Line level is the specified
strength
Strength may refer to:
Physical strength
*Physical strength, as in people or animals
* Hysterical strength, extreme strength occurring when people are in life-and-death situations
*Superhuman strength, great physical strength far above human c ...
of an audio signal used to transmit
analog audio
Analog recording is a technique used for the recording of analog signals which, among many possibilities, allows analog audio for later playback.
Analog audio recording began with mechanical systems such as the phonautograph and phonograph. L ...
between components such as
CD and
DVD
The DVD (common abbreviation for Digital Video Disc or Digital Versatile Disc) is a digital optical disc data storage format. It was invented and developed in 1995 and first released on November 1, 1996, in Japan. The medium can store any kin ...
players,
television set
A television set or television receiver, more commonly called the television, TV, TV set, telly, tele, or tube, is a device that combines a tuner, display, and loudspeakers, for the purpose of viewing and hearing television broadcasts, or using ...
s,
audio amplifier
An audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving loudspea ...
s, and
mixing console
A mixing console or mixing desk is an electronic device for mixing audio signals, used in sound recording and reproduction and sound reinforcement systems. Inputs to the console include microphones, signals from electric or electronic inst ...
s.
Line level sits between other levels of audio signals. There are weaker signals such as those from
microphone
A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public ...
s (microphone level or mic level) and
instrument pickups (instrument level), and stronger signals, such as those used to drive
headphones
Headphones are a pair of small loudspeaker drivers worn on or around the head over a user's ears. They are electroacoustic transducers, which convert an electrical signal to a corresponding sound. Headphones let a single user listen to an au ...
and
loudspeaker
A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or " ...
s (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 The output impedance of an electrical network is the measure of the opposition to current flow (impedance), both static ( resistance) and dynamic ( reactance), into the load network being connected that is ''internal'' to the electrical source. The ...
and output power capability.
Consumer electronic devices concerned with audio (for example
sound card
A sound card (also known as an audio card) is an internal expansion card that provides input and output of audio signals to and from a computer under the control of computer programs. The term ''sound card'' is also applied to external audio i ...
s) 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 connector
The XLR connector is a type of electrical connector primarily used in professional audio, video, and stage lighting equipment. XLR connectors are cylindical in design, and have three to seven connector pins, and are often employed for analog b ...
s. Professional equipment may also use unbalanced connections with (1/4 inch) TS phone jacks.
Nominal levels
![Line levels](https://upload.wikimedia.org/wikipedia/commons/4/46/Line_levels.svg)
A line level describes a line's
nominal signal level as a ratio, expressed in
decibel
The decibel (symbol: dB) is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a po ...
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: 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
DBM or dbm may refer to:
Science and technology
* dBm, a unit for power measurement
* DBM (computing), family of key-value database engines including dbm, ndbm, gdbm, and Berkeley DB
* Database Manager (DBM), a component of 1987's ''Extended Edi ...
). 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 V
RMS, equivalent to +6 dBV. Such higher output levels allow the CD player to bypass a preamp stage.
Expressed in absolute terms, a signal at is equivalent to a
sine wave
A sine wave, sinusoidal wave, or just sinusoid is a curve, mathematical curve defined in terms of the ''sine'' trigonometric function, of which it is the graph of a function, graph. It is a type of continuous wave and also a Smoothness, smooth p ...
signal with a peak amplitude (V
PK) of approximately , or any general signal at
root mean square
In mathematics and its applications, the root mean square of a set of numbers x_i (abbreviated as RMS, or rms and denoted in formulas as either x_\mathrm or \mathrm_x) is defined as the square root of the mean square (the arithmetic mean of the ...
(V
RMS). A signal at is equivalent to a sine wave signal with a peak amplitude of approximately , or any general signal at approximately 1.228 V
RMS.
Peak-to-peak (sometimes abbreviated as ''p-p'') amplitude (V
PP) 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 .
The line level signal is an
alternating current
Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC) which flows only in one direction. Alternating current is the form in whic ...
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
A
line driver
A line driver is an electronic amplifier circuit designed for driving a load such as a transmission line. The amplifier's output impedance may be matched to the characteristic impedance of the transmission line.
Line drivers are commonly used ...
is typically used to drive line-level analog signal outputs.
As cables between line output and line input are generally extremely short compared to the audio signal wavelength in the cable,
transmission line
In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...
effects can be disregarded and
impedance matching
In electronics, impedance matching is the practice of designing or adjusting the input impedance or output impedance of an electrical device for a desired value. Often, the desired value is selected to maximize power transfer or minimize signal ...
need not be used. Instead, line level circuits use the
impedance bridging
In audio engineering and sound recording, a high impedance bridging, voltage bridging, or simply bridging connection is one in which the load impedance is much larger than the source impedance. The load measures the source's voltage while minima ...
principle, in which a low impedance output drives a high impedance input. A typical line out connection has an output impedance from 100 to , 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
In electronics, a voltage divider (also known as a potential divider) is a passive linear circuit that produces an output voltage (''V''out) that is a fraction of its input voltage (''V''in). Voltage division is the result of distributing the in ...
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.
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 the 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
A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another.
Transducers are often employed at the boundaries of automation, measurement, and contr ...
, 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 out
![Line circle out](https://upload.wikimedia.org/wikipedia/commons/5/51/Line_circle_out.png)
Line-out symbol.
PC Guide color lime green.
Line outputs usually present a source impedance of 100 to 600
ohm
Ohm (symbol Ω) is a unit of electrical resistance named after Georg Ohm.
