Advent Day 21

+4dBu or -10dBV? How should I set up my outboard gear and what’s the difference between those two metrics? Is there any difference at all? On day 21 of our BOOM Library SFX Advent Calendar, our sound designers Axel and Michael deal with this seemingly never ending question and they give some examples on how to set up the hard- and software 🙂

dBu vs dBV vs dBFS and how to set up outboard gear:

Since we started to get more and more involved into outboard hardware gear, some studio basics pop up that we need to recall. We’ll be talking about analogue headroom, digital headroom, signal to noise ratio and how to setup an AD/DA converter with three examples. Let’s get started.

What is dBu and what is dBV?
Both are values that measure the strength of an electrical signal in volt.

dBu is a logarithmic voltage ratio with a reference voltage of
V₀ = 0.7746 volt ≡ 0 dBu

dBV is a logarithmic voltage ratio with a reference voltage of V₀ = 1.0000 volt ≡ 0 dBV

As we can see, both are logarithmic values, they only differ in their reference voltage. The difference between dBu and dBV is always 2.2185dB.

For example:
+20dBu = +17.7815dBV
-8dBV = -5.7815dBu

This means that any specification related to a difference in the signal strength is the same in both values (e.g. a headroom of +24dB is 24dB in dBu and dBV) therefore very often the value is specified only in dB.

What does +4dBu mean?
+4dBu is professional studio standard. It’s the alignment level for analogue systems and corresponds to 0VU. In other words: When your VU Meter shows 0VU you have a signal of +4dBu. For mixing in the analogue domain you should keep your average signal levels around 0VU. As most professional analogue equipment is designed to clip at +24 dBu you have a headroom of 20 dB when you keep your average signal level at +4dBu (equal to 0 VU):

(+24dBu analogue clipping point) minus (+4dBu alignment level) = 20dB headroom

The headroom allows you to catch peaks and transients without any clipping. The headroom specification always refers to difference of the alignment level and the maximum level before clipping.

What does -10dBV mean?
-10dBV is the alignment level used in hi-fi / semi-professional / home recording gear, which has a more sensitive input and a corresponding lower output.

How big is the signal difference between +4dBu and -10dBV?
We’re using the calculator on http://www.sengpielaudio.com/calculator-db-volt.htm

The page is an awesome reference for a bunch of acoustic related math and definitions. Check it out!

Let’s calculate in dBV:
+4dBu = 1.228 volts = 1.7815 dBV
-10dBV – 1.7815 dBV = – 11.7815 dB

The other way around in dBu:
-10dBV = 0.3162 volts = -7.7815 dBu
+4dBu – (-7.7815dBu) = +11.7815 dB

Conclusion:
the difference between +4dBu and -10dBV is 11.78dB.

What is digital Full Scale (dBFS)?
Compared to the analogue domain inside a DAW we are working with digital levels! The so called “digital Full Scale” (dBFS) therefore represents a digital level, which will be reached when the signal level in your DAW reaches 0dB. 0dBFS is the absolute maximum level for dBFS, any other levels (they are always lower than 0dBFS) are displayed with a minus at the beginning.

Attention: dBV and dBu (analogue levels) have nothing to do with dBFS (digital levels)! dBFS is a digital/binary number as dBu and dBV are volts! You can NOT convert dBu/dBV to dBFS.

What does this mean when working in a DAW?
If you want to use outboard gear in your DAW you have to use an AD/DA Converter to combine the digital with the analogue domain. The in- and outputs of a converter have a maximum input and output gain which will be reached when the signal level in your DAW reaches 0dB (remember: this is the “digital Full Scale” 0dBFS).

Here’s an example: Let’s say your converter has a max in- and output level of +19dBu at 0dBFS. This means you reach the alignment level of +4dBu for your analog gear when you turn down the signal in your DAW to -15dBFS. The maximum output level of +19dBu minus the +4dBu alignment level results in a headroom of +15dB.

Conclusion:
When you’re using outboard gear in your DAW, the possible headroom is dependent on the maximum in- and output level of your converter.

