Nominal Levels
A line level describes a line's nominal signal level as a ratio, expressed in decibels, against a standard reference voltage. The nominal level and the reference voltage against which it is expressed depend on the line level being used. While the nominal levels themselves vary, only two reference voltages are common: decibel volts for consumer applications, and decibels unloaded for professional applications.
The reference voltage for the decibel volt (0 dBV) is 1 VRMS, which is the voltage required to produce 1 milliwatt of power across a 1 kilohm load. The reference voltage for the decibel unloaded (0 dBu) is the voltage required to produce 1 mW of power across a 600 Ω load (approximately 0.7746 VRMS).
The most common nominal level for consumer audio equipment is −10 dBV, and the most common nominal level for professional equipment is 4 dBu. By convention, nominal levels are always written with an explicit sign symbol. Thus 4 dBu is written as +4 dBu.
Expressed in absolute terms, a signal at −10 dBV is equivalent to a sine wave signal with a peak amplitude of approximately 0.447 volts, or any general signal at 0.316 volts root mean square (VRMS). A signal at +4 dBu is equivalent to a sine wave signal with a peak amplitude of approximately 1.737 volts, or any general signal at approximately 1.228 VRMS.
'Peak-to-peak' amplitude (sometimes abbreviated as 'p-p') refers to the total voltage swing of a signal, which is double the peak amplitude of the signal. For instance, a signal with a peak amplitude of +/-0.5 V has a p-p amplitude of 1.0 V.
| Use | Nominal level | Nominal level, VRMS | Peak Amplitude, VPK | Peak-to-Peak Amplitude, VPP |
|---|---|---|---|---|
| ARD, Germany | +6 dBu | 1.550 (approximate) | 2.192 (approximate) | 4.384 (approximate) |
| Professional audio | +4 dBu | 1.228 (approximate) | 1.737 (approximate) | 3.474 (approximate) |
| Consumer audio | −10 dBV | 0.316 | 0.447 | 0.894 |
The line level signal is an alternating current signal without a DC offset, meaning that its voltage varies with respect to signal ground from the peak amplitude (for example +2.192 V) to the equivalent negative voltage (-2.192 V).
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