We should keep in mind that decibel measurements are descriptions of ratios. A decibel (dB) is a logarithmic ratio of two values. A decibel is a "dimensionless" value, meaning that it is just a number, not a unit. We would not say “10 dB" without first establishing a reference point, just as we wouldn’t tell a police officer we were only going "30 km" to describe our speed after being pulled over. We’d say we were going "30 km per hour".
- dBV: (with uppercase "V") 0dBV = 1 volt (AC), regardless of any impedance load.
- dBv: (with lowercase "v") 0dBv = 0.775 volts (AC) into a 600 Ω load.
- dBu: (with lowercase "u") 0dBu = 0.775 volts (AC) into an unterminated load (but derived from a 600 Ω load).
- dBm: (with lowercase "m") 0dBm = 1 mW per 0.775 volts (AC) into a 600 Ω load.
The dBu unit has been defined to replace the dBv unit and avoid confusion with the dBV unit.
dBFS : A dB reference level equal to "Full Scale". It is used in specifying A/D and D/A audio data converters, but also increasingly used to refer to signal levels in the digital domain since they are almost always referenced to the full digital value. Full scale refers to the maximum voltage level possible before "digital clipping", or digital overload of the data converter. The actual Full Scale voltage is fixed by the internal data converter design, and varies from model to model.
dB/Octave : A means of measuring the slope of a filter. The more dBs per octave, the sharper the filter slope.
Analog levels are usually measured in voltage ratios such as dBu (decibel unloaded) or dBV (decibel voltage). Since dBu and dBV both describe continuous positive and negative voltage changes, analog signal levels are expressed in average RMS levels.
Two internationally accepted standards have been used in the music industry: +4dBu for professional equipment, which is often referred to as studio level. And -10 dBV for home electronics, widely regarded as consumer level. This may sound like an insignificant detail, but to mix these standards up can have huge negative effects on your recording, because +4 dBu studio level is in fact about 12 dB louder than -10 dBV consumer level!
Line level is the specified strength of an audio signal used to transmit analog sound between audio components such as CD and DVD players, TVs, audio amplifiers, and mixing consoles.
Line levels and their approximate nominal voltage levels:
|Use||Nominal level||Nominal level, VRMS||Peak Amplitude, VPK||Peak-to-Peak Amplitude, VPP|
|Professional audio||+4 dBu||1.228||1.736||3.472|
|Consumer audio||-10 dBV||0.316||0.447||0.894|
In the digital realm, audio is depicted as a string of zeros and ones. Because of this binary nature, analog signal levels such as dBu cannot be used to express digital levels. Instead, digital levels use so-called dBFS (decibels relative to full scale) which have a defined maximum peak level and a fixed scale. Therefore 0 dBFS is the maximum level that can be achieved in a digital system such as a DAW or software plugin. Everything above 0 dBFS will cause an overload and therefore clip the digital signal. It is therefore important to stay below 0 dBFS and give your digital signals sufficient headroom.
dBu vs. dBFS
dBu indicate voltage ratios in RMS levels, while dBFS have a fixed scale and state peak levels.
It is not possible to compare both levels against each other. Put simply, 0 dBu are not 0 dBFS - and the other way around!
But even more importantly, since the input and output level of a digital signal is defined by the operating level of the converter, converting dBu to dBFS is not possible!
Hence, there is no formula or method to match dBu and dBFS. The only way would be to take signal and measure the actual voltage and compare it to the digital level. But this is not practical!