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Everything you've always wanted to know about LEVELS (III + FM3)

Tremonti

Fractal Fanatic
The topic of setting and monitoring levels in the Axe-Fx III and FM3 has confused me in the past. Writing an article about it has been on my to-do list for quite a while. So here it is. It covers what's useful to know, for me. Answering some of my own questions. It may answer some of yours.

I may have gotten some details wrong. No doubt members of the community will jump in. Maybe Fractal Audio will. Consider this a work in progress. After having reached a stable condition (the article, not me), I'll move it to the wiki.


About LEVELS in the Axe-Fx III and FM3

The Axe-Fx III and FM3 provide parameters at various places that control the level of the signal directly, as well as meters that display levels visually. This applies to the hardware and software. This document explains them, following the flow of the signal.

HARDWARE A/D INPUT LEVELS
  • The signal from the guitar enters the hardware through the instrument input. The first parameter in the Axe-Fx III that matters, is found here: Setup : I/O : Input. The A/D Input Levels parameters control the signal going into the analog-to-digital converter. Setting it right makes sure that minimal undesirable noise will enter the processor (aka signal-to-noise ratio or SNR).
  • The FM3 does not have Input Level parameters. Instead, it provides Setup : I/O : Audio : Input Pad parameters.
  • The INPUT LEDs on the hardware correspond with these parameters.
  • Setting the A/D Input Level / Input Pad parameters correctly means that - ideally - the red INPUT LED lights up occasionally (“tickle the red”). It’s common for a guitar not to hit red at all, which is nothing to worry about. If you have multiple guitars, just set the input parameter for the loudest one, and leave it there. Even when hitting red, the signal is never really clipping hard. There’s 6 dB of headroom, and then a soft-limiter kicks in.
  • IMPORTANT! The A/D Input Level / Input Pad parameters do NOT have an impact on volume, tone or the amount of amp gain. That’s because the processor compensates the selected setting. BUT: do not set it below 5% on the Axe-Fx III.
  • When you page right from the Home menu, you reach the Meters page. The ANALOG IN meters show the same thing as the INPUT LEDs on the hardware, without the green / orange / red colors.
  • After the A/D conversion, the signal (the note or chord you struck on your guitar) now arrives in the so-called digital domain.
DIGITAL AUDIO INPUT
  • Setup : I/O : USB/AES provides level controls for signal entering the processor through USB channels, and on the Axe-Fx III only: SPDIF and AES. Check these if you're connected to a computer and you get no sound from your DAW, YouTube, etc.
INPUT 1 GAIN
  • The Axe-Fx III provides a parameter to adjust ALL presets for variations in guitar output level (guitar pickups). It’s Setup : I/O : Input : Input 1 Gain. It trims the level of Input 1 before the start of the grid so, unlike the A/D Input Level parameters, it has an impact on blocks such as the virtual amplifier.
  • The FM3 doesn't provide this parameter.
PRESET: INPUT BLOCK
  • The signal enters the layout grid through an Input block. Like all blocks on the grid, it has a level parameter and 4 channels. This parameter lends itself well to adjust the signal for differences between guitars per preset, as an alternative to the global Input 1 Gain parameter mentioned above.
PRESET: AMP BLOCK
  • When it comes to levels, the Amp block on the grid is special. People often use the Amp Level parameter to set the overall level of the preset. This parameter controls the output of the Amp block and therefore does not have an impact on the gain or tone of the virtual amplifier.
  • The Amp block also has a parameter that controls the level of the signal at the input: Input Trim. It can be used to mimic the difference between the Low and High inputs on a real amplifier, or to control the virtual amplifier’s gain (instead of using Input Drive in the Amp block).
  • There's much more to the Amp block, like Master Volume, but that's beyond the scope of this article. More information
PRESET: MORE ABOUT BLOCKS
  • When you select a block on the hardware and press Edit, you’ll see a mini meter, indicating the left/right input signal, and left/right output signal. The software editors do not provide these mini meters. If the input mini meter hits red, it means that the output level of the preceding block is too hot. Blocks in the digital domain can’t really clip though; that can only happen at the final digital-to-analog conversion stage.
