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How to calibrate levels on the Focusrite ISA 2

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How to calibrate levels on the Focusrite ISA 2

Postby armans » Sat Jan 11, 2020 3:51 pm

Hello. I have found a tutorial on how to calibrate the levels on the Focusrite ISA 2 but was wondering if someone could give me a hand as i am not sure I am doing it right. My interface is a Clarett 4 which has mic/instrument inputs on the front and line inputs on the back. For the calibration I am using the front line inputs with max +26dbu inputs. To calibrate the meters this is the procedure I have followed:

1. I send a test tone to the line input of ISA Two from my DAW using a TRS cable out of my Clarett output 1

2. I connect the output of ISA Two to input one on the front of my Clarett interface

3. I keep the the mic/line gain knob on the Clarett all the way down

4. I monitor the input level in my DAW, and adjust the gain of ISA Two until the DAW input level reads 0dBFS.

Up until this point I have followed Focusrites instructions (I think) yet, this is when something strange happens, all of a sudden when increasing the gain there is a point at which the gain pops up immediately into 0dbfs. Is this normal? Shouldnt the accent be gradual? It is gradual for a while but when the level gets to around -18dbfs it jumps up all of a sudden.

I would appreciate your help
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Re: How to calibrate levels on the Focusrite ISA 2

Postby CS70 » Sat Jan 11, 2020 6:02 pm

A blind shot, but that kind of behavior suggests a feedback loop.

I'd check the routing on the Clarett to see how the i/o's you're using are connected (in the sense that they shouldn't be).
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Re: How to calibrate levels on the Focusrite ISA 2

Postby Hugh Robjohns » Sat Jan 11, 2020 6:54 pm

Yes, almost certainly a howl-round loop going on. Your input from the ISA is somehow getting routed back to the same output that you're sending tone from. Check the routing setup both in the DAW and in the clarett's control panel.
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Re: How to calibrate levels on the Focusrite ISA 2

Postby armans » Sat Jan 11, 2020 9:39 pm

CS70 wrote:A blind shot, but that kind of behavior suggests a feedback loop.

I'd check the routing on the Clarett to see how the i/o's you're using are connected (in the sense that they shouldn't be).
You guessed it. Thanks for your intuition.
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Re: How to calibrate levels on the Focusrite ISA 2

Postby armans » Sat Jan 11, 2020 9:42 pm

Hugh Robjohns wrote:Yes, almost certainly a howl-round loop going on. You input from the ISA is somehow getting routed back to the same output that you're sending tone from. Check the routing setup both in the DAW and in the clarett's control panel.
Thank you. There was a loop indeed.

But I still am a bit stuck with the meter calibration. Here are the steps as instructed by focusrite

1. Send a test tone to the line input of ISA Two. This can be achieved using the tone generator in your DAW, and the line output of your interface.

2. Connect the line output of ISA Two to a line input of your interface.

3. Monitor the input level in your DAW, and adjust the gain of ISA Two until the DAW input level reads 0dBFS.

4. Adjust the calibration knob on the rear panel of ISA Two until the ISA Two's meter value matches that of your DAW.


Number 4 is not clear to me. Adjust the calibration knob on the rear until the ISA two meter mathes that of your DAW but which one? The output or input?

thanks!
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Re: How to calibrate levels on the Focusrite ISA 2

Postby Hugh Robjohns » Sat Jan 11, 2020 9:58 pm

What Focusrite are suggesting you do is make the meter on the ISA2 show the same thing as the input meter of the DAW so that when you're recording, the preamp meter shows the same as the DAW's A-D input and there's no confusion as to when anything is going to clip!

An alternative, though, to tweaking the meter calibration on the ISA2 would be to tweak the interface's input level instead, so that the DAW's input meter matches the metering on the ISA2.

Two different ways of achieving the same practical outcome.

Use whichever method is easiest...
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Re: How to calibrate levels on the Focusrite ISA 2

Postby armans » Sat Jan 11, 2020 10:55 pm

Hugh Robjohns wrote:What Focusrite are suggesting you do is make the meter on the ISA2 show the same thing as the input meter of the DAW so that when you're recording, the preamp meter shows the same as the DAW's A-D input and there's no confusion as to when anything is going to .

