# What is the difference between dBFS and dBu?

What is the difference between dBFS and dBu for leveling when it comes to final mix and standards for cinematic release?

I know that each country is different. In South Africa, the spec for cinematic release is -2dBFS.

We use the -18dB as our 0 on the peak meter, but what is dBFS?

Does it mean theat we peak at -2dBFS on the peak meter with a dynamic range of about 11dB?

I hope this isn't to confusing. Thanks.

• Quick question on this topic- if -20 dBFS = +4 dBU, does that mean that you can only record up to +24 dBu before your digital signal clips? I know that is only a reference level so probably not, but is there a dBu maximum where clipping will occur? Thanks. Commented Mar 28, 2012 at 16:28
• no. this is just a reference point relating between two scales (digital and electrical respectively). Commented Mar 28, 2012 at 16:46
• ok but when recording to tape was there some sort of measurement as to when that would occur? Commented Mar 28, 2012 at 20:42

Ok the first and most important thing to realise is that dB is a relative scale, it is essentially a ratio but because we deal with such large ratios it is also a logarithmic scale to make the numbers more manageable.

So to use dB as a measurement we need to specify a reference point. For dBu, the reference - 0dBu is 0.775volt. Now dBu is a measurement of voltage or signal level. For dBfs, the fs stands for full scale and is only relevant in digital settings and so 0dBfs is when all the bits are a 1, ie it can't get any higher.

To relate the dBu and dBfs scales you need to establish what your headroom figure is going to be and like all good standards we have more than one. Some manufacturers set -20dBfs as 0dBu, the standard here in the Uk is -18dBfs is 0dBu. So you have to know what your metering has been set to, to be able to equate dBu ad dBfs.

Hope that helps,

Mike.