  • Page right on the Layout screen to reach the Meters page (not the same as the Meters page on the Home screen), and you’ll see those mini meters for the entire grid. Very handy to detect the cause of routing or level problems.
  • It’s good practice to aim for unity gain where possible, meaning that engaging and bypassing a block should not cause the sound to get softer or louder, unless that’s the goal.
PRESET: OUTPUT BLOCK
  • The signal exits the layout grid through an Output block. I’ve stated above that the Amp Level parameter is the main parameter to control the overall preset level, but Output Level can also be used as such. Especially because it provides additional functionality.
  • First, it lets you set individual output levels for each of the 8 scenes of the preset. Handy if you prefer to use dedicated scenes for soloing and such. Note that changing the output levels of individual scenes also affects the level of reverb and delay trails when switching between scenes, which may be undesirable.
  • Also, it provides meters that display the very important preset output level.
PRESET: LEVEL METERS
  • As written above, the Output blocks on the grid show vertical meters that display the preset output level.
  • The same meters, but now displayed horizontally, appear when looking at the Layout screen in “zoomed out” view. They are often referred to as VU meters which show the relative loudness of the preset.
  • The software editors show the same meters in the Preset Leveling window.
  • These meters, which all show the same thing, can be used to set and match the levels of presets for consistent sound. Ideally, the level of the preset should hover around the red lines in the meters. The VU meters are calibrated such that there is still 12 dB of headroom at the red line with the OUT knob (see below) at maximum.
GLOBAL EQ
  • Each Global EQ, found in Setup : Global, includes a level parameter. This lets you control the overall level of the outgoing signal through that particular output port. This does not affect AES, SPDIF and USB Audio.
NOMINAL OUTPUT LEVEL
  • Setup : Audio : Output Level lets you choose between -10 and +4. This is the overall nominal output level. The default is -10 dBv to reduce the number of support cases due to people overloading the inputs on consumer-grade interfaces, mixers, etc. Most professional gear runs at +4 dBu so you may want to change the level to +4 dBu in that case. The legacy Axe-Fx II is set to +4 dBu at default, so it is louder than the III at factory settings.
HARDWARE OUT KNOBS
  • Finally, the OUT knobs on the hardware let you adjust the overall volume for each pair of outputs. This happens after the digital-to-analog conversion, so these are analog controls. The exact position of the OUT knobs is shown as a percentage in Setup : Utility : ADC Levels. OUT 1 also controls the volume level of the headphones.
  • The OUTPUT LEDs on the Axe-Fx III correspond with the OUT knobs (post-fader). They can be seen as “levels into converters”. The FM3 has a single red CLIP LED instead.
  • Output clipping depends on two things: (1) the preset output level and (2) the position of the OUT knob.
  • If the meters indicate output clipping, action should be taken. Page right on the Layout screen, and check which block shows red output meters. For a quick temporary solution, turn down level in the Global EQ. Note that clipping can’t damage the device.
  • When you page right from the Home menu, you reach the Meters page. The ANALOG OUTPUT meters show the same thing as the OUTPUT LEDs on the hardware, but without the green / orange / red colors.
  • The maximum output level of the Axe-Fx III and FM3 is 22 dBu.
  • When using AustinBuddy’s presets, take note of his specific advice to match output levels with a DAW.
DOWNSTREAM GEAR
  • Powered monitors, amplifiers etc. provide levels controls of their own. This is beyond the scope of this article.
I/O LOOPS
  • I/O ports 3 and 4 on the Axe-Fx III, and I/O port 2 on the FM3, are designed for unity gain applications, such as effect loops. What comes in, goes out at the same level. To achieve this, turn the corresponding OUT knob fully clockwise.
A FINAL WORD
  • A level parameter is just that. It makes the signal louder or softer. It's digital, it has no sound of its own, it's neutral. Changing the value of a block’s level parameter will only change the sound (gain, distortion, tone, etc.) when that block is followed by non-LTI effects such as an Amp or Drive block.
Wait...what is a modeler?
 

