Why instrument volume in capture mode?

Can anyone explain why there is a need for an instrument input volume control in the capture process? I mean, isn’t the idea that what’s going into the instrument input and through the capture out is unity gain when everything is set to 0? So what capture out is sending to the amp should be the same level as guitar directly into the amp?
If I have a favourite setting on my amp that I want to capture, a setting that sounds great with my favourite guitar directly into amp, why would I want to change the instrument input gain in the capture window?

I guess for the same reason that there’s an input level on the QC paths: to allow for adjustments for either signals that are too hot or too weak for some reason. In you case 0dB is probably the correct setting.

See Capture Techniques - Quad Cortex Wiki for some additional information on QC captures.

I can see the need for an input level control in the signal path for a preset - to compensate for different guitars and pickups. But for the capture process?
Say you use a strat with low output pickups. If you adjust input volume in the capture window as close to clipping as possible, you will hit the amp quite differently than if you play directly through the amp. I just can’t see why you would want that.

Without knowing the technical details I would speculate that the amp is fed with a multitude of different levels during the capturing process and it’s about capturing the amp’s response correctly at any input level to be ready for any signal.
However, setting an appropriate input level for your guitar ensures that the guitar signal is converted from analog to digital at the highest possible resolution without wasting headroom in the input stage. It’s more about signal quality and signal to noise ratio. Less about unity gain. I might be wrong though.

according to NDSP support, the Inst Level is only for monitoring your reference device/comparing the capture to the original reference tone. The guitar signal itself is never involved in the actual capture progress

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Confirmed by measurement - it’s the same signal level no matter what - might help to understand why some captures are better than others, gain staging wise. I have a feeling a lot of people are misinterpreting the use of this control. Here’s my measurement of the capture signal at various INST levels:

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The first step in any amp or pedal capture is to set the controls on the amp or pedal to get the tone you want to capture. The guitar input in QC is just a convenience so that you can make these adjustments while everything is setup for the capture, and so that you can conveniently compare the capture results with the original source.

Setting the input to something other than 0dB would change how your guitar is hitting the amp and would influence setting the controls on the amp to get the tone you want to capture. You’d be changing the gain staging, either driving the amp harder or softer than you would by plugging the guitar directly into the amp. This could result in a different gain staging when using the capture.

There is however a reason you might want to adjust the input gain while setting up for a capture. Say you have a guitar that has excessively hot pickups and you have to always set it at -6dB on the QC input 1 to keep it from clipping. All your patches would be setup to account for this change in input gain. If you create a capture with that same guitar, you would want to set the guitar input to -6dB while making the capture so that you guitar would be driving the amp the same way it would be driving the capture block of that amp. Then the amp and capture would be seeing the same gain staging and the capture block would more closely match the tone you intended to capture.