Line level
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.
For the tool of the same name, see spirit level.
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 3.5 mm (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 6.35 mm (1/4 inch) TRS phone jacks or XLR connectors. Professional equipment may also use unbalanced connections with (1/4 inch) TS phone jacks.
Impedances[edit]
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 10 kΩ or more.[5]
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.[5]
The line input acts similarly to a high impedance voltmeter or oscilloscope input, measuring the voltage asserted by the output while drawing minimal current (and hence minimal power) from the source.
The high impedance of the line in circuit does not load down the output of the source device.
These are voltage signals (as opposed to current signals) and it is the signal information (voltage) that is desired, not power to drive a transducer, such as a speaker or antenna. The actual information that is exchanged between the devices is the variance in voltage; it is this alternating voltage signal that conveys the information, making the current irrelevant.
Line in[edit]
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 10 kΩ.
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 47 kΩ to over 1 MΩ. 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 to not load them excessively.
Line level in traditional signal paths[edit]
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.