Pro Tools reference level for +4dBm and phase shift

[L. Durham]

Hello all, this is my first post so I hope I'm doing it right!

I have been running some tests on my Pro Tools system. I have the native system and the Avid HD I/O 16X16 AD/DA converter.

I have been doing calibration in my studio where I interface the Pro Tools system with analogue equipment and have been left with some rather odd results regarding reference levels.

When using Pro Tools inbuilt signal generator generating a 1000hz sine wave, the level must be -17.2dbfs to have +4dbm at the converters output.

Here is the measurement set up... I have a calibrated AC microvolt meter (Levell TM3B) meter across the output of the converter. I also have a digital RMS voltmeter connected to it. The output is also being terminated with a 600ohm load. A 600ohm load and a reading of 1.228V, would be +4dbm (0.775vac being 0db (1mW). However, the output level from tools is saying -17.2dbfs when my external meters are saying +4dbm. I was expecting pro tools to read -18dbfs with +4dbm on its output.

Am I doing something wrong? Is there a calibration I am missing? Perhaps an analogue one? I have checked my external metering over and over and it is correct to within +/- 0.05db.

There are also some phase shift issues but Ill get to that later!

Looking forward to your replys!

Thank you, Lewis, London.

[DetroitT]

All channels same?
Try Drop the load. dbu

[L. Durham]

Yep, this is across all channels. Even without the 600ohm load on the converter. So, no load, 1.228v (+4dBu) and protools is reading -17.2dBfs.

[DetroitT]

So when set to -18dbfs the generator at patchbay or HDI/O
Reads +3.2dbu correct?
Output Trims on the back HDI/O ever used? (Could be adjusted there)

[Darryl Ramm]

^^ exactly. Have you calibrated the HD IO Outputs?

[L. Durham]

HD I/O output calibration I have not adjusted. As all the channels are doing it, I cant see why all the calibrations would have moved by the same amount. Its like it was factory adjusted that way.

Has anyone else found the calibration pots to be off and needed adjusting. I would imagine most people would not really notice (or accuratly measure) the output level to within .1 of a dB. I only did because I was calibrating a set of VU's and wanted it to be spot bollock.

[DetroitT]

Backside trimers are just for this.
It’a drag with I/O’s like the Omni that can only rough in and not be exact to the hundredth.
Avid Factory may just be a robot setup slightly differently.

Think of it like a factory formatted hard drive.
Format fresh is the way to go.

Your going about this correctly BTW
T

[Rich Breen]

Originally Posted by L. Durham

...
Am I doing something wrong? Is there a calibration I am missing? Perhaps an analogue one? I have checked my external metering over and over and it is correct to within +/- 0.05db.

There are also some phase shift issues but Ill get to that later!

...

So as others have pointed out this is just a cal issue. What's the phase issue?

[Darryl Ramm]

One guess here as to why things are all calibrated about the same but off is someone tried to calibrate this previously with an (effectively infinite impedance) multimeter and not across a 600 ohm load. That will push things ~0.7db lower, (as you are seeing) with what I believe is the ~50 ohm output impedance of the HD IO.

And what phase shift? Measured how? All you can do is typically invert the phase by wiring the balanced connections backwards.