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Scarlett 2i2 vs 18i8 gain range

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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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Re: Scarlett 2i2 vs 18i8 gain range

Postby fladd » Sat Dec 30, 2017 1:32 am

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 :-)
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Re: Scarlett 2i2 vs 18i8 gain range

Postby fladd » Tue Jan 02, 2018 6:30 pm

No response from Focusrite anymore. Guess they are also confused.
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Re: Scarlett 2i2 vs 18i8 gain range

Postby fladd » Sat Jan 06, 2018 11:13 pm

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?
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Re: Scarlett 2i2 vs 18i8 gain range

Postby Wonks » Sat Jan 06, 2018 11:18 pm

Got a DI box?
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Re: Scarlett 2i2 vs 18i8 gain range

Postby fladd » Sat Jan 06, 2018 11:50 pm

Wonks wrote:Got a DI box?
No. Is that needed?
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Re: Scarlett 2i2 vs 18i8 gain range

Postby fladd » Mon Jan 08, 2018 11:37 am

I now conducted a further test with all inputs on both interfaces: https://we.tl/Ok3LWDLn3Q.

A sine wave from Z3ta+ running on iPad was recorded through the iPad's speakers into a Shure SM58. In the first run, the sine wave hit the microphone at 76.5 dBA (measured with an external volume meter). In the first run, the sine wave hit the microphone at 88.5 dBA (measured with an external volume meter).

Notably, in the second run, I tried to match the dB reading of the 2i2 in Reaper to that of what Focusrite had in their test (i.e. 5.0dB).

The results are consistent. The 18i8 is around 10.5dB quieter. I am not sure how Focusrite got their results (I wrote them again, so maybe I will know soon).

Is there a way to calculate, based on the measured volume, the specs of the SM58 and the specs of the interfaces, what the expected level in the DAW should be?
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Re: Scarlett 2i2 vs 18i8 gain range

Postby Hugh Robjohns » Mon Jan 08, 2018 11:59 am

fladd wrote:Is there a way to calculate, based on the measured volume, the specs of the SM58 and the specs of the interfaces, what the expected level in the DAW should be?

The sensitivity of the SM58 is published by Shure... but it's not actually relevant here as it is common to all your tests, and you are complaining about the different sensitivities of the two interfaces, not the mic in comparison to other mics.

However, 76.5dB SPL at the microphone should provide an electrical output of about -70dBu, and 88.5dB SPL would produce about -58dBu. Those signal levels will be raised by the analogue gain in the interface, and then converted into a digital signal according to the converter alignment as previously discussed.

Unfortunately, there is no way to calculate the digital signal level recorded in the DAW from a given sound pressure level reaching the mic from first principles because, as I explained much earlier, Focusrite don't state the internal alignment of the analogue/digital signal levels through the converters in the two interfaces.

What you can do, though, is measure the interface's overall conversion level -- as you have done -- by noting what digital level (in dBFS) is achieved for a certain analogue input signal level with the user gain control at a certain setting.

I'm still at a loss as to what exactly you're trying to achieve through all this, though.

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Re: Scarlett 2i2 vs 18i8 gain range

Postby Matt Houghton » Mon Jan 08, 2018 12:36 pm

James Perrett wrote: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.

Point taken, James. As Hugh said above, striking the right balance between being diligent and informative, and being repetitive and long-winded can be tricky. But I do try at the very least to include references/links to relevant previous reviews when I have to keep things brief!
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Re: Scarlett 2i2 vs 18i8 gain range

Postby fladd » Mon Jan 08, 2018 11:21 pm

Hugh Robjohns wrote:
fladd wrote:Is there a way to calculate, based on the measured volume, the specs of the SM58 and the specs of the interfaces, what the expected level in the DAW should be?

The sensitivity of the SM58 is published by Shure... but it's not actually relevant here as it is common to all your tests, and you are complaining about the different sensitivities of the two interfaces, not the mic in comparison to other mics.

However, 76.5dB SPL at the microphone should provide an electrical output of about -70dBu, and 88.5dB SPL would produce about -58dBu. Those signal levels will be raised by the analogue gain in the interface, and then converted into a digital signal according to the converter alignment as previously discussed.

Unfortunately, there is no way to calculate the digital signal level recorded in the DAW from a given sound pressure level reaching the mic from first principles because, as I explained much earlier, Focusrite don't state the internal alignment of the analogue/digital signal levels through the converters in the two interfaces.

What you can do, though, is measure the interface's overall conversion level -- as you have done -- by noting what digital level (in dBFS) is achieved for a certain analogue input signal level with the user gain control at a certain setting.

I'm still at a loss as to what exactly you're trying to achieve through all this, though.

H

Thanks for the explanation.

Given the measurements of the Focusrite support, together with the statement that their measurements closely match the specs (whatever these are), one has to come to the conclusion that one of my two Focusrite interfaces is not functioning correctly (since my measurements are off by ~ 6 dB). What I am trying to achieve is to figure out which one.

Of course one could argue that it does not matter, as I can always increase digital gain, but in that case I would not understand what the point of analogue gain is in the first place (or even more so, why most people want a lot of it), because you can always increase the signal in the digital domain after recording it. Why would one need a preamp at all then?
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Re: Scarlett 2i2 vs 18i8 gain range

Postby James Perrett » Mon Jan 08, 2018 11:55 pm

fladd wrote: Why would one need a preamp at all then?

Most mics have a wider dynamic range than all but the very best A/D convertors can accommodate. In addition, A/D convertors often require a signal of a few volts to achieve full scale. The preamp therefore has to change the output level of the mic to match the input level of the A/D and ensure the optimum gain structure.

Mics with digital outputs exist but they are expensive compared to their analogue equivalents which reflects the need for a high grade A/D convertor.
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