Ohm or OHM may also refer to:
People
* Georg Ohm (1789–1854), German physicist and namesake of the term ''ohm''
* Germán Ohm (born 1936), Mexican boxer
* Jörg Ohm (b ...
s. The voltage can reach 2 volts
peak-to-peak
The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of ampl ...
with levels referenced to −10
dBV (300 mV) at . The
frequency response
In signal processing and electronics, the frequency response of a system is the quantitative measure of the magnitude and phase of the output as a function of input frequency. The frequency response is widely used in the design and analysis of sy ...
of most modern equipment is advertised as at least 20 Hz to 20 kHz, which corresponds to the conventional
range of human hearing
Hearing range describes the range of frequencies that can be heard by humans or other animals, though it can also refer to the range of levels. The human range is commonly given as 20 to 20,000 Hz, although there is considerable variati ...
. Line outputs are intended to drive a load impedance of
10,000 ohms; with only a few volts, this requires only minimal current.
Connecting other devices
Connecting a low-impedance load such as a loudspeaker (usually ) to a line out will essentially
short circuit
A short circuit (sometimes abbreviated to short or s/c) is an electrical circuit that allows a current to travel along an unintended path with no or very low electrical impedance. This results in an excessive current flowing through the circuit ...
the output circuit. Such loads are around 1/1000 the impedance a line out is designed to drive, so the line out is usually not designed to source the current that would be drawn by a 4 to 8 ohm load at normal line out signal voltages. The result will be very weak sound from the speaker and possibly a damaged line out circuit.
Headphone outputs and line outputs are sometimes confused.
Different make and model headphones have widely varying impedances, from as little as to a few hundred ohms; the lowest of these will have results similar to a speaker, while the highest may work acceptably if the line out impedance is low enough and the headphones are sensitive enough.
Conversely, a headphone output generally has a source impedance of only a few ohms (to provide a bridging connection with 32 ohm headphones) and will easily drive a line input.
For similar reasons, "wye"-cables (or "Y-splitters") should not be used to combine two line out signals into a single line in.
Each line output would be driving the other line output as well as the intended input,
again resulting in a much heavier load than designed for. This will result in signal loss and possibly even damage.
An active mixer, using for example
op-amps,
should be used instead.
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
![Line circle in](https://upload.wikimedia.org/wikipedia/commons/1/1a/Line_circle_in.png)
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 input
A DI unit (direct input or direct inject) is an electronic device typically used in recording studios and in sound reinforcement systems to connect a high-output impedance, line level, unbalanced output signal to a low-impedance, microphone lev ...
" 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
Acoustic sounds (such as
voice
The human voice consists of sound made by a human being using the vocal tract, including talking, singing, laughing, crying, screaming, shouting, humming or yelling. The human voice frequency is specifically a part of human sound production in ...
s or
musical instrument
A musical instrument is a device created or adapted to make musical sounds. In principle, any object that produces sound can be considered a musical instrument—it is through purpose that the object becomes a musical instrument. A person who pl ...
s) are often recorded with
transducer
A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another.
Transducers are often employed at the boundaries of automation, measurement, and contr ...
s (
microphone
A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public ...
s 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
A preamplifier, also known as a preamp, is an electronic amplifier that converts a weak electrical signal into an output signal strong enough to be noise-tolerant and strong enough for further processing, or for sending to a power amplifier an ...
or "preamp",
which boosts the signal to line level.
After manipulation at line level, signals are then typically sent to a
power amplifier
An audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup, to a level that is high enough for driving loudspea ...
, where they are amplified to levels that can drive
headphone
Headphones are a pair of small loudspeaker drivers worn on or around the head over a user's ears. They are electroacoustic transducers, which convert an electrical signal to a corresponding sound. Headphones let a single user listen to an au ...
s or
loudspeaker
A loudspeaker (commonly referred to as a speaker or speaker driver) is an electroacoustic transducer that converts an electrical audio signal into a corresponding sound. A ''speaker system'', also often simply referred to as a "speaker" or " ...
s. These convert the signals back into sounds that can be heard through the air.
Most
phonograph
A phonograph, in its later forms also called a gramophone (as a trademark since 1887, as a generic name in the UK since 1910) or since the 1940s called a record player, or more recently a turntable, is a device for the mechanical and analogu ...
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
RIAA equalization is a specification for the recording and playback of phonograph records, established by the Recording Industry Association of America (RIAA). The purposes of the equalization are to permit greater recording times (by decreasi ...
to the signal as well as boosting it to line level.
See also
*
Nominal level
Nominal level is the operating level at which an electronic signal processing device is designed to operate. The electronic circuits that make up such equipment are limited in the maximum signal they can handle and the low-level internally genera ...
*
Alignment level
The alignment level in an audio signal chain or on an audio recording is a defined anchor point that represents a reasonable or typical level. It does not represent a particular sound level or signal level or digital representation, but it can b ...
*
Microphone
A microphone, colloquially called a mic or mike (), is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public ...
*
Preamplifier
A preamplifier, also known as a preamp, is an electronic amplifier that converts a weak electrical signal into an output signal strong enough to be noise-tolerant and strong enough for further processing, or for sending to a power amplifier an ...
*
Amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...
*
Soundcard
A sound card (also known as an audio card) is an internal expansion card that provides input and output of audio signals to and from a computer under the control of computer programs. The term ''sound card'' is also applied to external audio ...
References
External links
*
Conversion of dBu to volts, dBV to volts, and volts to dBu, and dBV
{{DEFAULTSORT:Line Level
Electronic engineering
Audio engineering
Sound
Consumer electronics
Signal cables