A decibel (dB) is a ratio unit for expressing signal amplitudes. If the amplitudes are expressed in voltage: dB=20 log (V1/V2). If the amplitudes are expressed in power: dB=10 log (P1/P2). In digital audio systems, decibels are often expressed in dBFS, the voltage value below the value produced by a full scale digital audio signal. Decibels relative to an arbitrary reference value, dBr, require that the reference value be stated. Unbalanced consumer signals are most commonly referred to 1 V, and are called dBV (0 dBV = 1 V). These units are used to specify consumer format unbalanced signal levels. The standard consumer level is -10 dBV. Balanced professional signals are typically referred to 0.775 V RMS, and are called dBu. A balanced 0 dBu signal is 0.775 V. Standard professional audio signals are +4 dBu.