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Scarlett 2i2 vs 18i8 gain range

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Re: Scarlett 2i2 vs 18i8 gain range

Postby Hugh Robjohns » Thu Dec 28, 2017 7:40 pm

ef37a wrote:I was referring to the Radial Key Largo article which said "attest to the quality of the Key Largo's Class-A audio circuitry". Search as I might I can find no such reference in the manual although it does say "audio grade operational amplifiers" .

Ah. A Bob Thomas review. Dunno about the class-A reference. As you say, nothing on the website about it, so whether it's a faux pas, or Bob was told something separately I don't know. I'll make enquiries in the new year.

I am aware of the 'offset bias' trick but it compromises headroom I understand and I can see no good reason for it?

It does, but not significantly or problematically in most cases if used carefully... At least according to Mr Neve!

I cannot recall any review in SOS ... any reviewer ever commenting that a product suffered from low level crossover distortion?

:-D suffered, or benefitted?

Shame that you get lumbered mostly with the esoteric and expensive stuff...

It's no lumber, really! ;-)

The gear that really needs 'sorting out' is down at the 100+ quid region.

Yes and no... At the budget end of the market the cost restrictions inherently restrict the potential performance too.

H
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Re: Scarlett 2i2 vs 18i8 gain range

Postby James Perrett » Thu Dec 28, 2017 10:50 pm

Hugh Robjohns wrote:
There was a recent SOS review of a Focusrite 8 channel preamp/convertor and, given my previous experience, I searched carefully for a reference to the minimum gain spec. There was no mention of it which I found surprising given that it is an issue with at least 3 Focusrite products that I know about and probably more that I don't.

Obviously I can't comment on another reviewer's review content; we each write about the things we see as relevant and important in a given product, and that will be informed by the anticipated use of the product and previous experience -- the latter being a very individual thing, of course.

After writing that I went back and searched for the review on the web site. It was Matt's review of the Scarlett 8 Pre that I was thinking of. However, now that I was able to see all the web links in the review I could trace back through Matt's history of the various Focusrite preamps and I see he mentions the issue in a previous review. This doesn't help those of us who mainly read the print edition but at least I can now see that he has covered the problem previously.

Numbers would still be useful though - I seem to remember in the ancient past that some magazines would have 2 sections to a review with a technical evaluation done by another reviewer - although I could see that could get a bit onerous if Hugh got lumbered with that job for every review.
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Re: Scarlett 2i2 vs 18i8 gain range

Postby fladd » Fri Dec 29, 2017 12:29 am

Hugh Robjohns wrote:
fladd wrote:If I understand you correctly, then there is no such thing as absolute gain.

Yes, there's absolute gain -- how much bigger or smaller the signal is made, overall -- but that is the sum of all gain stages, including that of the digital conversion process. In the case of your two interfaces, I suspect the 2i2 effectively has less analogue gain but more digital gain than the 18i8.

Let me try and explain it with some numerical examples ...

Let's say the output voltage from your microphone is 3mV (0.003V).

If the first preamp boosts that by 46dB the signal level will be raised to 0.6V (in round numbers).

If the second preamp boosts it by 50dB it would produce an analogue output signal level of 0.95V.

...and in the analogue world, the latter is obviously (4dB) louder than the former, as you would expect.

However, let's say that these analogue signals are fed into different A-D converters, and the first converter is set up so that it reaches 0dBFS (clipping level) with an input signal level of 1V. In that case, the input of 0.6V produces a digital output level of -4.4dBFS.

But let's say the second interface is set up with 6dB more headroom so that it delivers 0dBFS only when the input voltage reaches 2.0V. In that case the input signal of 0.95V produces a digital output of -6.5dBFS.

...and consequently the output from the second interface is actually 2dB lower in the digital domain than the that from the first preamp, despite that first model having 4dB less analogue gain.

And the reason is that although it has 4dB less analogue gain, it effectively has 6dB more digital gain (from the conversion alignment) and thus 2dB more gain overall from analogue input to digital output.

This seems to have several very important and rather surprising implications:

(a) For a manufacturer to provide, and for a customer to read the maximum gain value (in isolation) is completely uninformative.

Well, not completely uninformative. It is still useful to know the available analogue gain range. 40dB is about the minimum you can get away with with close-miked sources. 60dB is typically required for quieter sources or less sensitive mics, and 75dB or more for very insensitive mics with distant placements... for example.

(b) Statements such as "an SM7b needs at least 60dB of gain" are entirely meaningless.

Without a context, arguably, yes. Although in the context of a typical analogue preamp/mixer, it's a handy rule of thumb to estimate the amount of gain required in a typical application.

(c) There is no a priori way of knowing if an interface/preamp will provide enough amplification for a given microphone.

