Thoroughly confused about gain staging and about to give up

The more I read about, or watch videos about, gain staging, the more confused I get, especially when trying to create a preset in the QC

Firstly, I have read on this forum that the input gain should be 0.0 yet, I have watched videos on YouTube re: QC, that you need to adjust the input gain so that it is right below clipping (into the red)

Secondly, when it comes to gain staging, I keep seeing/hearing that -18dB is the magic number and that each FX block, amp, etc., should be set so that there is an average of -18dB ---- that leads me to the question: how is this accomplished? Also, if -18dB is the target when recording, is all the extra gain available on amps and FX pedals superfluous?

Or, do I set the FX/amp/etc. any volume I want and then turn the output block down so it averages -18dB?

I really am about to give up - I just donā€™t ā€œgetā€ how to create a QC preset so that every single block averages -18dB

donā€™t worry about the individual blocks, the gain/output level is cumulative and the more blocks you add, the hotter the signal gets (in most cases)

Concentrate more on the Output Block and dialing in for your desired overall level at that point. It may require some adjustments earlier in the path, but probably not every single point.

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Hmmm, so clipping within the blocks is perfectly fine, so long as I turn the output block to average -18dB
Or, is this -18dB ā€œruleā€ I keep hearing about not important?

well, I wouldnā€™t say clipping is ā€˜fineā€™ necessarily, just that you donā€™t need to obsess about metering out the levels to an exact, predetermined value in each block if youā€™ve got a bunch of blocks. Iā€™d still aim for keeping each block under control with safe headroom, donā€™t drive them too hard.

Iā€™ll defer to others to comment on the ā€˜-18dB factor.ā€™
I like to get as much out of a preset as I can, before clipping, but I usually just earball it.

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Ignore all the stuff youā€™ve readā€¦

Set input gain to 0dB.
Add whatever blocks you want with all output gains set to 0dB unless you really need to change it.
Set the output of the last block so that the output meters show your loudest signal is below clipping (not hitting the end of the meter or going red).

In between blocks the levels are just tone preference. Someone might want to crank a distortion pedal output to drive an amp block harder. Someone else might want to keep everything pristine clean, with no drive at all. Itā€™s all down to what you want.

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That video is mine, Iā€™ve received a lot of kickback on that, but a lot of the people also donā€™t RTFM, just because the meter is going into the red does not mean itā€™s clipping. In the case of the QC, clipping is shown in this manner:

Page 24:
In order to keep your signal healthy, Input and Output blocks will
turn red if their assigned I/Os are clipping.

That being said, I run my QC with the input at 0.0 now

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For guitar plugins to behave like the modeled amps, they must be driven from the audio interface at a level that would correspond to a guitar straight into the real amp. If the interface gain is too high, the amp will distort too quickly, If it is too low, the amp plugin will be cleaner than the real amp with the same settings.

There are two independent controls to adjust for the gain staging, the first is the input gain of the audio interface input, and the second is the digital input gain control in the plugin. These have to be adjusted together to provide the proper gain staging from the guitar into the plugin. Hereā€™s the approach:

  1. Set the audio interface input gain to 0dB using a high-Z input
  2. Open the Interface and Amp Sim Input Level Table spreadsheet and get the gain setting for your interface and amp modeler. Set the plugin input gain to this setting. This table essentially captures information about the levels the amp sim was designed to see.
  3. Increase the gain on the audio interface input until you are just under the clipping level on your audio interface when you play as loud as possible.
  4. Reduce the plugin input gain to compensate for the same amount of gain you added at the audio interface input

This will maximize the signal to noise ratio in the audio interface, using the maximum number of bits in the interface bit depth, and will still give the plugin the input level it was designed to receive from the guitar.

See https://www.youtube.com/watch?v=gJ59h7xfvdI for a detailed explanation.

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I have seen that video but, that video pertains to amp sim plugins, not the Quad Cortex itself

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I had a tendency to overthink the issue of input levels and gain staging, but now I actually totally relax and leave the input gain at 0.0.
Sometimes I have a capture or amp model where I feel I drive it too hot with my humbuckers, then I put a gain block before and reduce a bit.

I donā€™t see why I should worry so much more than with real amps or pedals.
As long as I have no digital clipping somewhere, I donā€™t care about the levels.

With a real amp, I would also adjust the settings to my taste and to the specific guitar. I do the same on my modeled amps.

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OP, youā€™re WAY overthinking it. Just plug in and play. Sure; if you crank the output volume on every block, youā€™ll run into issues, but for the most part you shouldnā€™t ever have to worry about dBs.

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The OP is right to be confused. The manual doesnā€™t mention how the QC input is factory calibrated nor how to set it. Our experience with other gear and the reinforcement of untold numbers of YouTube videos have us maximizing the input level to the ADCs. Which is going to look like a boosted signal to the device blocks.

All this misinformation and resulting confusion is really unfortunate. Yet understandable.

One would expect the manual to address this issue at some point. Version 3.1 and weā€™re still expecting.

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The confusion of late is only related to using plugins on the computer via an interface. This might be the first thread ever to apply this confusion to a hardware modeler. Iā€™ve run across a couple presets that were too hot, but itā€™s easy enough to turn the output volume down on the amp or cab block (usually the culprits).

