Line level is a term used to denote the strength of an audio signal used to transmit analog sound between audio components such as CD and DVD players, TVs, audio amplifiers, and mixing consoles, and sometimes MP3 players.
In contrast to line level, there are weaker audio signals, such as those from microphones and instrument pickups, and stronger signals, such as those used to drive headphones and loudspeakers. The strength of the various signals does not necessarily correlate with the output voltage of a device; it also depends on the source's output impedance, or the amount of current available to drive different loads.
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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 a computer sound card are generally unbalanced, with a TS connector of 6.35 mm (1/4"), 3.5 mm (1/8" miniature) or 2.5 mm (3/32" subminiature). The connections on most other consumer equipment use RCA jacks. In most cases changing the volume setting on the source equipment does not vary the strength of the line out signal.
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 [dBV] for consumer applications, and decibels unloaded [dBu] for professional applications.
The reference voltage for the decibel volt (0 dBV) is 1 VRMS, which is the voltage required to produce 1 milliwatt [mW] of power across a 1 kiloohm [kΩ] load.[1] 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).[2]
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 values are twice the peak values.
Digitised values run from 0 for zero voltage up to the maximum designed value for the circuit. There is no absolute maximum, and it depends on the circuit design.
| Use | Nominal level | Nominal level, VRMS | Peak Amplitude, VPK |
|---|---|---|---|
| ARD, Germany | +6 dBu | 1.550 (approximate) | 2.192 (approximate) |
| USA professional audio | +4 dBu | 1.228 (approximate) | 1.737 (approximate) |
| Consumer audio | −10 dBV | 0.316 | 0.447 |
The line level signal is an alternating current signal, meaning that its voltage varies for example from −2.192 V to +2.192 V. [3]
Impedance bridging is employed to ensure that very little power is transferred and the line in circuit does not load down the output of the source device. When a line out signal, with its output impedance of around 100 Ω, is connected to a line in with an input impedance of 10 kΩ, most of the voltage appears across the input resistance and almost none of the voltage is dropped across the output. In effect, the output impedance of the source, and the input impedance of the line in 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.
Line-out symbol. PC Guide color Lime green.
The signal out of line out remains at a constant level, regardless of the current setting of the volume control. You can connect recording equipment to line out and record the signal, without having to listen to it through the device's speaker, and without the loudness of the recording changing if you change the volume control setting of the device while you are recording.
The impedance is around 100 Ω, the voltage can reach 2 volts peak-to-peak with levels referenced to -10 dBV (300 mV) at 10 kΩ, and frequency response of most modern equipment is advertised as 20 Hz - 20 000 Hz (although other factors influence frequency response). This impedance level is much higher than the usual 4 - 8 Ω of a speaker or 32 Ω of headphones, such that a speaker connected to line out essentially short circuits the op-amp. Even if the impedances would match, yielding the theoretical maximum power transfer of 50%, the power supplied through line out is not enough to drive a speaker.
Line-in symbol. PC Guide color Light blue.
Line in expects the kind of voltage level and impedance that line out provides. You can typically connect the line out connector of one device with the line in of another. However, doing this with a straight cable directly connected to both devices and having both devices on AC power, you may run into a ground loop; although some devices provide isolation by using an opto-isolator, which does not create a physical connection between the devices.
A line input has a high impedance of around 10 kΩ, as is often labeled as "Hi-Z" input (Z being the designator for impedance).
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". After manipulation at line level, signals are then typically sent to a device known as a power amplifier, where they are amplified to levels that can drive headphones or loudspeakers, which convert the signals back into sounds that can be heard through the air.
Most phonographs also have a low output level and require a preamp; typically, a home stereo amplifier will have a special phono input with a built-in preamp, which is much more sensitive than a line-level input. The phono preamp applies RIAA equalization to the reproduced sound.
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.