Line level is the specified strength of an audio signal used to transmit analog sound between audio components such as CD and DVD players, television sets, audio amplifiers, and mixing consoles.

Generally, line-level signals sit in the middle of the hierarchy of signal levels in audio engineering. There are weaker signals, such as those from microphones (Mic Level/Microphone 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

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 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&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 (by convention, decibel values are written with an explicit sign symbol). For consumer equipment it is , which is used to reduce manufacturing costs.

Expressed in absolute terms, a signal at is equivalent to a sine wave signal with a peak amplitude (V<sub>PK</sub>) of approximately , or any general signal at root mean square (V<sub>RMS</sub>). A signal at is equivalent to a sine wave signal with a peak amplitude of approximately , 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 has a of .

{| 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|

| align=left| 1.228

| align=left| 1.736

| align=left| 3.472

|-

| Consumer audio

| align=right|

| 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 ) to the equivalent negative voltage ().

Impedances

As cables between line output and line input are generally extremely short compared to the audio signal wavelength in the cable,

transmission line effects can be disregarded and impedance matching need not be used.

Instead, line level circuits use the impedance bridging principle,

in which a low impedance output drives a high impedance input.

A typical line out connection has an output impedance from 100 to 600 Ω, with lower values being more common in newer equipment.

Line inputs present a much higher impedance, typically or more.

The two impedances form a voltage divider with a shunt element that is large relative to the size of the series element, which ensures that little of the signal is shunted to ground and that current requirements are minimized.

Most of the voltage asserted by the output appears across the input impedance and almost none of the voltage is dropped across the output. A large resistor in series with each output can be used to safely mix them together, but must be appropriately designed for the load impedance and cable length.

Line in

28px &nbsp;&nbsp; 42px &nbsp;&nbsp; 35px &nbsp;&nbsp; 35px &nbsp;&nbsp; 35px &nbsp;&nbsp; Line-in symbol. PC Guide color light blue.

It is intended by designers that the line out of one device be connected to the line input of another. Line inputs are designed to accept voltage levels in the range provided by line outputs.

Impedances, on the other hand, are deliberately not matched from output to input.

The impedance of a line input is typically around .

When driven by a line output's usual low impedance of 100 to 600 ohms, this forms a bridging connection in which most of the voltage generated by the source (the output) is dropped across the load (the input), and minimal current flows due to the load's relatively high impedance.

Although line inputs have a high impedance compared to that of line outputs,

they should not be confused with so-called Hi-Z inputs (Z being the symbol for impedance) which have an impedance of to over . These Hi-Z or instrument inputs generally have higher gain than a line input.

They are designed to be used with, for example, electric guitar pickups and direct injection boxes.

Some of these sources can provide only minimal voltage and current and the high-impedance input is designed not to load them excessively.

Line level in traditional signal paths

Acoustic sounds (such as voices or musical instruments) are often recorded with transducers (microphones and pickups) that produce weak electrical signals. These signals must be amplified to line level, where they are more easily manipulated by other devices such as mixing consoles and tape recorders. Such amplification is performed by a device known as a preamplifier or preamp,

which boosts the signal to line level.

After manipulation at line level, signals are then typically sent to a power amplifier, where they are amplified to levels that can drive headphones or loudspeakers. These convert the signals back into sounds that can be heard through the air.

Most phonograph cartridges also have a low output level and require a preamp; typically, a home stereo integrated amplifier or receiver will have a special phono input. This input passes the signal through a phono preamp, which applies RIAA equalization to the signal as well as boosting it to line level.

See also

  • Alignment level
  • Amplifier
  • Microphone
  • Nominal level
  • Preamplifier

References

  • Conversion of dBu to volts, dBV to volts, and volts to dBu, and dBV
  • Conversion of voltage V to dB, dBu, dBV, and dBm
  • The Decibel