Ok. Then in the 1st step when it says to send a test tone to the Isa it doesnt say what level it should be. Can it be any level?
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Re: How to calibrate levels on the Focusrite ISA 2

Postby Hugh Robjohns » Sun Jan 12, 2020 12:40 pm

The standard reference level for this kind of thing is -20dBFS which typically equates to +4dBu (0VU) in the analogue world... but it can be anything that is convenient. All you're trying to do is match meter readings, after all.

I'd avoid sending peak level (0dBFS) tones though as they can hurt and/or do damage...
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Re: How to calibrate levels on the Focusrite ISA 2

Postby armans » Mon Jan 13, 2020 7:40 am

Hugh Robjohns wrote:The standard reference level for this kind of thing is -20dBFS which typically equates to +4dBu (0VU) in the analogue world... but it can be anything that is convenient. All you're trying to do is match meter readings, after all.

I'd avoid sending peak level (0dBFS) tones though as they can hurt and/or do damage...
If my max outputs on the Clarett are +18dbu then wouldbt 4dbu be -14dbfs?

This is the sine tone I was using and sending to the input of my ISA 2. I then routed that back into my Clarett 4 into input 1 and set my Focusrite ISA to line level. I Then minitored the input in my DAW and raised the gain on ISA 2 until I got 0dbfs in my DAW input. Once it reached 0dbfs I then used the knob on the back of ISA 2 to make the meters on the ISA 2 the same as the input in my DAW (0dbfs). Supposedly that is what I should have done yet 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?

By the way, as a test when I realized the levels were off I went and turned the calibration knob all the way anti clockwise to 16dbu = 0dbfs and this seems to be much closer to showing real dbfs levels of what I am recording so I must be doing something wrong.
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Re: How to calibrate levels on the Focusrite ISA 2

Postby ef37a » Mon Jan 13, 2020 8:17 am

To avoid any problems with feedback loops put the tone on a phone or other external playback device. Even cassette would do, you don't need the signal to be ultra clean.

The ISA2 outputs 'pro' levels . The Claret's inputs do not accept pro level, quite. Better than many 'prosumer' AIs but not quite +4dBu with 24dB of headroom. I also note the pre amp's meters are scaled in dB fs which is odd for an analogue device? Would make some sense if there was an A/D retrofit card option but there is not AFAICS?

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Re: How to calibrate levels on the Focusrite ISA 2

Postby armans » Mon Jan 13, 2020 10:34 am

ef37a wrote:The ISA2 outputs 'pro' levels . The Claret's inputs do not accept pro level, quite. Better than many 'prosumer' AIs but not quite +4dBu with 24dB of headroom.

The Claretts front inputs offer +26dbu max line input! That is with the gain all the way down. I measured this and it is correct.

ef37a wrote:I also note the pre amp's meters are scaled in dB fs which is odd for an analogue device? Would make some sense if there was an A/D retrofit card option but there is not AFAICS?

Dave (aka ecc83)

It may be odd but it definitely helps me because I can look at the meter lights from far away and see what level I am getting in my DAW.
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Re: How to calibrate levels on the Focusrite ISA 2

Postby ef37a » Mon Jan 13, 2020 10:53 am

I had assumed you were using the rear inputs so as to bypass the Claret pre amps and keep them free for other purposes? The rear ins have a maximum input level of +18dBu (iirc from HR thread?)

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Re: How to calibrate levels on the Focusrite ISA 2

Postby Hugh Robjohns » Mon Jan 13, 2020 12:14 pm

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
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Re: How to calibrate levels on the Focusrite ISA 2

Postby armans » Mon Jan 13, 2020 4:06 pm

ef37a wrote:I had assumed you were using the rear inputs so as to bypass the Claret pre amps and keep them free for other purposes? The rear ins have a maximum input level of +18dBu (iirc from HR thread?)

Dave.
On the Clarett 4 USB the max input is +18dbu
On the Clarett 4 thunderbolt version the max input on the rear is +10dbu
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Re: How to calibrate levels on the Focusrite ISA 2

Postby ef37a » Mon Jan 13, 2020 4:42 pm

armans wrote:
ef37a wrote:I had assumed you were using the rear inputs so as to bypass the Claret pre amps and keep them free for other purposes? The rear ins have a maximum input level of +18dBu (iirc from HR thread?)

Dave.
On the Clarett 4 USB the max input is +18dbu
On the Clarett 4 thunderbolt version the max input on the rear is +10dbu

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

(BTW Hugh. I had forgotten about "ballistics"! No, two different meters are never going to 'track')

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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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