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Scarlett 2i2 vs 18i8 gain range

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Re: Scarlett 2i2 vs 18i8 gain range

PostPosted: Fri Dec 29, 2017 1:36 pm
by fladd
Mmh interesting. What I still don't understand then: What is the benefit of having more analogue gain but also more digital headroom? For istance, 40dB of gain and aligning to 4dB or 50dB of gain and aligning to 14dB would result in the exact same signal level in my DAW, right? If that is true then I don't understand why Focusrite would choose to add more gain ( which probably makes the preamps more expensive in production) only to later lower the gain again during conversion, instead of just having a weaker gain (and potentially cheaper) preamp and not lower the digital gain that much during conversion... The user does not seem to benefit from the moor gain. Or in case of the 18i8, the new gain staging is even much worse than one their low cost devices, if I understand it correctly.

Re: Scarlett 2i2 vs 18i8 gain range

PostPosted: Fri Dec 29, 2017 2:06 pm
by Hugh Robjohns
fladd wrote:What I still don't understand then: What is the benefit of having more analogue gain but also more digital headroom?

It all about optimising the gain structure to minimise noise and maximise headroom, while still providing a sufficiently wide adjustable (analogue) gain range to cope with a variety of different microphone sensitivities and likely applications.

I can't tell you why Focusrite have made the specific choices they have in these two products.... but it will be based heavily on customer feedback as to what people want/need and what works best for the majority of customers.

H

Re: Scarlett 2i2 vs 18i8 gain range

PostPosted: Fri Dec 29, 2017 3:33 pm
by RoadieChauffeur
fladd wrote:What I still don't understand then: What is the benefit of having more analogue gain but also more digital headroom?
As Hugh says, the design choices can be more influenced by commercial than technical considerations - the end aim being to make something that can be sold at a profit and people will buy, rather than a technically perfect device. Cost engineering considerations will also come into play, which can include things as odd as 'we can get this part cheap because we buy lots of them for something else we make'.

On the wider subject of gain, in devices that take an analogue input and give a digital output, you are quite right to be a bit confused by the specifications (and Hugh is equally right to be annoyed by the the fact that half these specs are usually missing). At the end of the day, both signals represent the sound, but the magnitude of both is meaningless and only relevant to other parts of the same signal, apart from some conventions on signal levels that help us all join bits of kit together.

When the analogue signal is put into an ADC in the same device, the signal level (and hence the analogue gain required) at the conversion point is arbitrary and irrelevant to the outside world. It would be just as possible (but pushing what is sensible!) to have an ADC with a full scale input of 1mV as an ADC with a full scale input of 100V - the latter would obviously require huge amounts of gain in the analogue stage but you could/should end up with exactly the same numbers come out of the digital stage. The choices here a very much about what is easiest to do for the lowest noise.

As both you and Hugh have alluded to, the range of gain available is what gives you flexibility to cope with a range of source levels. But again, this is mostly a feature of normal designs. It is perfectly possible (again, not necessarily sensible or practical) to build an interface with zero analogue gain, but an adjustable input scale on the ADC - i.e. you could change the reference voltage for full scale from a few mV to a few V say. Give or take a few technical considerations, you'd end up with exactly the same data coming out the end!

Re: Scarlett 2i2 vs 18i8 gain range

PostPosted: Fri Dec 29, 2017 4:38 pm
by fladd
So all things considered, I should get better (less noisy) recordings with the 2i2 then (especially with gain hungry mics and quiet sources)?

Re: Scarlett 2i2 vs 18i8 gain range

PostPosted: Fri Dec 29, 2017 6:48 pm
by James Perrett
fladd wrote:So all things considered, I should get better (less noisy) recordings with the 2i2 then (especially with gain hungry mics and quiet sources)?

You could have just looked at the input noise spec to reach the same conclusion. The 18i8 claims an input noise of -122dBu at maximum gain while the 2i2 claims >-127dBu at 60dB gain. While the gain figures may be pretty meaningless in this context as they could send any level they like to the A/D convertor, the noise figures are absolute values that are easily compared.

Re: Scarlett 2i2 vs 18i8 gain range

PostPosted: Fri Dec 29, 2017 10:45 pm
by Wonks
But your recording space will still be far noisier, so unless you are recording something quiet a long way away with a dynamic mic, then the space will be far noisier than any mic or pre-amp noise if you've used sensible input gain levels.

Re: Scarlett 2i2 vs 18i8 gain range

PostPosted: Fri Dec 29, 2017 11:06 pm
by Hugh Robjohns
fladd wrote:So all things considered, I should get better (less noisy) recordings with the 2i2 then (especially with gain hungry mics and quiet sources)?

I doubt you'd actually be able to tell any difference in 'the noise' between the two interfaces in normal use simply because, as Wonks points out, the ambient acoustic room noise is likely to far outweigh the electronic noise floor contorbuted by the preamps.

Okay, so it appears that the EIN noise floor specs are lower for the 2i2, and it would be logical therefore that this model should be notionally quieter (electronically) for a given digital output level.

However, the important fact to remember is that neither of these interfaces is designed or intended for use with gain-hungry mics or quiet sources; they both have too little gain for that, quite intentionally. They're actually intended for common close-miking techniques on vocals, guitars etc, in typical project studio applications with common dynamic and capacitor (condenser) mics -- and from the feedback I've received they both seem to meet those needs pretty well.

