I can't watch this video at the moment but the thumbnail seems to suggest that turning the gain down isn't the right idea. I thought you said the guy who said to turn the gain up is wrong
Simple life?
- Just keep the gain all the way down, use the
database here and it'll be fine for the most part.
If you really want to go down the rabbit hole...
There are two separate issues here at play
1. Accuracy
An amp responds to the strength of the incoming signal, shaping the tone differently at lower versus higher input levels. This is why single-coil pickups typically sound cleaner and less gainy than high-output pickups like EMG 81 humbuckers.
For a digital model to behave the same way, a model will assume a mapping of digital level to real real-world voltage. For example, it looks like the Neural DSP plugins assume that a 0dBFS signal corresponds to a real world 12.2 dBu signal.
To calibrate our input chain, we need to know what input voltage level results in which digital level. A lot of interfaces will list something like "Maximum Input Level (at minimum gain)" spec - for example the
Scarlett 2i2 4th gen specifies +12dBu for its instrument inputs. That means that a 12dBu signal presented at the instrument input, with the gain knob all the way down, will result in a 0 dBFS signal.
This is why, when you specify "Neural DSP" and "Scarlett 4th gen" in the
database here it will tell you to adjust the input gain by -0.2dB, because Neural DSP plugins, expect a 12.2 dBU signal to generate 0dbFS, not a 12dBu signal, and assumes you have the interface channel gain knob all the way down.
2. Noise and SNR considerations
A separate matter is how to capture an input signal so that the system SNR is maximized.
Broadly speaking, in a guitar recording chain with an interface we have:
- Guitar/source noise — EMI/hum, and thermal noise inherent to the pickups, wiring, and environment. The only way to improve this source SNR is by changing pickups, reducing sources of EMI, playing harder, etc.
- Preamp noise — noise contributed by the input stage of the audio interface, which is largely determined by the interface’s circuit design and components.
- ADC noise — noise introduced by the analog-to-digital conversion process, referenced to full scale.
ADC noise
- ADC noise is fixed relative to full scale
- Increasing the analog signal level increases the signal-to-noise ratio at the point of conversion. So when people say adjust your interface gain knob but leave enough headroom to not clip your signal - this is the SNR that is being optimized.
Does this matter?
Yes, because the goal is to feed the signal into a digital amp model, some of which apply very large amounts of gain. At that stage,
all noise is amplified: guitar noise, preamp noise,
and ADC noise. This is why differences in gain staging can become audible.
Here's a concrete example.
I use a
MOTU M2, which I measured. With the input gain knob fully counter-clockwise, a
+14.6 dBu input signal produces
0 dBFS in the digital domain.
From the
database here,
Neural DSP plugins expect +12.2 dBu to correspond to 0 dBFS. To align these references, I set the plugin’s input gain to
+2.4 dB, effectively calibrating the input chain.
Next, to “optimize” the recording level of my
PRS Silver Sky Core, I increased the MOTU’s input gain but ensured the signal still did not clip. At that setting a
+0.8 dBu input produces
0 dBFS (verified using a calibrated test signal). To maintain the same effective operating point in the plugin, I compensated by setting the Neural DSP input gain to
–11.4 dB.
In other words, both setups are
nominally level-matched at the plugin input, but they differ in how much analog gain is applied before conversion.
Using the
Lead 2 preset in the
John Mayer X plugin, I can hear the difference in perceived noise.
In the first half of the example, the MOTU M2 input gain is fully down.
In the second half, the Motu M2 input gain is increased as described above but compensated at the plugin by lowering its input gain to remain calibrated.
In both cases, I start with the guitar volume fully off, then bring it to full, roll it back to 5, then to 2, and finally off again.