HD I/O calibration question

[jbreher]

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

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.

[Darryl Ramm]

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).

[tlindaas]

Calibrated my HD I/Os today, adjusted the output trim pots to 1.228 volts using the Fluke 115 true RMS multimeter. But just for fun I wanted to also check the voltage readings on two cheaper multimeters I have, and got 1.250 and 1.215 volts for my old and new one respectively (see attached picture).

By nudging the level on Pro Tool's signal generator, it is easy to see what those numbers mean in practice. It is not a lot. So in practical terms then, the old one reads approximately 0.2 db too high, and the new one 0.1 db too low. The old one is not a true RMS multimeter, but the new one is, and it is like 10% of the price of the Fluke 115.