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Old 10-07-2019, 06:30 PM
jbreher jbreher is offline
Join Date: Oct 2019
Location: USA
Posts: 13
Default Re: HD I/O calibration question

Cool. While my initial question is now answered, and my HD I/O now seemingly properly calibrated (using Signal Generator at Peak), let's get anal-rententive for a moment here.

Originally Posted by Darryl Ramm View Post
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.
I'm not sure I understand this. Unless you are assuming a 50 ohm output impedance for the HD I/O outputs. And also assuming a 600 ohm input impedance for what appears to be a postulated (electromechanical?) VU meter. I can't see where else the 50/600 term in the denominator might come from.

I'm-a gonna open another thread on calibrating an Avid (actually digidesign) Pre.
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Old 10-07-2019, 07:05 PM
Darryl Ramm Darryl Ramm is offline
Join Date: Nov 2010
Location: USA
Posts: 13,442
Default Re: HD I/O calibration question

If you want to measure VU you should measure it into 600 ohms, 0VU only equals +4dBu at that load spec.

Wether analog or digital the meters will often have that load, or do a correction... but some screw that up. The difference is small if the output is low enough impedance. And yes a HD IO has about 50 ohm output impedance, even then it’s splitting hairs. But it is nice to do stuff abd get all the numbers to work out.

... but this is in the weeds, the reason I was asking what you were measuring was more to avoid other confusion, e.g. if you had an oscilloscope and were measuring peak-peak voltage or had it in 50 ohm input impedance mode (a mistake I have made a few times).
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