Where do you guys get this stuff?
The VU meter came about in the 1930's to help set levels for the installation of early telephones. Its purpose was to show a technician where to set the audio level so as not to get into distortion. That is why the numbers above 0 dB are in red. Red is to be avoided!
The recording guys picked it up from the telephone guys and look at the sorry state its in now.
In analog circuits there is some distorted head room above the ideal standard at 0 dB. But if you used an ADC, all information above 0 dB would not exist because it would exceed the bit capability of the ADC. There is no headroom above 0 dB for an ADC.
The real definition for 0VU (0 dB VU) is as follows and I quote:
"Technically speaking, 0VU is equal to +4 dBm, or 1.228 volts RMS across a 600 ohm load."
http://www.mediacollege.com/audio/monitoring/vu.html Look it up elswhere if you don't believe them.
Notice the 1.228V RMS. Any input that can handle 1.228V RMS or more will handle 0VU. The UCA222 (and I am sure many other lowly sound cards) will handle 1.25V RMS. That is more than the +4 dBU standard. So the UCA222 will handle +4 dBU (which is the voltage reference at any load not just at 600 ohms).
Designers of digital gear have designed the analog front end of ADCs with pads and gain circuits that are transparent to the user so as to mimic old fashioned analog gear. This allows the user to treat the system like old time gear and push signals into distortion to get that artistic sound they want. So be it. But for goodness sake, that has nothing to do with how the stuff actually works or the voltage it operates at.
You cannot put a 22 dBU signal directly through a circuit that is powered with 5V and get a good result. That is true. But with a 20 dB pad you can make it handle the signal and still have 2 dBU to spare. All that is happening is the signal is being crushed by 20 dB so the peaks are not overloading the ADC. But it is still a 5V system.
This is as far as I am going with this discussion. But for those of you who want to know how it actually works, here are some audio recording type URLs to get you started. To learn about how ADCs work and the voltages available to sound cards, you are on your own.
Good luck!
http://www.sengpielaudio.com/calculator-db-volt.htm
http://www.sengpielaudio.com/calculator-gainloss.htm
http://www.sengpielaudio.com/calculator-transferfactor.htm