Fotukito

Member
Great, useful description @yek; added to my own crib sheets!

FWIW the compensated A/D input gain that 'has no effect on the tone' seemed like black magic to me... how can that be?? So in an exchange with FAS support I ultimately came to this layman's understanding, using FX3 Input 1 as an example:

Conceptually the initial signal flow into Input 1 is:
1. Guitar analog
2. Trim pot (controlled by Input 1 / Instrument on I/O > Input > A/D Input Levels)
3. A/D
4. Post A/D gain compensation (internally controlled by FX firmware)
5. Input 1 Output Level
6. Input 1 Gain (added in fw 12.12)

FAS has established that x peak-to-peak analog signal from some reference guitar with a pre-A/D headroom of ~6dB at a Trim setting of 50% (appears to be the factory default) needs to translate to y dBFS into the downstream amp, so it will be processed by subsequent sim blocks to sound equivalent to that same real guitar plugged into the equivalent real-world effects and amps modeled by the Axe. When a user plugs in their own guitar and adjusts Trim, it enforces this reference relationship by proportionally (inversely) adjusting the ‘Post-A/D gain compensation’.

Let's say my guitar generates a signal that's 3dB weaker than whatever guitar FAS (conceptually) used to establish their reference. If I plug in my weaker guitar and do nothing to the default Trim setting, then there will be (y minus 3dB) going downstream to the sim blocks, accurately modeling the sound of my weaker guitar (as compared to the 'reference guitar').

If I increase the Trim to add about 3dB (targeting ‘tickling the red’ at ~-6dB) before the A/D (and thus improving my input S/N), the Axe decreases the post-A/D gain compensation by 3dB so there’s still (y minus 3dB) going to the downstream sims. My guitar sounds the same.

(The opposite occurs for a hotter guitar.)

The result is the Trim only affects S/N at the A/D, and downstream signal level is always true to the actual guitar in use.

This is so clever!
 

jefferski

Fractal Fanatic
Wait...what is a modeler?
It's a big collection of obscure Cold War era electronic parts all contained in a heavy wooden box, which tries to replicate the sound of (one) of the amps in a Helix, Kemper, or AxeFx. Due to issues like manufacturing tolerances, many of these "modelers" don't really sound like the ones in the AxeFx but some people swear that they're really close.

A couple of issues that come up with these is that if you want to own "models" of a number of amps you need a very large room to keep them in, soundproofing to not disturb your wife, kids, or neighbors, and you can't really take a lot of them to a local pub gig although if you're happy with just one then you can "get by." They're also really heavy and you need to figure in the expense of employing a "roadie" to help you move them around, or a "chiropractor" to help you recover.
 

Smittefar

Fractal Fanatic
  • Finally, the OUT knobs on the hardware let you adjust the overall volume for each pair of outputs. This happens after the digital-to-analog conversion, so these are analog controls. The exact position of the OUT knobs is shown as a percentage in Setup : Utility : ADC Levels. OUT 1 also controls the volume level of the headphones.
  • The OUTPUT LEDs on the Axe-Fx III correspond with the OUT knobs (post-fader). They can be seen as “levels into converters”. The FM3 has a single red CLIP LED instead.
  • Output clipping depends on two things: (1) the preset output level and (2) the position of the OUT knob.
These two statements seem to be contradictive.

This is something, I would like to understand in more detail. I have a few presets with very clean sounds, where I struggle with getting enough loudness before peak clipping. The second statement above seems to indicate that I can get more clean headroom by switching from -10 dBV output setting to +4 dBu setting and then adjust the Out1 to compensate for the level change.

I would love to have clarification of this

EDIT: And thanks for doing this - I think this one of the places, where most users get lost.
 

yek

Moderator
Moderator
Switching to +4 will do nothing to solve that clipping.

