Fader Unity and ADC Conversion Level

[DFT]

Hi ...

I'm setting up analog summing. My DAC conversion level is -16 dBfs for +4 dBu/ 0 dBvu.

With a -16 dBfs level sent out I get -4 dBvu with faders at unity.

I'd like to adjust PT so my faders sitting at unity provide 0 dBvu level from my DAC. This would both improve resolution and give me an easier reference for hitting analog gear.

Is it possible to adjust how PT assigns dBfs to faders? And if so, how? It is possible that this question exposes a great deal of ignorance.

FWIW I'm on Ultimate 12.6.

Thanks much in advance!

[LDS]

Woooah Nelly. Yeah, there are a number of misconceptions here.

Firstly, digital audio... the dBFS thing is incredibly rigid, defined by the bit depth of a digital audio system. You can't just change it. Digital clipping at 0dBFS is literally defined by running out of bits, and reaching the upper (or lower for a negative audio peak) limits of the binary number the digital audio system can represent. Within that framework, a unity fader specifically means 0dB of gain applied to the signal passing through it. It doesn't get louder... or quieter... the input is the same amplitude as the output.

Secondly, resolution of a digital audio system doesn't alter with volume. A 24 bit system is a 24 bit system. The quantisation noise is always described as half the value of the least significant bit. That doesn't change with volume. When you work with digital audio... you're a farmer who sells eggs. Only whole eggs. Nobody wants broken or half eggs. Whether you have 1000 eggs or 50 eggs or just 1 egg to sell, the resolution of your counting system remains absolutely constant. Digital audio is exactly the same, it just uses binary - a base 2 counting system, instead of the usual day to day base 10 decimal counting system we are most familiar with.

Thirdly, the calibration practise you mention is entirely about aligning the headroom of your analogue equipment, to the headroom of your fixed level digital system. Digital clips at 0dbFS. Professional analogue equipment usually has around +20dBu of headroom. Maybe slightly lower at +18dBFS. Calibrating it to -16dBFS = +4dBu means the point of clipping both analogue and digital parts of your system is relatively similar, 0dbFS = +20dBu.

So, with that out of the way the issues you are facing exist somewhere within the analogue side of your system. Either the converters aren't actually set to -16dBFS = +4dBu, or you have some cabling issues (unbalanced cables will drop a signal by -6dB on balanced inputs), or a gain knob is turned down somewhere.

[daeron80]

I think PT uses -20 dBFS = 0VU, doesn't it? Which might explain why your 16 dB headroom gear sees it as -4 (20-16=4). The old digidesign 888|24 interfaces used to have a calibration trim pot for each input and output so you could choose your own 0VU reference within certain limits. I think the factory settings was -18. That proved to be not quite enough for all professional purposes. You really need 20 dB of headroom to be sure you won't clip.

[Rich Breen]

Originally Posted by DFT

...
Is it possible to adjust how PT assigns dBfs to faders?...

Pro Tools doesn't "assign" anything - dBFS is an absolute measurement of dB below Full Scale.

Unclear what in your system is doing the A/D/A conversion, but that's what needs to be adjusted for desired nominal level. Most professional converters allow for this adjustment.

[Rich Breen]

Originally Posted by Rich Breen

Pro Tools doesn't "assign" anything - dBFS is an absolute measurement of dB below Full Scale.

Unclear what in your system is doing the A/D/A conversion, but that's what needs to be adjusted for desired nominal level. Most professional converters allow for this adjustment.

And just to clarify a couple things: You can of course set what Pro Tools calls "0 VU" in its meter preferences page, but this has nothing to do with actual output level - that is set at your A/D/A converters. For example, Avid HD I/Os have trimmers on the back of the unit for in and out levels, and come from the factory set to -18dBFS = +4dBu (at least mine did).