Line level
Encyclopedia
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, TV
s, audio amplifier
s, and mixing console
s, and sometimes MP3 players.
In contrast to line level, there are weaker audio signals, such as those from microphone
s and instrument pickups, and stronger signals, such as those used to drive headphones
and loudspeaker
s. 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.
, with a TS connector of (1/4"), (1/8" miniature) or (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.
as a ratio, expressed in decibel
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 reference voltage for the decibel volt is , which is the voltage required to produce of power across a load. The reference voltage for the decibel unloaded is the voltage required to produce of power across a load (approximately ).
The most common nominal level for consumer audio equipment is , and the most common nominal level for professional equipment is . By convention, nominal levels are always written with an explicit sign symbol. Thus is written as .
Expressed in absolute terms, a signal at is equivalent to a sine wave
signal with a peak amplitude of approximately , or any general signal at root mean square
(VRMS). A signal at is equivalent to a sine wave signal with a peak amplitude of approximately , 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.
The line level signal is an alternating current
signal, meaning that its voltage varies for example from −2.192 V to +2.192 V.
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 , 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.
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 , and frequency response
of most modern equipment is advertised as (although other factors influence frequency response). This impedance level is much higher than the usual of a speaker
or of headphones, such that a speaker connected to line out essentially short circuit
s 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 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 , as is often labeled as "Hi-Z" input (Z being the designator for impedance
).
sounds (such as voice
s or musical instrument
s) are often recorded with transducer
s (microphone
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
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 loudspeaker
s, which convert the signals back into sounds that can be heard through the air.
Most phonograph
s 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.
, 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.
Compact Disc
The Compact Disc is an optical disc used to store digital data. It was originally developed to store and playback sound recordings exclusively, but later expanded to encompass data storage , write-once audio and data storage , rewritable media , Video Compact Discs , Super Video Compact Discs ,...
and DVD players, TV
Television
Television is a telecommunication medium for transmitting and receiving moving images that can be monochrome or colored, with accompanying sound...
s, audio amplifier
Audio amplifier
An audio amplifier is an electronic amplifier that amplifies low-power audio signals to a level suitable for driving loudspeakers and is the final stage in a typical audio playback chain.The preceding stages in such a chain are low power audio amplifiers which perform tasks like pre-amplification,...
s, and mixing console
Mixing console
In professional audio, a mixing console, or audio mixer, also called a sound board, mixing desk, or mixer is an electronic device for combining , routing, and changing the level, timbre and/or dynamics of audio signals. A mixer can mix analog or digital signals, depending on the type of mixer...
s, and sometimes MP3 players.
In contrast to line level, there are weaker audio signals, such as those from microphone
Microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
s and instrument pickups, and stronger signals, such as those used to drive headphones
Headphones
Headphones are a pair of small loudspeakers, or less commonly a single speaker, held close to a user's ears and connected to a signal source such as an audio amplifier, radio, CD player or portable Media Player. They are also known as stereophones, headsets or, colloquially, cans. The in-ear...
and loudspeaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
s. 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
Output impedance
The output impedance, source impedance, or internal impedance of an electronic device is the opposition exhibited by its output terminals to an alternating current of a particular frequency as a result of resistance, inductance and capacitance...
, or the amount of current available to drive different loads.
Overview
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 unbalancedUnbalanced line
In Electrical engineering, an unbalanced line is a transmission line, usually coaxial cable, whose conductors have unequal impedances with respect to ground; as opposed to a balanced line.Microstrip and single-wire lines are also unbalanced lines....
, with a TS connector of (1/4"), (1/8" miniature) or (3/32" subminiature). The connections on most other consumer equipment use RCA jacks
RCA connector
An RCA connector, sometimes called a phono connector or cinch connector, is a type of electrical connector commonly used to carry audio and video signals...
. In most cases changing the volume setting on the source equipment does not vary the strength of the line out signal.
Nominal levels
A line level describes a line's nominal signal levelNominal 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 output and the low-level internally-generated electronic noise they add to the signal...
as a ratio, expressed in decibel
Decibel
The decibel is a logarithmic unit that indicates the ratio of a physical quantity relative to a specified or implied reference level. A ratio in decibels is ten times the logarithm to base 10 of the ratio of two power quantities...
