Quick quick quick 'cause I never find time.
It's a draft, I'll update this message when I get more time (+PC BSOD prevented more typing).
I performed measurements of the 2i2 inputs mostly using my Motu Ultralite mk5 as the generator, but also the Focusrite 2i2 itself (therefore in a loopback) as its DAC is better than its ADC.
Before measurements, I installed the Focusrite Control 2 Software.
As opposed to output volume knob, the input gain knobs are digital and so will increase or decrease the input gain in 1dB steps. It's therefore same as doing in the Software.
This interface has only 2 inputs. So when using the front inputs to connect an instrument like a guitar or a line level one (like a synthesizer), it will disconnect the respective rear XLR Microphone input. So it's kind of ok to have the microphone inputs at the back of the interface and the instruments at front since we are most likely to switch often the latter.
Amir tested at 48kHz Microphone inputs only and with an input gain of obviously +9dB. So I tested at 96kHz with different input gains and all the input types, to complete measurements.
Focusrite 2i2 Gen4 - Measurements (Line input)
I could not saturate the input as my interface does not output more that 21dbu. So the Focusrite saturates above that and probably at the specified 22dBu (9.75Vrms).
The interface will exhibit its best performances with an input gain of +6dB (whatever is the input, by the way). Here is 4Vrms input with +6dB gain applied:
We get a
SINAD of 101.2dB in that case. Distortion is dominated by H3 with input 1 but it is H2 with input 2, which is funny. If someone can hear the influence of that, using one input or the other could change the final result, and considering the planned utilization, it could even be useful.
Here is the proof, with 2Vrms input in line 2 and still +6db gain at the input, we get:
SINAD reaches nearly
102dB which means we have consistent performances with lower input voltage. And we see that now it's H2 dominated. Distortion is very low anyways. This is very good for the main purpose of a low cost interface.
Quick view on the THD+N vs Frequency, at 96kHz (instead of 48kHz that Amir did), noise and distortion included up to 30kHz:
It's better than what Amir reported, but this is Line Input, not Microphone input that he used (see below for the same with Microphone input).
Focusrite 2i2 Gen4 - Measurements (Microphoe input)
With 2Vrms at the input and +6dB input gain, we get:
This
100dB SINAD.
Increasing the input gain (+8dB) to reach -1dBFS at the input does not please our interface:
We lost 4dB in SINAD (for only additional 2dB gain input), we are just at CD Audio performance, not too bad for a Microphone, that said.
Now again, +6dB gain input is the comfort zone of the 2i2, especially on distortion perspective. Try to stay as much as possible close to that and adjust in your DAW to the final desired level for your mix.
Now if I try to replicate the measurement of Amir, 1.7Vrms at the microphone input and with +9dB input gain, so that we get -0.14dBFS in the dashboard (as in his):
Voilà, our interface is unhappy with high level internal digital input, from an increased input gain.
On a noise perspective, you will be increasing the noise a little up to +12dB input gain. After that, the more gain the more noise and it's quasi linear from +20dB input gain (i.e. +1dB input gain = +1dB noise).
Quick view on the THD+N vs Frequency, at 96kHz (1.7Vrms input which Amir used but at +6dB gain instead of +9dB), noise and distortion included up to 30kHz:
This is nearly same performance as with line input, so it’s good.
Focusrite 2i2 Gen4 - Measurements (Instrument input)
Before I start talking about the Instruments input, I need to mention that the Focusrite 2i2 Gen4 does not appreciate at all to be connected with an unbalanced cable at the input unless... we push the "Inst" button to go Instrument input (at the front of the 2i2 or in the Focus Control 2 computer interface).
Indeed, when using line input with unbalanced connectors (TS cable or RCA to TS), the distorsion increases a lot. Pressing the "Inst" button immediately reduces the distortion. The Instruments input are designed to address much lower voltage sources. A guitar, for instance, outputs few mV, maybe 1V max, and uses a TS connector.
When the "Instrument" input is engaged, the interface automatically adjust the input gain to +7dB minimum.
So let's feed the 2i2 with 100mVrms and the input gain adjusted to +31dB, we get:
Forget about the mains components (50Hz and 150Hz), they are due to the absence of decent galvanic isolation between the two computers I used.
We have a THD of -89dBr and the SINAD is only 83dB because of the low signal level and the huge gain at the input (which inevitably increase the noise level too).
With higher input level, like 250mVrms, and input gain adjusted at +23dB:
You see the noise is lower (-92dB) relative to the signal because the amplitude of the signal is higher, and the input gain is lower. But the THD has increased to -78dBr. So now the SINAD (78dB) is dominated by distortion instead of noise.
Even if this looks like high harmonic distortion (and it is, per our standards of DAC resolution of today) this is not so a problem when recording an instrument which has a lot of harmonic content by definition, much much higher than this unwanted distortion. In other words, it will remain hidden.
So this instrument input does not like high level signals. Anything playing above 0dBU (0.77Vrms) will upset it, and I'd recommend using the line input, rather that instrument input, in that case. But do not use unbalanced connectors with the line input.
I hope all of this helps the future users.
Cheers