There are several different calibration guidelines throughout the professional audio world how to setup your headroom. Two examples:

1.) European & UK calibration for Post & Film: −18 dBFS = 0 VU = +4 dBu

2.) American Post: −20 dBFS = 0 VU = +4 dBu

The downside of outboard gear: Signal to Noise ratio
Now let’s get a bit more into detail about the usage of digital headroom.

Attention: This is only related to the usage of an AD/DA Converter and not to the usage of plug-ins (which might require headroom for the plug-ins to work correctly).

Compared to the analogue domain there is no real need for a strict digital headroom in a way of an always unused “forbidden” zone.
You should keep your digital meters close to 0 dBFS and decide how much headroom you need.

Why is that?

Analogue gear, beside all its beauty, comes with a downside: The signal to noise ratio. Unfortunately all outboard gear produces noise. Though the noise is very subtle (common values for signal to noise ratio are between -90 and -80 dB) it can become an issue if you’re using a stack of units. We do use several compressors and equalizers and each time we increase the gain on one unit we do not only increase the level of the signal but also the noise of the unit and all the other units in the chain running into this specific unit as well. This increases the noisefloor and it can reach audible levels.
Even worse: If you’re using more headroom than you require you have to bring the levels back up at the end of your chain to reach digital fullscale. This raises the noisefloor even further. And if you’re using a digital peak limiter in your DAW to further decrease the dynamic range of your audio material you’ll raise the noisefloor again.

Conclusion:
You should choose your personal required headroom by adjusting the in- and output levels on your converter and by keeping your maximum digital levels as high as you can without clipping the converter.

What happens when I mix +4dBu with -10dBV units?
If you use a -10dBV device in a chain with other outboard gear that’s designed for +4dBu you can easily clip the device as the -10dBV unit is not designed to handle the high signal levels a +4dBu unit can produce (remember, these can handle signals up to +24dB).

On the other hand, if you use a +4dBu unit in an -10dBV chain, the signal will be 11.79dB lower than what the input is designed for. This will bring up issues especially if you want to process the dynamics. The 11.79dB lower signal makes it hard for the threshold detector to use its full capacity, so you have to crank up the threshold on your comp/limiter to make the unit start to work and yo
u’ll never reach the full gain reduction the unit was designed for. So both is not optimal and you might want to test carefully what actually works better in your specific case.

Here are some examples with stuff we use and also might work for other units with similar settings of course.

RME Fireface UC
As current temporary interface (our Avid Omni interface is currently being customised and will be back in a few days) we are using an RME Fireface UC. In the level settings of the “Fireface USB Settings” you can select the following values for the outputs:

• +10 dBV (reference level at 0dBFS = 4.2 dBu)
• +4 dBu (reference level at 0dbFS = 13dBu)
• “Hi Gain” (reference level at 0dbFS = 19dBu)

and for the inputs:

• -10dBV (reference level at 0dBFS = 4.2 dBu)
• +4dBu (reference level at 0dBFS = 13dBu)
• “Lo Gain” (reference level at 0dBFS = 19dBu)

The naming convention is a bit weird though, as -10dBV, +4dBu,”Hi gain” and “Lo gain” don’t show the reference levels at 0 dBFS.

To make sure you are running the Fireface correctly you have to choose the corresponding in- and outputs. Which is obviously -10 dBV for in- and output, +4dBU for in- and output or “High gain” for the output and “Lo Gain” for the input.

The settings result in the following headroom (Remember: Headroom is the difference between the alignment level and the maximum level before clipping).

High Gain: +19dBu @ 0dBFS
Headroom at:
alignment level +4dBu = 15 dB headroom
alignment level -10dBV = 29 dB headroom

+4dBu: +13dBu @ 0dBFS
Headroom at:
alignment level +4dBu = 9dB headroom
alignment level -10dBV = 23dB headroom

-10dBV: +4.2dBu @ 0dBFS (this is a setting one would use for a -10dBV chain)
Headroom at:
alignment level +4dBu = 0.2 dB headroom
alignment level -10dBV = 14.2 dB headroom

Depending on how well you know your gear or what you want to achieve, you want to use “High Gain” for outboard gear to drive them hot. Most gear reacts differently with higher input gain resulting in a stronger coloration which in other tasks might not be desired (e.g. mastering). Depending on the material and how hot we want to run the outboard gear, we mostly use +19dBu @ 0dBFS or +13dBu @ 0dBFS. But this is something everyone has to find out for themselves, depending on the wanted sound and gear used.

