a buddy asked for a transcript, so for the benefit of those who prefer to read at their own pace:
There is so much written online about this that is either incorrect, or poorly explained. What does -18dBFS mean? Why do we see this mentioned so often?
In the analog domain, a constant sine wave that has a voltage of 1.23V can be described as having a signal level of 4dBu.
1.228V RMS sine wave=4dBu
A 4dBu (1.228V RMS) sine wave is commonly used in the analog domain as a REFERENCE LEVEL to calibrate equipment. This is helpful, because anyone can make a sine wave and measure it, and its a constant volume rather than something dynamic. 4dBu is the most common standard for professional LINE LEVEL gear. When people talk about line level, it is relative to this 4dBu value (
https://en.wikipedia.org/wiki/Line_level). Line level is what most studio gear operates at - compressors, EQ's, reverbs, tape machines, mixing consoles, studio monitors, converters etc.
Very often,
analog VU meters are calibrated so that a 4dBu sine wave shows 0VU on the meter.
How does this relate to digital/dBFS?
There is no fixed relationship between an analog voltage, and the level it is represented as digitally. Different analog-digital converters use their own definition of what a 4dBu sine wave appears at in dBFS.
On one A/D converter, a 4dBU (1.228V) sine wave may appear as -20dBFS. On another converter, the same 4dBu sine wave may appear as -10dBFS. Some converters even let you adjust this relationship to whatever value you like.
Its quite common for converters to use the definition: 1.228V RMS=4dBu=-18dBFS (If your converter line inputs have 22dBu of maximum input level, then 4dBu+18dB=22dBu). With a 4dBu sine wave there is 18dBFS of headroom.
The idea of this is to allow for signals that are above 0VU (such as fast peaks that are not easily detected on a VU meter) to avoid clipping.
Not all converters use this value, though. Some have more headroom, some have less.(e.g. UA Apollo often uses 20dBu max input level, so 16dB of headroom above 4dBu). As long as you aren't clipping, or recording WAY too quiet, you generally don't have to worry about this.
Why do people mention it then?
- when combining analog equipment with their DAW
- when using analog modelled plugins, and expecting them to behave like their analog counterparts
- familiarity for people who are used to working with analog metering.
THE PROBLEM
Analog equipment uses a consistent reference for setting your volume: 0VU=4dBu.
You can walk into any studio in the world, and play a 1.228V sine wave into their analog equipment and meters, and (if calibrated) it will display 0VU.
The meter shows the volume of any signal compared to a 4dBu sine wave.
As we know, A/D converters DON'T have a fixed dBFS reference for a 4dBu sine wave. When a plugin company makes an analog model of a piece of equipment, they need to decide on what reference level a 4dBu sine wave shows at as a dBFS level. Plugin manufacturers do not know what converters everyone is using and converters themselves all have different reference levels, so there is no perfect solution.
Many plugins use 4dBu=0VU=-18dBFS as their internal operating level, but lots of other plugins use different internal operating levels to this.
Don't assume that its always -18dBFS=4dBu=0VU.
Its quite common for developers to use a different dBu to dBFS reference for a number of reasons. Perhaps they are modelling a piece of HW, using converters that have more (or less) headroom than this. Sometimes, they adjust the internal operating level, as they expect the plugin to be used on a mix bus where average levels are typically louder.
UAD's various tape machines are calibrated for -12dBFS=4dBu for this reason. It helps you from inadvertently driving them too hard in "typical" use. Waves's Tape Machine plugins use 4dBu=-18dBFS. A -12dBFS sine wave is 6dB hotter on the Kramer Master Tape meters (than UAD Studer A800).
Reducing the plugin input level by 6dB means our -12dBFS sine wave is now -18dBFS, and shows as 0VU in the plugin.
I have tested tape machine plugins by companies like Softube, IK, Fuse, Black Rooster Audio, Slate Digital etc. They all use different internal operating levels, so don't assume its -18dBFS.
Another reason for a plugin manufacturer to use various internal operating levels is when a piece of equipment has a limited range of usable headroom. For instance, UAD modelled their LA-2A plugins at 4dBu=-12dBFS. This means, to drive the plugin harder, you would have to add 6dB more level than a plugin calibrated for -18dBFS=4dBu.
Ever wondered why some plugins saturate easier than others? Could it be down to the reference level it was modelled at?
It's not just tape machines or LA2A plugins that vary, ALL analog modelled plugins use a variety of values. -18dBFS=4dBu is just one of them.
If you are using a plugin on your mix bus that uses an internal reference level of -16dBFS=4dBu, you can put a VU meter before it and adjust it to this specification. Now you can adjust the volume of your mix so that the average volume is around 0VU. If your mix is roughly around 0VU, and your meter and plugin calibration values match, then the level you are hitting the plugin with would be typical to how you would use the hardware.
If most of your plugin chain is operating at -18dBFS=4dBU and one plugin is at -15dBFS, you may want to increase the input of that plugin by 3dB, and reduce its output by 3dB. This would be the most similar equivalent to using the real hardware in the same chain. If you don't do this, it's like boosting or attenuating between each piece of gear. It might sound good, but you'll get a different response to what would happen with hardware.
VU meters (analog or digital) have their own pro's and cons (BBC engineers called them "Virtually Useless" meters). They can be useful for full mixes, as well as sounds that are long and sustaining like bass or vocals. They are very poor for showing you any valuable information on signals with a lot of transient detail (like drums, acoustic guitar, percussion etc).
Setting your input levels with a VU meter is often a poor choice compared to a peak meter when recording to digital, especially for transient heavy or uncompressed sources. If you are familiar with how VU meters behave, and also know how much headroom your converters have when your VU meters show 0VU, then using VU meter plugins may offer some familiarity. Generally you will want to use a combination of various types of metering, depending on what kind of audio you are working with, and what you are trying to achieve.
Be suspicious of people online who suggest setting input levels for recording digitally using a VU meter. There is also a lot of misinformation about what -18dBFS refers to. -18dBFS=4dBu=0VU ALWAYS refers to a sine wave, as a constant reference point.dBFS is a measure of PEAK
dBu is a measure of VOLTAGE (average)
To establish a relationship between digital and analogue levels, we have to use a constant voltage like a sine wave. dBFS meters only tell us information about the peaks in the signal. VU is more of a measure of the average loudness and does not respond to quick peaks.
They each have different purposes - dBFS is helpful for digital because we have to keep the signal peaks from clipping. But that's all it really tells us - how close the peaks of a signal are to 0dBFS.
When "-18dBFS" is mentioned, it only refers to SINE WAVES used for calibration. Your real world signals will have a variety of levels, because you are working with complex sounds, rather than just sine waves. Use -18dBFS (or whatever value) for your sine wave when calibrating, and then you can essentially ignore the specific levels as long as you aren't clipping.
Use various types of meters to keep things so whatever plugins you are using are happy.
If a plugin is calibrated for -18dBFS, and your mix is around 0VU on a meter calibrated to -12dBFS, you may want to lower your signal a bit. A snare drum at -18dBFS is very unlikely to be equivalent to 0VU or 4dBu - this definition only works for sine waves.
Once you are calibrated, use your different meters with common sense.
Avoid clipping your inputs and outputs.
