Because it’s really easy to see a level on an analogue meter, VUs are great for calibrating levels in a studio. An with our attenuator, it’s easy to use them for digital levels too.
What is 0VU?
0 on a VU meter is at it’s most sensitive, so it’s a great position to calibrate to. A needle width at 0VU is about 0.1dB, so you can be very accurate.
But what level is 0VU exactly?
Audio started out in the 1900’s mostly on radio, and telephones, and a lot of the standards we have today originated then. dBm was defined for these markets. dBm is a power measurement , but can be measured in volts. 0dBm is the level across a 600 ohm resistor, when you dissipate 1mW of power.
600 ohms was the standard termination resistance for long telephone transmission lines, and by measuring the voltage across that terminating resistor, you could work out how much loss was occurring in the wires, and boost the output to compensate.
0dBm worked out at about 775mV RMS (it was a sinewave signal), but if you attached a 1930’s rectifier meter (in order to read an AC signal) to the equipment wires, the rectifiers would distort the signal.
RCA, who were a dominant radio equipment manufacturer, solved this by adding a resistor in front of the meter, the distortion vanished, however the added resistor reduced the level shown by the meter. In order to make everything work RCA suggested that 0 on the meters should be +4dbm, hence 0VU was born.
0VU also happened to be a good indicator of the modulation levels in radio. If a signal went over 0VU (100% modulation) it would start to distort in transmission.
So 0VU = +4dBm, or in today world, without 600 ohm loads, +4dBu
What is 0dBFs?
Digital equipment has a maximum number it can store. it is actually a peak level, but it stands for 0dB (the maximum) Full scale ( Fs). You cannot store any number bigger than this, but you can store smaller numbers, so all digital levels are 0dBFs or less like -16dBFs.
This is great in a computer, but at some point it needs to be converted into analogue by a DAC. The analogue gain of the DAC determines how many dBu you get when you output 0dBFs.
What is a good studio level?
In most studios, 0dBFs represents an analogue level of about +14dBu to +24dBu.
So what level should you choose?
You can chose any level you fancy within your own studio, but note the following:
- Common levels are + 18dBu and +22dBu. If you need to conform to others studios, choose one of these levels
- +22dBu is often used in recording to cope with higher, non compressed levels
- +18dBu is often used in mastering to work with a variety of outboard
- Normal professional equipment clips at about +28dBu
- Normal unbalanced, or consumer equipment clips at about + 22dBu
- iPhones etc clip at about +10dBu
- Older vintage gear likes levels around +14dBu
- Newer gear will work better around + 18dBu
- Most active speakers, and speaker amplifiers are very, very loud when fed with +10dBu
If you can’t decide, go for +18 or +22dBu
Setting up a DAC with our VUs
As long as our VUs have been calibrated, you can use them to setup a DAC to your level. This is the procedure and maths to do it:
- Decide your studio level: say +18dBu = 0dBFs
- Deduct 4dB off this : = + 14dBu, call this Level A
- Choose the nearest attenuator setting on our meter that is under Level A, and turn the VU to this attenuation: =12dB
- Subtract this attenuation from Level A, to get a negative number: 12dB-14dB= -2dB
- Output a 1KHz sinewave from your DAW, to the DAC, at the above level: -2dBFs
- Connect or route the DAC to the VU meter. Connect it up as you’d normally use it, so any slight cable or gear losses get compensated too
- Adjust the levels on the DAC, till the VU meter shows 0VU
The above shows the levels for +18dBu, if you wanted +22dBu, you would set the VU to 15dB attenuation and output -3dBFs ( +22-4 = +18. 15-18 = -3dB)
Setting up an ADC with our VUs
Once you’ve setup the DAC, you can setup the ADC. Assuming you want to calibrate the ADC to the same level:
- Output a 1KHz, -1dBFs sinewave from your DAW to the DAC
- Connect the DAC outputs to the ADC inputs. Connect it up as you’d normally use it, so any slight cable or gear losses get compensated too
- Connect the ADC digital output to a DAW digital input, and route it to a channel with meters on it
- Adjust the levels on the ADC until the DAW shows a level of -1dBFs
Why -1dBFs? Because 0dBFs is a maximum, it’s easier to adjust levels if you have a bit of headroom to adjust around. with -1dBFs, you won’t accidentally clip the ADC when trimming it, and get false readings.
Recalibrating our VUs
If you have a calibrated DAC, you can repeat the setup a DAC process above, but trim the VU to show the correct levels.