RME ADI-8 QS
Using an RME ADI-8 QS, an 8 channel AD/DA converter, you can select 4 different reference levels (+4.2/+13+19/+24 dBu) at 0dBFS. This is straightforward and pretty much the same as for the Fireface.

Let’s assume for now, that the in- and output levels are set to the same levels to keep it simple. Doing some math again:

+24dBu @ 0dBFS
Headroom at:
alignment level +4dBu = 20dB headroom
alignment level -10dBV = 34dB headroom

+19dBu @ 0dBFS
Headroom at:
alignment level +4dBu = 15 dB headroom
alignment level -10dBV = 29 dB headroom

+13dBu @ 0dBFS
Headroom at:
alignment level +4dBu = 9dB headroom
alignment level -10dBV = 23dB headroom

+4.2dBu @ 0dBFS (this is a setting one would use for a -10dBV chain)
Headroom at:
alignment level +4dBu = 0.2 dB headroom
alignment level -10dBV = 14.2 dB headroom

Avid Omni
The Omni indeed is a bit tricky to set up. The setup is frustratingly bad communicated in the manual or any resources available from Avid, but this still makes sense when thought about and compared to other measurements. For the inputs you can select either +4 dBu (+24dBu @ 0dBFS)
-10 dBV (+10dBu @ 0dBFS)

For the output settings you have the following choices.

+10 dBu (for speakers)
+24dBu (for line-level devices)

Let’s see what happens, when we combine the different in- and outputs.
Remember: Headroom = (maximal output before clipping) minus (alignment level)

Output +10dBu, Input -10dBV
This is a setting one would use for a -10dBV chain. Attention: When selecting the -10dBV input your alignment level is -10dBV and not +4dBu!

Let’s do the math again:
10 dBu equals +7.7815dBV
headroom = (+7.7815dBV) – (-10dBV) = 17.7815dB

Output +24dBu, Input +4dBu
This is a setting one would use for a +4dBu chain. When selecting the +4dBu input your alignment level is +4dBu and not -10dBV!
headroom = (+24dBu) – (+4dBu) = 20dB

It’s obvious to see that these combination are meant to work together. See what happens when you choose another combination.

Output +10dBu Input +4dBu
Headroom: (+10dBu) – (+4dBu) = 6dB

By selecting +4dBu input setting the input is calibrated to a +24dBu @ 0dBFS signal. But with the setting of +10dBu output, you would get a very low output signal, being 14 dB lower than what the input expects.
This could even work in very few situations, but first of all, you would most likely drive the outboard gear not hot enough. Or you would have to raise the levels of the hardware by 14dB to close the gap between the output and input of the Omni. This would obviously increase the noise floor.

Output +24dBu Input -10dBV
+24dBu is equal to 21,78 dBV
Headroom = (+21.78dBV) – (-10dBV) = 32dB

By selecting -10dBV input setting the input is calibrated to a +10dBu @ 0dBFS. Having an output of +24dBu, you get a 14dB higher signal than what the input of the Omni expects. To avoid clipping the input, you would either have to lower the gain by 14dB on your outboard gear or lower the maximum output signal of your DAW to -14dBFS, resulting in an unused 14dB in your output D/A converter.

Those are some very typical examples of what you will stumble upon and we think they give a very clear basis on how to level your equipment. We tried to break it down as clearly as possible and once understood, this really is pretty easy. Of course this can only be a guideline or a try to make it understandable and it can be creatively useful to drastically use different settings – however, this shouldn’t be the case unless the basics are there. We hope this helps the one or another person out there.

Thanks Axel & Michael.

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