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  #1  
Old 10-06-2019, 06:12 PM
jbreher jbreher is offline
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Default HD I/O calibration question

I am wanting to calibrate my HD I/O. I am trying to follow the instructions in the user guide, employing the (admittedly possibly modified) template 'Calibration Session (8ch)' found in the '/Applications/Avid/Pro\ Tools/Pro\ Tools\ Utilities/Calibration\ Mode\ Sessions/HD\ IO\ Calibration\ Session' folder.

The process seem logical to me. My confusion: upon opening the Signal Generator plugin, the settings are:
Sine (good)
Frequency 1000Hz (good)
Level -20.0 dB (I'll change to -18, as I'll be rock focused)
Mode: Peak (WTF!?)

Should this not be outputting my cal level (-18 dB) RMS, rather than Peak? I'll be trying to hit 1.228 Vrms at each output, not Vpeak, right?

I don't know the entire history of this session template. Someone may have modified it from RMS to Peak. But I would have expected it to come up RMS.

Can someone resolve my confusion?
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  #2  
Old 10-06-2019, 06:41 PM
Darryl Ramm Darryl Ramm is offline
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Default Re: HD I/O calibration question

Pick what is easier for you. If you have a rms voltmeter use rms, if you have oscilloscope then use peak. I've never looked at that session, just created what is needed on the fly.
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  #3  
Old 10-06-2019, 07:46 PM
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Default Re: HD I/O calibration question

Hi, welcome to the community.

As said you need to know what you are measuring
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  #4  
Old 10-06-2019, 09:06 PM
jbreher jbreher is offline
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Default Re: HD I/O calibration question

Thanks, but I really don't find either response helpful. It may be that I have poorly framed the question.

Avid provides a calibration procedure for the HD I/O in the User Guide. Unfortunately, the procedure is incomplete.

I understand that I am to choose my desired headroom, and reference my standard operating level to that value. The user guide indicates that the device was calibrated at the factory for 18 dB headroom at +4dBu. I am happy to work at that reference. I wish to ensure that this particular unit is properly calibrated to that level.

The User Guide, as mentioned above, provides a calibration procedure. Using this procedure, I expect to be able to calibrate the unit for 18 dB headroom at +4dBu. It explains how to use the provided template -- and even to create such a session from scratch -- to accomplish this task. This procedure uses the Signal Generator plugin. It describes how to set the Signal Generator plugin - with the rather egregious omission of whether to use RMS or to use Peak.

So that is the totality of my reframed question - when employing the procedure in the HD I/O User Guide, do I set the Signal Generator plugin to RMS or to Peak?

Since initial post, I went ahead and set the Signal Generator to RMS, and ran the procedure. After completion, I had the puzzling result of the input channels metering below the output channels. So I subsequently set the Signal Generator to Peak, and ran the procedure a second time. Upon completion, the meters read a consistent level of -18 for all inputs as well as all outputs. This I take as a strong clue that when using the defined procedure to calibrate the HD I/O, the Signal Generator must be set to Peak.

Or am I missing something?
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  #5  
Old 10-06-2019, 11:48 PM
Darryl Ramm Darryl Ramm is offline
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Default Re: HD I/O calibration question

You explained what you are setting the plugin to but you never explained **exactly** how you are measuring the output value? Do you have an actual VU meter? Or are you measuring voltage? What *exact* device are you using and what exact level are you looking for? Are you factoring in load impedance corrections?

You want what calibration level? 18dB headroom above 0dbFS = 0VU = +4dBu

dBFS is always peak... from the literal meaning of "full scale" So if you can really measure 0VU on a real VU meter then set the signal generate to use peak--the template was correct. There are no other things to think about (even with VU itself being defined against a V rms reference that does not matter, since here you are using 0VU as the reference measured on a VU meter).

But just calculate the numbers needed if you want to read any other how, or read at a different calibration point to make turning the pot easier... e.g. aim exactly for 2V pp on an oscilloscope.

The calibration instructions could be better in my option. How many places have a VU meter handy vs. a digital multimeter? And which is likely better calibrated/easier to set things to more closely across the channel?. So just work out what to do yourself. And think it through carefully... are you are using a VU meter what impedance is it calibrated for 600 Ohms actual spec definition.. Or... ? Do you want you want to correct for source/load impedance?, ... the 600 ohm VU spec load impedance across the ~50 ohm HD IO Output impedance(thanks BScout ) .. then you can factor in the ~0.7 dB difference if measuring voltages on an effectively infinite impedance volt meter or oscilloscope (make sure any scope is in a high input impedance mode)).

Last edited by Darryl Ramm; 10-07-2019 at 12:10 AM.
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  #6  
Old 10-07-2019, 01:00 AM
jbreher jbreher is offline
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Default Re: HD I/O calibration question

Thanks for the continued discussion.

Quote:
Originally Posted by Darryl Ramm View Post
You explained what you are setting the plugin to but you never explained **exactly** how you are measuring the output value?
OK.

Quote:
Do you have an actual VU meter? Or are you measuring voltage? What *exact* device are you using
No VU meter - that seems yet another variable that would require calibration first. I am measuring voltage. I am using a Hewlett-Packard 973A handheld true RMS DMM. The probes are across hot and cold of each channel in turn. At least for the outputs. The input calibration -- as per the procedure defined in the HD I/O User Guide -- employs the internal SW metering with the previously calibrated outputs.