"Levels into converters" comes from this:

"The front panel meters indicate the level INTO the D/A converters. The only place the system can clip is at the converters so the meters let you know when you are in danger of clipping the converters. A signal is generated internally. That signal can be any value from 0 (negative infinity dB) to a thousand dB, in theory. That signal is then multiplied by the "gain" of the output level knob (0 to 1). The knobs have an audio taper but that's irrelevant. If, after applying that gain, the signal exceeds 1.0 the converters will clip and red LEDs on the meters will light. If you think about the meters as level into the converters it all makes sense because that's what it is. To further demonstrate this increase the Boost/Pad setting for Output 3/4 and watch what happens. The output meters are the analog of the input meters. They indicate converter levels. Nothing more, nothing less. And what they do is extremely important and convenient. source
 

Smittefar

Fractal Fanatic
OK, but then this statement

Finally, the OUT knobs on the hardware let you adjust the overall volume for each pair of outputs. This happens after the digital-to-analog conversion ...

is incorrect. The output knobs is digital scaling before the digital-to-analog conversion.

What I was suggesting above is this:
I prefer to run my Out 1 knob all the way up, simply because it is the must reproducible setting. I never have to consider whether I changed it by mistake. If I select +4 dBU and run it all the way up, the signal is a bit hot for our digital mixer. Therefore, I have chosen to run Out 1 all the way up and use the -10 dBV setting. This is fine for our mixer.

Now, if I change the setting back to +4dBU, I will have to reduce Out 1 to compensate for the level added by the +4 dBu setting. Reducing Out 1 will buy me more headroom, if Out 1 is a scaling before the D/A converter. I will have to play around with this a bit.
 

yek

Moderator
Moderator
OK, but then this statement
Finally, the OUT knobs on the hardware let you adjust the overall volume for each pair of outputs. This happens after the digital-to-analog conversion ...
is incorrect. The output knobs is digital scaling before the digital-to-analog conversion.

I don't think so. The output knobs control analog circuitry, after the D/A converter, IIRC.
 
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Smittefar

Fractal Fanatic
That's what I always thought as well - but the other statement says that it is digital scaling before D/A conversion. It cannot be both ... I think :D
 

mr_fender

Fractal Fanatic
The digital signal level that goes to USB or SPDIF is not affected by the output knobs. Not sure about the signal after that which feeds the D/A converters. @FractalAudio would have to clarify that one. I always assumed the Out knobs were on the analog side like on the II.
 
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l4mpi

Member
I don't think so. The output knobs control analog circuitry, after the D/A converter, IIRC.
That's not how I'm understanding the quoted post from Cliff. These sentences in particular:

A signal is generated internally. [...] That signal is then multiplied by the "gain" of the output level knob (0 to 1). [...] If, after applying that gain, the signal exceeds 1.0 the converters will clip and red LEDs on the meters will light.
Seems like the gain is applied before the converters, as otherwise it would have no influence on the signal clipping, right? Furthermore, it's stated that the meters show levels into the converters, so the fact that the meters change when you turn the knob seems to be another indication that the knobs operate in the digital domain.
 

moshwitz

Inspired
Sup? ;-)

PRESET: MORE ABOUT BLOCKS
  • When you select a block on the hardware and press Edit, you’ll see a mini meter, indicating the left/right input signal, and left/right output signal. The software editors do not provide these mini meters. If the input mini meter hits red, it means that the output level of the preceding block is too hot. Blocks in the digital domain can’t really clip though; that can only happen at the final digital-to-analog conversion stage.
  • Page right on the Layout screen to reach the Meters page (not the same as the Meters page on the Home screen), and you’ll see those mini meters for the entire grid. Very handy to detect the cause of routing or level problems.
  • It’s good practice to aim for unity gain where possible, meaning that engaging and bypassing a block should not cause the sound to get softer or louder, unless that’s the goal.

