Line level is the specified strength of an audio signal used to transmit analog audio between components such as CD and DVD players, television sets, audio amplifiers, and mixing consoles. Line level sits between other levels of audio signals. There are weaker signals such as those from microphones (microphone level or mic level) and instrument pickups (instrument level), and stronger signals, such as those used to drive headphones and loudspeakers (speaker level). The "strength" of these various signals does not necessarily refer to the output voltage of the source device; it also depends on its output impedance and output power capability. Consumer electronic devices concerned with audio (for example sound cards) often have a connector labeled line in and/or line out. Line out provides an audio signal output and line in receives a signal input. The line in/out connections on consumer-oriented audio equipment are typically unbalanced, with a (0.14 inch, but commonly called "eighth inch") 3-conductor TRS minijack connector providing ground, left channel, and right channel, or stereo RCA jacks. Professional equipment commonly uses balanced connections on (1/4 inch) TRS phone jacks or XLR connectors. Professional equipment may also use unbalanced connections with (1/4 inch) TS phone jacks. 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: for consumer applications, and for professional applications. The decibel volt reference voltage is . The decibel unloaded reference voltage, , is the AC voltage required to produce of power across a impedance (approximately ). This awkward unit is a holdover from the early telephone standards, which used 600 Ω sources and loads, and measured dissipated power in decibel-milliwatts (dBm). Modern audio equipment does not use 600 Ω matched loads, hence dBm unloaded (dBu). The most common nominal level for professional equipment is (by convention, decibel values are written with an explicit sign symbol). For consumer equipment, a convention exists of , originally intended to reduce manufacturing costs. However, consumer equipment may not necessarily follow that convention. For example, a standard CD-player output voltage has emerged of around 2 VRMS, equivalent to +6 dBV. Such higher output levels allow the CD player to bypass a preamp stage. Expressed in absolute