MusicWater
Banned
It neither knows nor cares. Nor does your DAW, for that matter.
To simplify just a bit to aid clarity:
The ADC divides the input voltage by its rail voltage, which gives it a number between 0 and 1*. This is the value that it sends to the computer, which gets stored on your hard drive and manipulated in your DAW. Your DAW then does that 20 log x thing to that number and shows you the answer on the meter.
Send the file to me. My DAW sees the same number between 0 and 1, does that 20 log x thing and shows me the same thing on my meter that you saw on yours.
Now I send it to an output. My DAC multiplies that number between 0 and 1 by it's rail, and the actual output voltage will only be the same as what you put into if the rail on my DAC is accidentally the same as that on your ADC.
Master a song to 0dbfs and load it on your phone. Your phone will try to get as loud as it can, not as loud as your computer can.
It works on the other end also, though. 1/17 is quite a bit bigger than 1/35.
Which is to say (the part of whatshisname's post that made sense) that if I tell you that I record to -10dbfs in my DAW, it doesn't tell you anything about the actual voltage I'm running on the analog path in unless I also include data about my converter.
My converters (Fostex 2424LV, Tascam US1641, PodStudioUX1) don't have analog meters. The 2424LV has digital meters which read exactly the same as my DAW, but I know from experience that it runs a little hot.
*Anything bigger than 1 is clipped off somewhere along the line, of course. Also, of course, there are rounding errors introduced - the noise that I was talking to the "number guys" about.
Edit - Actually, now that I've looked back at my converter specs, let me get even more specific. If I take any line level source, and split it to both the Fostex and the Tascam, the Fostex will always be 4db hotter in DAW.
Edit again - Wait, let me get even more specific since I think you'll get a kick out of this story.
The Fostex machine feeds my studio computer. This is where I "prototype" my live rig for my most recent show. We rehearsed through it, tweaked things, played again, tweaked a little more and then decided it was fine and time to test it on the live machine. Saved the entire Reaper file to my USB stick, stuck it in the laptop, and opened it. All the same plugins live on both machines. Everything else the same. Unplugged pedalboard from Fostex, plugged into the Tascam and it immediately felt just a tad anemic and weak. I couldn't get it to hit the amp or work the edge of the overdrive the way I had gotten used to. I never really memorized the exact difference between the two, and didn't bother to go look it up at the time. I just turned up the amp sim's In knob until it did what I wanted it to do. Turned out to be right around 4db.
Hey ashcat_lt, thanks for chiming in and explaining why what I wrote always was correct. This thread will be a learning lesson for some X million engineers out there...
Here is what PrismSound write in their Orpheus manual:
Analog Line Inputs
Electronically balanced, with fully-balanced analog signal path
Input sensitivity:
Switchable ‘+4dBu’ (0dBFS=+18dBu) or ‘-10dBV’ (0dBFS=+6dBu)
Analog Line Outputs
Electronically balanced, with fully-balanced analog signal path
Output amplitude:
Switchable ‘+4dBu’ (0dBFS=+18dBu) or ‘-10dBV’ (0dBFS=+6dBu)