Thanks for posting this Yek,,, I had made a "Wish" post for axe-edit about wishing to have these meters included for the EXACT reasons you have listed up above and practically get crucified for asking for it being told how "They" do it, or "The only important meters are........" or "use the ones in your DAW they are better anyway", that kind of thing. I'm not always at my DAW,, and do a good bit of preset building /editing on the front panel as I have since the standard/Ultra days through having the Axe-II and now with the Axe-III,, and yeah,, I know you can't really Clip the blocks,,But I like to see the meter going into a block from the previous block, cause being older sometimes my eyes hurt, and I'll bring a laptop( with no DAW installed) with me for axe-edit instead of always looking at the screen on the unit,, just for looking at a bigger screen.

Anyway,,, Thanks for posting this ;-)

MOSHON
DAVE
 

Smittefar

Fractal Fanatic
I managed to make a small experiment that made it clear to me that adjustments to Out 1 are applied in the digital domain before the D/A converters - the output LEDs reflect the level after the Out 1 scaling.

I pulled up a preset with no amp block, just a cab block with an acoustic sim IR. This is a preset, where I have always struggled with output clipping.

I put the looper in front and recorded a riff that produced output clipping. Then I recorded three passes in my DAW (reaper) using the analog output of the AxeIII. I lined up the recordings and normalized all the recordings to better see the clipping. You see the normalization gains encircled in red
Clipping_w_AxeIII_Out1.png
  1. Out 1 setting -10 dBV and knob all the way up. The waveform is clearly clipped at the two markers. The red circle shows that 10 dB was added to the clip to achieve peak normalization, so I was far away from clipping my analog inputs
  2. Out 1 setting -10 dBV and the knob at noon. Level is reduced by ~12 dB but (22 dB needed to achieve Peak normalization) - There is no clipping to be seen
  3. Out 1 setting +4dBu and the knob set to have the same level as pass 1 (10 dB needed for Peak Normalization) - There is no clipping to be seen.
Also, the red Output LEDs on the front panel did not light up during passes 2 and 3.

Question: Where can I see my Out 1 knob setting. I want to write it down, so I can get back to this exact setting. EDIT: NEVERMIND: Of course, Yek already put it in the OP. My Out 1 is set to 50%.
 
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polanoid

Member
A couple of issues that come up with these is that if you want to own "models" of a number of amps you need a very large room to keep them in, soundproofing to not disturb your wife, kids, or neighbors, and you can't really take a lot of them to a local pub gig although if you're happy with just one then you can "get by." They're also really heavy and you need to figure in the expense of employing a "roadie" to help you move them around, or a "chiropractor" to help you recover.
I also found some of those modelers are really expensive, especially compared to the original.
 

sprint

Fractal Fanatic
I often observe that bypassing the Cab block increases level quite a bit in my presets, and have wondered if I'm compromising something somehow by using the amp block level knob for preset levelling as these adjustments are varying the Cab block input level more or less randomly - am I changing the intensity of the cab block model, or of the amp speaker page parameters by making overall level adjustments using amp block level.
 

l4mpi

Member
I often observe that bypassing the Cab block increases level quite a bit in my presets, and have wondered if I'm compromising something somehow by using the amp block level knob for preset levelling as these adjustments are varying the Cab block input level more or less randomly - am I changing the intensity of the cab block model, or of the amp speaker page parameters by making overall level adjustments using amp block level.
I'm far from an expert on this but I don't think the input level matters for an IR.
One laymans explanation for IRs is to think of the output signal as the sum of playing the IR waveform at each sample (e.g. 48000 times a second at 48kHz), multiplied by the value of the original signal at that sample. Not sure how dumbed down that explanation is but if it's approximately correct then changing the level before the IR would just change the level of the output signal by the same value times a constant factor depending on the selected IR, and should not influence tone (except for the usual psychoacoustic effects from adjusting volume). Other things in the cab block such as the preamp could have a nonlinear response though, so it might make a difference if you touched those parameters.
 
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