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How to calibrate levels on the Focusrite ISA 2

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Re: How to calibrate levels on the Focusrite ISA 2

Postby Hugh Robjohns » Mon Jan 13, 2020 4:45 pm

armans wrote:If my max outputs on the Clarett are +18dbu then wouldbt 4dbu be -14dbfs?

Sorry, I missed this question earlier.

Yes, if the unit's meter is scaled such that 0dBFS equates to +18dBu, then +4dBu, being 14dB lower, should also equate to -14dBFS. Your maths is working... :lol:

But the question here is, are we talking about specific analogue signal levels, or typical programme reference levels? In your case, which simply concerns meter calibrations, I'd suggest the only thing of interest is the meter reference level... And when working in the digital domain, it is usual to take -20dBFS as that reference level.

It really doesn't matter -- in your specific situation -- what the analogue signal level is required to achieve that display on the 'digital' meter in the ISA, or the meter in the DAW. So the analogue level between the ISA2 and interface could be -2dBu or 0dBu or +4dBu... it really doesn't matter in any practical sense. All that matters is that the two meters tell you the same story.
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Re: How to calibrate levels on the Focusrite ISA 2

Postby armans » Tue Jan 14, 2020 5:29 pm

ef37a wrote:Ah, so even more 'sumer' than 'pro'! That is not even 20dB headroom for -10dBV.

Really? Does that mean my audio inteface is rubbish?
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Re: How to calibrate levels on the Focusrite ISA 2

Postby armans » Tue Jan 14, 2020 5:32 pm

Hugh Robjohns wrote:The max output level from the ISA2 is +24dBu and by default its metering is calibrated such that 0dBFS on the meter occurs at +22dBu (but the adjusting range spans +16 to +24dBu).

However, this is a bit of a problem since the front line input of the Clarett, with the knob fully anticlockwise expects +26dBu to reach 0dBFS... so the ISA's meters and those in the DAW can never be aligned on tones when using that input with the gain knob fully down.

You could align them with the input gain turned up on the interface, but then you'll need to recalibrate every time you use it which is not practical.

So, you'd be better off using one of the rear panel line inputs to receive the ISA2 since the max level here is +18dBu, and you can definitely match the ISA2 meters to that.

when I switch over to using a microphone the levels are nowhere near being calibrated. My microphone level is way too hot in my DAW compared to what the meter on the ISA 2 is showing me. Any ideas what the problem could be?

Yes... different meter ballistics. We align meters using steady tones because that's the only way we can. Different meters respond very differently with programme material because some are true-peak reading, some are quasi-peak reading, some display the true-RMS level, some a quasi-RMS level, and some now indicate the perceived loudness... They all have different attack (integration) and recovery times, too... so no two different meter types will ever show the same thing on programme audio material.

As a result, you have to be able to recognise and interpret what the different meters are telling you, and use their information accordingly.

Of course, if you're recording with a sensible headroom margin it really doesn't matter anyway -- the meter is only a rough guide to getting the signal sitting somewhere reasonably sensible between the noise floor and the clipping point.

From your description, I'd hazard a guess that the ISA2 is showing something close to the peak levels, while your DAW is showing something more similar to the VU or an RMS meter.

At the end of the day, though, the meter is just an aid. Tweak its calibration knob until you get consistent recording levels, in that what you think you're recording from the ISA2 looks roughly the same on the DAW... Take the pragmatic approach!

H

Understood thank you. Do you think there is any downside to using the front inputs as opposed to the back ones other than the obvious fact that I sacrifice the front inputs which have preamps in them. The Front inputs go through the preamp circuit so I am guessing the back inputs were made for using with external preamps and line level signal except I can't understand why Focusrite chose to have only +10dbu max input there since most pro gear uses +24dbu
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Re: How to calibrate levels on the Focusrite ISA 2

Postby Hugh Robjohns » Tue Jan 14, 2020 5:48 pm

armans wrote:Do you think there is any downside to using the front inputs as opposed to the back ones other than the obvious fact that I sacrifice the front inputs which have preamps in them.

Only the practical issue of meter alignment as already discussed. Potentially, there would be fractionally more noise and distortion by going through the extra preamp stage, but it would be negligible.

I am guessing the back inputs were made for using with external preamps and line level signal except I can't understand why Focusrite chose to have only +10dbu max input there since most pro gear uses +24dbu

It does seem odd... but I guess they were expecting people to connect semi-pro equipment with a nominal -10dBV operating level.
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Re: How to calibrate levels on the Focusrite ISA 2

Postby ef37a » Wed Jan 15, 2020 9:30 am

Can someone post a link to the Claret 4 TBs specifications please?
I'll be jiggered if I can find it!

