is showcased in many factory presets. Search the Factory presets page for "filter".
Tips and tricks
Use a Filter and Scene Ignore to adapt a preset to multiple guitars
Use a Filter instead of a Cab block
A Filter can be used to create an "amp+cab-in-the room" sound.
Fade-in when engaged
Firmware 21.02 for the Axe-Fx III:
"Changed bypass/engage speed for the following blocks: Drive, Filter, Graphic EQ, Parametric EQ, Tremolo and Wah. These blocks now bypass/engage with a gentle fade as rapid bypass/engage with these types of blocks can sound abrupt."
Some filter types are also available in other effect blocks, such as the Dynamic Distortion block.
Attach the global LFO controller or the LFO in the Filter block to the Frequency parameter to generate frequency sweeps, as demonstrated in many Factory presets.
Envelope Filter (Auto Wah)
Fractal Audio processors don't provide an Auto Wah effect block. You can achieve this effect by attaching the Envelope controller to a Filter or a Wah block.
Try a factory preset such as the Psychadelic Duck, Frenetica, Track Wah.
Attack is the time it takes to go from silence to maximum level. Release is the time it takes to drop from sustain level to silence after being released.
80's rock guitar sound
"This was a common technique in the 80's when tracking. If you have an Axe-Fx or other modeler with EQ options you can try it yourself. Put an EQ or Filter block before the amp. A parametric is best. Set the type to Peaking, Frequency to 1 kHz and Q to around 1 and gain to around 6 dB to start. Experiment with the parameters."
The Axe-Fx III, FM9 and FM3 let you obtain the same result using Pre EQ in the Amp block.
Eric Clapton's mid-boost
To emulate Eric Clapton's mid-boost knob on his guitars:
"Use the Filter block. Since the max boost is 25 dB set the filter order to 4th which will give you a maximum of 24 dB. Set the type to Peaking. Set freq to 500 Hz. Vary gain and Q to taste." 
Set the Filter to "Null", set its Level to where you like it, put it at the end of the signal chain, and assign a switch. Now you have a simple, low-CPU, clean boost at your disposal.
Finding the resonant frequency of a guitar cabinet
Read this: Finding the resonant frequency with a solid-state amplifier
Alternative to Boost in the Amp block
The Boost parameter in the Amp block boosts the signal at the input of the Amp block with 12 dB. If you rather have an adjustable boost, use a Filter block before the Amp block, set to "Null", with Level at the desired value.
Boston / Rockman sound
Put a Filter block in front of a Plexi amp model, select Peaking, set Frequency at around 800 Hz, Q at 0.707 and Gain at 12 dB.
Boosting mids with FRFR amplification
A mid-boost can be help a guitar cut through the mix when using FRFR amplification.
Try a Filter at the end of the grid, select Peaking, Frequency at 770 hz, Q at 0.35, Gain between 2 and 4 dB.
Alternative to Crossover block
There's no Crossover block in the FM3. Filters can be used to achieve something similar. Check out the FM3 factory preset "Polyfuzz Crossover".