I refer you to my earlier answer re typical 40/60/75dB gain range applications.

(d) Comparing the amount of amplification between interfaces/preamps is not possible without actually testing them against each other.

This one is definitely true because, as I said in my previous response, few manufacturers specify the the analogue-digital conversion alignment, and thus the gain through to the digital domain.

Out of interest, what sort of digital level difference are you getting from your two Scarlett interfaces?

H

Thanks again for the detailed response.

I just did a measurement with a SM58 and the difference is 10.5 dB. That is, the 18i8 at full gain is 10.5 dB quieter than the 2i2 at full gain, according to Reapers's meters.

Given that this is quite a substantial difference, I went back to the Focusrite website to look at the specs again. The 2i2 has a max input level of 4 dB. The 18i8 has a max input level of 8.5 dB. If both devices had the same amount of gain, you would expect the 18i8 to be 4.5 db quieter than the 2i2. Given that it is 10.5 dB quieter, I have to conclude that the 18i8 actually has 5 dB gain less than the 2i2. This means that one of the two specs seems definitely wrong. The question is of course which one. Either the 18i8 has actually only 41 dB of gain (instead of the stated 46), or the 2i2 has actually 51 dB of gain (instead of the stated 46).

Are my conclusions correct?
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Re: Scarlett 2i2 vs 18i8 gain range

Postby fladd » Fri Dec 29, 2017 1:15 am

I should also mention that the pad is not engaged. Engaging the pad will result in a difference of 20.5dB.
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Re: Scarlett 2i2 vs 18i8 gain range

Postby Hugh Robjohns » Fri Dec 29, 2017 1:41 am

fladd wrote:I just did a measurement with a SM58 and the difference is 10.5 dB. That is, the 18i8 at full gain is 10.5 dB quieter than the 2i2 at full gain, according to Reapers's meters.

Thanks. Yes, a significant difference, I agree.

The 2i2 has a max input level of 4 dB. The 18i8 has a max input level of 8.5 dB. If both devices had the same amount of gain, you would expect the 18i8 to be 4.5 db quieter than the 2i2.

I don't think your thought process holds water. The max input level indicates the available headroom margin of the input stage, but (a) your test signal is (hopefully) nowhere near those maximum input levels...and (b) the max input levels aren't directly related to the available gain. The gain controls the output level, which might reach as much as +24dBu, for example. So while there is 4.5dB difference in the input headroom, that doesn't relate to the overall system gain in any way.

I have to conclude that the 18i8 actually has 5 dB gain less than the 2i2.

No, the 18i8 has about 11dB less overall gain -- you've already proved that. But since it has 4dB more analogue gain (apparently) it must have a digital conversion alignment which is around 15dB lower, or in other words, around 15dB more digital headroom.

This means that one of the two specs seems definitely wrong.

No, it means the spec is incomplete.

Either the 18i8 has actually only 41 dB of gain (instead of the stated 46), or the 2i2 has actually 51 dB of gain (instead of the stated 46).

Well, it is entirely possible that the actual analogue gain ranges don't match the stated values in the control knobs... but I'd be surprised if that was the case.

What's really needed here is a proper measurement of the actual overall gain from analogue input to digital output. It would be easy to achieve if you have a calibrated analogue test oscillator....
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Re: Scarlett 2i2 vs 18i8 gain range

Postby fladd » Fri Dec 29, 2017 1:36 pm

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.
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Re: Scarlett 2i2 vs 18i8 gain range

Postby Hugh Robjohns » Fri Dec 29, 2017 2:06 pm

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.

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Re: Scarlett 2i2 vs 18i8 gain range

Postby RoadieChauffeur » Fri Dec 29, 2017 3:33 pm

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!
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Re: Scarlett 2i2 vs 18i8 gain range

Postby fladd » Fri Dec 29, 2017 4:38 pm

So all things considered, I should get better (less noisy) recordings with the 2i2 then (especially with gain hungry mics and quiet sources)?
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Re: Scarlett 2i2 vs 18i8 gain range

Postby James Perrett » Fri Dec 29, 2017 6:48 pm

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.
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Re: Scarlett 2i2 vs 18i8 gain range

Postby Wonks » Fri Dec 29, 2017 10:45 pm

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.
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Re: Scarlett 2i2 vs 18i8 gain range

Postby Hugh Robjohns » Fri Dec 29, 2017 11:06 pm

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.

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Re: Scarlett 2i2 vs 18i8 gain range

Postby fladd » Sat Dec 30, 2017 12:55 am

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.
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Re: Scarlett 2i2 vs 18i8 gain range

Postby fladd » Sat Dec 30, 2017 1:04 am

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/.
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Re: Scarlett 2i2 vs 18i8 gain range

Postby James Perrett » Sat Dec 30, 2017 1:27 am

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.
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