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Re: How to calibrate levels on the Focusrite ISA 2

Postby Hugh Robjohns » Wed Jan 15, 2020 11:09 am

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Re: How to calibrate levels on the Focusrite ISA 2

Postby ef37a » Wed Jan 15, 2020 11:16 am

Hugh Robjohns wrote:https://customer.focusrite.com/sites/customer/files/focusrite/downloads/27309/clarett-4pre-user-guide-en.pdf

Page 17

Yes, got that Hugh but it states "Max input +26dBu" Where is the specc for the fixed inputs at +10dBu?

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Re: How to calibrate levels on the Focusrite ISA 2

Postby Hugh Robjohns » Wed Jan 15, 2020 11:51 am

Sorry Dave... that's all I could find.
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Re: How to calibrate levels on the Focusrite ISA 2

Postby ef37a » Wed Jan 15, 2020 3:39 pm

Hugh Robjohns wrote:Sorry Dave... that's all I could find.

So, unless the correct figures can be found we could be doing Focusrite a grave injustice?

+10dBu seems a very poor headroom figure for even a sub £100 interface.

Was there a typo about or a misread number? Thinking on, much of this confusion would be saved if mnfctrs always gave us "XdBu in produces Y dB fs."

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Re: How to calibrate levels on the Focusrite ISA 2

Postby Hugh Robjohns » Wed Jan 15, 2020 4:16 pm

I've no idea where the idea of a +10dBu max line input came from. It seems very odd to me.

All the hardware facilities and other input and output specs appear identical between the USB and TB models, so I'd have expected the rear panel line inputs to have the same, entirely reasonable, +18dBu max input level.

H
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Re: How to calibrate levels on the Focusrite ISA 2

Postby armans » Thu Jan 16, 2020 9:37 am

ef37a wrote:Can someone post a link to the Claret 4 TBs specifications please?
I'll be jiggered if I can find it!

Dave.

It was replaced by the new usb one. I have the manual at home but even there it does not state the max input for the back inputs. Focusrite only put the max input specs for the front inputs into the manual. I wonder why, maybe because they prefer not to show it.
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Re: How to calibrate levels on the Focusrite ISA 2

Postby armans » Thu Jan 16, 2020 4:40 pm

ef37a wrote:
So, unless the correct figures can be found we could be doing Focusrite a grave injustice?

+10dBu seems a very poor headroom figure for even a sub £100 interface.

From focusrite:

If you have the Clarett 4Pre USB then you are correct, the maximum input is +18dBu, but if you have the Clarett 4Pre this is +10dBu.
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Re: How to calibrate levels on the Focusrite ISA 2

Postby armans » Thu Jan 16, 2020 4:42 pm

ef37a wrote:
Yes, got that Hugh but it states "Max input +26dBu" Where is the specc for the fixed inputs at +10dBu?

Maybe they left it out on purpose?
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Re: How to calibrate levels on the Focusrite ISA 2

Postby armans » Thu Jan 16, 2020 4:50 pm

Hugh Robjohns wrote:I've no idea where the idea of a +10dBu max line input came from. It seems very odd to me.

All the hardware facilities and other input and output specs appear identical between the USB and TB models, so I'd have expected the rear panel line inputs to have the same, entirely reasonable, +18dBu max input level.

H
I purchased 2 X -15db in-line attenuators. I received them today and was able to calibrate the meters using the back inputs but I must say I am rather dissapointed at how useful the meters will be anyway. What you said about different meters showing different things must be the reason. When I use a steady tone the meters show one thing but when I actually play my guitar and record, the meters in my DAW show much higher peak levels than what the ISA 2 shows me. I think my DAW shows peak values while the Focusrite shows average levels because the meter is always much lower than what I am recording
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Re: How to calibrate levels on the Focusrite ISA 2

Postby Hugh Robjohns » Thu Jan 16, 2020 5:11 pm

armans wrote:When I use a steady tone the meters show one thing but when I actually play my guitar and record, the meters in my DAW show much higher peak levels than what the ISA 2 shows me.

Welcome to the world of 'standards'... :crazy: We all love standards in the audio industry. That's why we have so many of them... all different...

A meter is just an indicator, and with experience you get to learn how to interpret what different meters are indicating.

So you just need to learn what an acceptable peak level is on the ISA meter and reference to that.

...and remember that working with a generous headroom margin always makes life much easier when recording as you don't need to worry about peak levels when you know nothing you do will cause an overload. In that situation, the meter is really just confirming the signal is present and correct.
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