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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? |
#2
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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
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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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Janne What we do in life, echoes in eternity. |
#4
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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? |
#5
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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; 10-06-2019 at 11:10 PM. |
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Re: HD I/O calibration question
Thanks for the continued discussion.
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Thanks for your help. Last edited by jbreher; 10-07-2019 at 08:04 AM. |
#7
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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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Scott Gatteño |
#8
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Re: HD I/O calibration question
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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. |
#9
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Re: HD I/O calibration question
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#10
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Re: HD I/O calibration question
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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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Scott Gatteño |
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