H

Re: Scarlett 2i2 vs 18i8 gain range

PostPosted: Sat Dec 30, 2017 12:55 am
by fladd
James Perrett wrote:
fladd wrote:So all things considered, I should get better (less noisy) recordings with the 2i2 then (especially with gain hungry mics and quiet sources)?

You could have just looked at the input noise spec to reach the same conclusion. The 18i8 claims an input noise of -122dBu at maximum gain while the 2i2 claims >-127dBu at 60dB gain. While the gain figures may be pretty meaningless in this context as they could send any level they like to the A/D convertor, the noise figures are absolute values that are easily compared.

Mmh, not sure where you got the noise figure of 122 dB for the 18i8 from. The official specs at the Focusrite website quote an EIN of 127 for the 18i8 (https://us.focusrite.com/usb-audio-inte ... 8#techspec). Are these measurements you did yourself?

I also don't quite understand where you got the 60dB measurement for the 2i2 from (it does not appear in the official specs on the Focusrite website, nor does the preamp of the 2i2 deliver 60dB of gain anyway: https://us.focusrite.com/usb-audio-inte ... ifications).

But in any case, I would have certainly expected better specs on the upper range devices...
I also wrote Focusrite support to ask about this issue, and they seem to be confused, too, stating that the preamps should be pretty closely matched.

Re: Scarlett 2i2 vs 18i8 gain range

PostPosted: Sat Dec 30, 2017 1:04 am
by fladd
Hugh Robjohns wrote:
fladd wrote:So all things considered, I should get better (less noisy) recordings with the 2i2 then (especially with gain hungry mics and quiet sources)?

I doubt you'd actually be able to tell any difference in 'the noise' between the two interfaces in normal use simply because, as Wonks points out, the ambient acoustic room noise is likely to far outweigh the electronic noise floor contorbuted by the preamps.

Okay, so it appears that the EIN noise floor specs are lower for the 2i2, and it would be logical therefore that this model should be notionally quieter (electronically) for a given digital output level.

However, the important fact to remember is that neither of these interfaces is designed or intended for use with gain-hungry mics or quiet sources; they both have too little gain for that, quite intentionally. They're actually intended for common close-miking techniques on vocals, guitars etc, in typical project studio applications with common dynamic and capacitor (condenser) mics -- and from the feedback I've received they both seem to meet those needs pretty well.

H

Yes, you are of course right. I also do intend to use the 18i8 in this way. My quietest mic is an SM58. I was just a bit surprised to see this huge difference in (digital) level, as I always assumed that, if anything, the higher range model should deliver a "hotter" signal.

Btw. if anyone is interested, I uploaded the test recordings (since Focusrite support also wanted them). I recorded the output of my mobile phone's built in speakers into an SM58 with the mic and mobile phone position (and volume) constant between the recordings. I merely unplugged the mic cable from one interface and put it into the other. Both interfaces where at max gain. I know this was not the best test scenario, but it was the most constant test I could do where I am right now (not at home), and it should be good enough for measuring the difference in level. Here are the recordings: http://stuff.fladd.de/scarlett/.

Re: Scarlett 2i2 vs 18i8 gain range

PostPosted: Sat Dec 30, 2017 1:27 am
by James Perrett
fladd wrote: Are these measurements you did yourself?

I did a very quick Google search and those figures appeared in the text that Google displayed for

https://uk.focusrite.com/usb-audio-inte ... ifications

and

https://uk.focusrite.com/usb-audio-inte ... cification

I realise that I attributed the 60dB gain condition to the wrong device though.

Re: Scarlett 2i2 vs 18i8 gain range

PostPosted: Sat Dec 30, 2017 1:32 am
by fladd
James Perrett wrote:
fladd wrote: Are these measurements you did yourself?

I did a very quick Google search and those figures appeared in the text that Google displayed for

https://uk.focusrite.com/usb-audio-inte ... ifications

and

https://uk.focusrite.com/usb-audio-inte ... cification

I realise that I attributed the 60dB gain condition to the wrong device though.

Oh, I see. The specs you looked at are from the 1st gen 18i8, though. Mine is a 2nd gen 18i8. Apparently the noise figure has changed.

The 60dB are still strange, since the old 18i8 also only reached 55dB :-)

Re: Scarlett 2i2 vs 18i8 gain range

PostPosted: Tue Jan 02, 2018 6:30 pm
by fladd
No response from Focusrite anymore. Guess they are also confused.

Re: Scarlett 2i2 vs 18i8 gain range

PostPosted: Sat Jan 06, 2018 11:13 pm
by fladd
Mmh, I now did get an answer from the Focusrite support. They also measured it with the same two devices, sending a sine wave with a generator and got different results.

For the 18i8 (2nd gen), they found the level to be -9.2dBFS. For the 2i2 (1st gen), they found the level to be -5.0dBFS. They then state that this closely matches the spec (not sure which ones exactly).

That is quite some difference to what I measured (4.2dB vs. 10.5dB difference). I will repeat my test in a different way tomorrow. They suggested to measure a sine wave from a third interface connected to the XLR inputs. I don't have a third interface, but I have a Circuit and a JD-Xi, I only don't know if connecting them to the XLR inputs will be safe (since these inputs are meant for much lower level mic signals).

What do you think?

Re: Scarlett 2i2 vs 18i8 gain range

PostPosted: Sat Jan 06, 2018 11:18 pm
by Wonks
Got a DI box?

Re: Scarlett 2i2 vs 18i8 gain range

PostPosted: Sat Jan 06, 2018 11:50 pm
by fladd
Wonks wrote:Got a DI box?
No. Is that needed?