Quote:
and what exact level are you looking for?
I am seeking to calibrate to -18dBFS corresponding to my nominal operating level of 0VU or +4dBu.

Quote:
Are you factoring in load impedance corrections?
No. I am measuring open-circuit levels on the outputs (or rather feeding the high Z input of my DMM). My assumption is that the output Z of the HD I/O is low enough in comparison to the input Z of any input I might feed with it so as to have a negligible effect upon the output voltage.

Quote:
You want what calibration level?
The factory setting of nominal 0VU or +4dBu corresponding to -18 dBFS is what I would like to calibrate to.

Quote:
dBFS is always peak... from the literal meaning of "full scale" So if you can really measure 0VU on a real VU meter then set the signal generate to use peak--the template was correct.
Yeah, I think I came around to that conclusion. With the exception that it was set up for a calibration level that does not correspond to the factory settings (i.e. -20 dBFS rather than -18 dBFS). I understand that there are numerous environments where -20 is more appropriate, but it just seems odd to ship a cal template that is in conflict with stated factory setting.

Quote:
There are no other things to think about
Manifestly, the results differ -- at least following the procedure in the user guide -- whether one employs Peak or RMS measurement of the signal source. As it should.

Thanks for your help.

Last edited by jbreher; 10-07-2019 at 09:04 AM.
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  #7  
Old 10-07-2019, 10:23 AM
originalscottyg originalscottyg is offline
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Default Re: HD I/O calibration question

My understanding is that the Signal Generator plugin allows you to select dB-RMS or dB-PEAK. These are the same for a pure sine wave, but different for more complex waveforms. When measuring voltage with a DMM, you are generally measuring Volts RMS (TrueRMS on a Fluke). The dBFS (peak) value of -20 or -18 will be referenced to an RMS voltage value at the converter output, in this case +4dBu or 1.228VRMS.
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  #8  
Old 10-07-2019, 11:55 AM
Darryl Ramm Darryl Ramm is offline
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Default Re: HD I/O calibration question

Quote:
Originally Posted by originalscottyg View Post
My understanding is that the Signal Generator plugin allows you to select dB-RMS or dB-PEAK. These are the same for a pure sine wave, but different for more complex waveforms. When measuring voltage with a DMM, you are generally measuring Volts RMS (TrueRMS on a Fluke). The dBFS (peak) value of -20 or -18 will be referenced to an RMS voltage value at the converter output, in this case +4dBu or 1.228VRMS.
No. Time to revisit basic definitions. Which is where I started here. If you remember what things actually are you can just calculate and do anything you want.

Vrms and Vpeak is not the same for a sine wave. Vrms is the equivalent power that would be delivered if the signal was DC into the same impedance.

You integrate over all the different signal patterns and can work out the difference for each between Vpeak and Vrms. Which gives...
Sine: Vrms ~0.71 Vpeak ~ -3 dB
Square: Vrms = Vpeak = 0dB
Triangle: Vrms ~ 0.58 Vpeak ~ -4.7 dB
Ramp: Vrms ~ 0.58 Vpeak ~ -4.7 dB (same as a triangle, you can argue this via symmetry)

And be careful if you are measuring Vpeak or Vpp on an oscilloscope.

Just play with the signal generator and you can see what is happening. If you set 0dBFS peak all the waveform signals exactly hit the digital extremes. If you then change to rms the signals (except square wave) will clip, by varying amounts, the exact dB differences above. Look at waveform levels, don't get confused by different Pro Tools meters.

But again, the original specific question is actually simple, if the goal is to get an internal dBFS signal and compare it to an external level. You just work in peak.
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  #9  
Old 10-07-2019, 12:16 PM
Darryl Ramm Darryl Ramm is offline
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Default Re: HD I/O calibration question

Quote:
Originally Posted by jbreher View Post
Thanks for the continued discussion.

No VU meter - that seems yet another variable that would require calibration first. I am measuring voltage. I am using a Hewlett-Packard 973A handheld true RMS DMM. The probes are across hot and cold of each channel in turn. At least for the outputs. The input calibration -- as per the procedure defined in the HD I/O User Guide -- employs the internal SW metering with the previously calibrated outputs.

I am seeking to calibrate to -18dBFS corresponding to my nominal operating level of 0VU or +4dBu.
You are fine if setting against dBu. Just to be anal-retentive here. You would be off about a massive ~ 0.7dB = 20 * log (1/(1+50/600)) by ignoring the 600 ohm load impedance if you were aiming for a 0VU reference. If I was writing the calibration procedures they would offer either a multimeter or VU meter, and mention VU vs dBu differences. Also likely the instructions would never get finished...
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  #10  
Old 10-07-2019, 12:45 PM
originalscottyg originalscottyg is offline
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Default Re: HD I/O calibration question

Quote:
Originally Posted by Darryl Ramm View Post
Vrms and Vpeak is not the same for a sine wave. Vrms is the equivalent power that would be delivered if the signal was DC into the same impedance.
I never said V-peak and V-RMS are the same for a sine wave. But you are right that I was incorrect about the dBFS measurement being the same. (I seem to recall an instance where a sine wave measured the same, but a complex waveform didn't.) I apologize for disseminating incorrect information.

Either way, the proper way to calibrate converters is to set the digital value in the DAW to a peak dBFS level (e.g. -20 or -18), then measure the V-RMS with a TrueRMS voltmeter.
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