Editing Line level (section)

== Nominal levels ==
[[File:Line levels.svg|thumb|center|500px|Voltage vs. time of sine waves at reference and line levels, with V<sub>RMS</sub>, V<sub>PK</sub>, and V<sub>PP</sub> marked for the +4dBu line level.]]

A line level describes a line's [[nominal level|nominal signal level]] as a ratio, expressed in [[decibel]]s, 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: {{nowrap|[[Decibel#Voltage|decibel volts]] (dBV)}} for consumer applications, and {{nowrap|[[decibel#Voltage|decibels unloaded]] (dBu)}} for professional applications.

The decibel volt reference voltage is {{nowrap|1 V<sub>RMS</sub> {{=}} {{nowrap|0 dBV}}}}.<ref>[http://www.tangible-technology.com/audiobasics/levels/level.html#SIDEBAR__calculating_dB_and_Power Tangible Tech Audio Basics]</ref>  The decibel unloaded reference voltage, {{nowrap|0 dBu}}, is the AC voltage required to produce {{nowrap|1 mW}} of power across a {{nowrap|600 Ω}} impedance (approximately {{nowrap|0.7746 V<sub>RMS</sub>}}).<ref name="handbook">{{cite book|title=Handbook for Sound Engineers: The New Audio Cyclopedia, Second Edition|editor=Glenn M. Ballou|year=1998|publisher=Focal Press|isbn=0-240-80331-0|pages=761}}</ref>  This awkward unit is a holdover from the early telephone standards, which used 600&nbsp;Ω sources and loads, and measured dissipated power in decibel-milliwatts ([[dBm]]).  Modern audio equipment does not use 600&nbsp;Ω matched loads, hence ''dBm unloaded'' (''dBu'').

The most common nominal level for professional equipment is {{nowrap|+4 dBu}} (by convention, decibel values are written with an explicit sign symbol). For consumer equipment it is {{nowrap|−10 dBV}}, which is used to reduce manufacturing costs.<ref>{{Cite book|url=https://books.google.com/books?id=TIfOAwAAQBAJ&q=pickup|title=The Audio Expert: Everything You Need to Know About Audio|last=Winer|first=Ethan|publisher=Focal Press|year=2013|isbn=978-0-240-82100-9|location=|pages=107}}</ref>

Expressed in absolute terms, a signal at {{nowrap|−10 dBV}} is equivalent to a [[sine wave]] signal with a peak amplitude (V<sub>PK</sub>) of approximately {{nowrap|0.447 volts}}, or any general signal at {{nowrap|0.316 volts}} [[root mean square]] (V<sub>RMS</sub>).  A signal at {{nowrap|+4 dBu}} is equivalent to a sine wave signal with a peak amplitude of approximately {{nowrap|1.736 volts}}, or any general signal at approximately 1.228&nbsp;V<sub>RMS</sub>.

Peak-to-peak (sometimes abbreviated as ''p-p'') amplitude (V<sub>PP</sub>) 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 {{nowrap|±0.5 V}} has a {{nowrap|p-p amplitude}} of {{nowrap|1.0 V}}.

{| class="wikitable"
|+ Line levels and their approximate nominal voltage levels
! Use
! Nominal level
! Nominal level, V<sub>RMS</sub>
! Peak amplitude, V<sub>PK</sub>
! Peak-to-peak amplitude, V<sub>PP</sub>
|-
| Professional audio
| align=right| {{nowrap|+4 dBu}}
| align=left| 1.228
| align=left| 1.736
| align=left| 3.472
|-
| Consumer audio
| align=right| {{nowrap|−10 dBV}}
| align=left| 0.316
| align=left| 0.447
| align=left| 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 {{nowrap|+1.5 V}}) to the equivalent negative voltage ({{nowrap|−1.5 V}}).<ref>[http://mitat.tuu.fi/?p=45 Oscilloscoped measurement for line level signal]</ref>