StudioLive-series mixers meter the channels and the main and subgroup buses using two different reference scales. The channel levels are measured in dBFS and the output levels are measured in dBu. To understand how this impacts the metering on your StudioLive, you must first be aware of the difference between dBu and dBFS and how both are measured.
dB is an abbreviation for “decibel.” One decibel is equal to one-tenth of a Bel. When a measurement is stated in dB, it is describing the ratio between two levels: the level being measured and the level being referenced. Without a reference point, the absolute dB value cannot be determined.
dBFS, or “decibels relative to full scale,” is used to measure amplitude levels in digital systems that have a maximum peak level (the point at which the A/D converter will clip). When measuring in dBFS, 0 dBFS cannot be exceeded. 0 dBFS can equal +10 to +24 dBu, depending on the device’s maximum peak level. For StudioLive-series mixers (and AI consoles), 0 dBFS equals +18 dBu in the subgroups and +24 dBu in the main output meters.
For StudioLive RMAI and RMLAI rack mount mixers, all outputs are +24 dbu measured outputs.
dBu measures decibels relative to 0.775 volts with an open or unloaded circuit. (The “u” in “dBu” stands for unloaded.) While 0.775 volts may seem rather arbitrary, it is the voltage level that delivers 1 mW in a 600Ω resistor, which is the standard reference impedance in a telephone audio circuit. (The Bel was named after Alexander Graham Bell). Because the StudioLive subgroup and main outputs are electrical, rather than digital, it is more advantageous to meter them relative to voltage.