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Actually Iā€™ve been scratching my head publicly about the QC input misalignment for months and months. And the truth is that many YouTube videos incorrectly telling us to maximize the input level before clipping on the QC input meter have been around for a while, too.

This is not about output level. Itā€™s about the amp models (and probably any device block, I would imagine) expecting a lower input signal than you get turning up the input till your guitar peaks at full-scale on the QCā€™s input meter. There appears to be a bunch of headroom on the input that weā€™re supposed to leave be if we want the digital signal chain on the QC to see the level that its various devices were modeled on.

Bottom line is that if you want the digital models to react like the real ones, leave the input knob set to 0.0. That aligns our real world to the virtual, digital world inside the QC. Turning the input knob up is like putting a boost in front of everything on your screen.

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Leaving at 0 is all fine if your pickups are producing a solid signal, for my low-ish gain tele boosting the input on the I/O screen (12 db in my case) then reducing by the same amount under the input gate control totally improved how it plays, more responsive and dynamic especially to rolling the volume and tone and doesnā€™t get choked by the gate anymore.

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This also improves the noise floor, because something happens/changes in the A/D conversion at +9,3db. You can clearly hear it when slowly turning the global input gain up step by step. Therefore I have my global input gain at +9,3db and the gain on my input blocks at -9,3db. But that doesnā€™t falsify the above argument of the blocks behaving the most realistic when they see a overall gain of Ā±0,0db - because that holds true here (+9,3-9,3=0)

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Well, I really thought I had it, leave the QC input gain at 0.0

Now, I am confused again and reaching for my acoustic (no pickup installed) guitar

This is really interesting. With 24 bits and 21st-century electronics at work, there should be no practical way that signal-to-noise or anything else is noticeably improved, especially when single-coil pickups are as noisy as they are.

As for noise gating, that should be a simple gain issue; turning up the QC input should have the same effect as turning down the gate threshold.

But you guys seem to be benefiting. Anybody know what Iā€™m missing, here? Iā€™d love to know. Iā€™m going to give it a try and see for myself, but Iā€™d love to understand what the heck is going on.

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For A2D inputs and D2A output, there are typically two gain controls, one before the converter and another after the converter. The design of the converters establishes the input/output dynamic range, typically around 114 dB which is very good. However, to get that dynamic range, you need to be pushing the input into the converters as high as you can without clipping. This uses the maximum number of bits in the converter and minimizes the noise floor.

As you push the input into the A2D converter higher, you need to reduce the gain after the converter to compensate, so that the resulting input level into the downstream blocks doesnā€™t change. The input trim controls on Fractal Audio devices like the FM9 do this automatically. During setup you adjust the input trim as high as you can without clipping on the meters. Changing the input trim doesnā€™t change the input into the amp blocks, it just optimizes the noise floor.

For Quad Cortex, we donā€™t know what the Input level controls. Is it the level into the A2D converter? Is it the level out of the A2D converter? Or is it both like the Fractal Audio input trim? Given that the QC Input Level does change the overall input gain, it appears the control is either before or after the A2D converter. We donā€™t know for sure which.

If Input Level is before the A2D converters (the most likely case), then you might want to push the input level as high as possible without clipping, and then use a gain block early in the signal path to reduce the gain by the same amount to preserve overall unity gain. This would maximize the dynamic range and minimize the noise floor, duplicating the behavior of the FM9 Input Level.

If the Input Level is after the A2D converters, then it would be just a digital clean boost which wouldnā€™t be very useful.

Iā€™m guessing that the QC guitar input was designed to be optimized for guitar input levels with the Input Level set at 0 dB. But guitar pickup levels can vary quite a bit. Low output single coil pickups might not push the A2D converters high enough to minimize noise and loose some of the available dynamic range. High output humbuckers might push the input into the A2D converters into clipping, something that should be carefully avoided. Digital clipping is not pleasant.

So the Input Level can be used to adjust for situations where the guitar pickup output is significantly different than typical levels. Whether you need to use a gain block to preserve unity gain or not depends on what tone you are looking for. If you want your low output pickup to behave the same in QC as it would into the front of a guitar amp, then youā€™d need to use the gain block to preserve unity gain after bumping up the input level. If on the other hand, you want that low output pickup to behave like other higher output guitars, then leave the QC Input level boost.

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My guess is that it works a bit like the input trim you described, but itā€™s not continuous.
If you just put some high gain amp and a cab on row 1, you can hear the noise floor even without a guitar.
From 0 to 9.3dB you turn up the gain and you can hear the noise floor get louder. At 9.3dB, like DavidRSTV described, you can suddenly hear the noise drop. There are more breakpoints like this somewhere around 12+dB and after that.

So I think the QC does only increase the gain on the digital side up to a certain limit and then increase the analog side gain step-wise to maximize the dynamic range and compensate on the digital side.
In other words, the input gain grows as you turn up the knob, but the analog input signal is ā€œrescaledā€ every few dB.

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Why canā€™t a development engineer from Neural come and explain the signal path and for what scenario did they design the inputs and blocks to work best?

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