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 reference voltage for the decibel volt is , which is the voltage required to produce of power across a load. The reference voltage for the decibel unloaded is the voltage required to produce of power across a load (approximately ).
The most common nominal level for consumer audio equipment is , and the most common nominal level for professional equipment is . By convention, nominal levels are always written with an explicit sign symbol. Thus is written as .
Expressed in absolute terms, a signal at is equivalent to a sine wave
Sine wave
The sine wave or sinusoid is a mathematical function that describes a smooth repetitive oscillation. It occurs often in pure mathematics, as well as physics, signal processing, electrical engineering and many other fields...
signal with a peak amplitude of approximately , or any general signal at root mean square
Root mean square
In mathematics, the root mean square , also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. It is especially useful when variates are positive and negative, e.g., sinusoids...
(VRMS). A signal at is equivalent to a sine wave signal with a peak amplitude of approximately , 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 ARD (broadcaster) ARD is a joint organization of Germany's regional public-service broadcasters... , Germany |
1.550 (approximate) | 2.192 (approximate) | |
| USA professional audio | 1.228 (approximate) | 1.737 (approximate) | |
| Consumer audio | 0.316 | 0.447 |
The line level signal is an alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
signal, meaning that its voltage varies for example from −2.192 V to +2.192 V.
Impedances
Impedance bridgingImpedance bridging
In electronics, especially audio and sound recording, a high impedance bridging, voltage bridging, or simply bridging connection is one which maximizes transfer of a voltage signal to the load...
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 , 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
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
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...
, the voltage can reach 2 volts peak-to-peak with levels referenced to -10 dBV (300 mV) at , and frequency response
Frequency response
Frequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...
of most modern equipment is advertised as (although other factors influence frequency response). This impedance level is much higher than the usual of a speaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
or of headphones, such that a speaker connected to line out essentially short circuit
Short circuit
A short circuit in an electrical circuit that allows a current to travel along an unintended path, often where essentially no electrical impedance is encountered....
s the op-amp
Operational amplifier
An operational amplifier is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output...
. 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
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
AC power
Power in an electric circuit is the rate of flow of energy past a given point of the circuit. In alternating current circuits, energy storage elements such as inductance and capacitance may result in periodic reversals of the direction of energy flow...
, you may run into a ground loop
Ground loop (electricity)
In an electrical system, a ground loop usually refers to a current, almost always unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops created by improperly designed or improperly installed...
; although some devices provide isolation by using an opto-isolator
Opto-isolator
In electronics, an opto-isolator, also called an optocoupler, photocoupler, or optical isolator, is "an electronic device designed to transfer electrical signals by utilizing light waves to provide coupling with electrical isolation between its input and output"...
, which does not create a physical connection between the devices.
A line input has a high impedance of around , as is often labeled as "Hi-Z" input (Z being the designator for impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
).
Line level in traditional signal paths
AcousticAcoustics
Acoustics is the interdisciplinary science that deals with the study of all mechanical waves in gases, liquids, and solids including vibration, sound, ultrasound and infrasound. A scientist who works in the field of acoustics is an acoustician while someone working in the field of acoustics...
sounds (such as voice
Register (music)
In music, a register is the relative "height" or range of a note, set of pitches or pitch classes, melody, part, instrument or group of instruments...
s or musical instrument
Musical instrument
A musical instrument is a device created or adapted for the purpose of making musical sounds. In principle, any object that produces sound can serve as a musical instrument—it is through purpose that the object becomes a musical instrument. The history of musical instruments dates back to the...
s) are often recorded with transducer
Transducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
s (microphone
Microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
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
Preamplifier
A preamplifier is an electronic amplifier that prepares a small electrical signal for further amplification or processing. A preamplifier is often placed close to the sensor to reduce the effects of noise and interference. It is used to boost the signal strength to drive the cable to the main...
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 loudspeaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
s, which convert the signals back into sounds that can be heard through the air.
Most phonograph
Phonograph
The phonograph record player, or gramophone is a device introduced in 1877 that has had continued common use for reproducing sound recordings, although when first developed, the phonograph was used to both record and reproduce sounds...
s 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
RIAA equalization
RIAA equalization is a specification for the correct recording of gramophone records, established by the Recording Industry Association of America...
to the reproduced sound.
Information transfer
These are voltage signals (as opposed to current signals) and it is the signal information (voltage) that is desired, not power to drive a transducerTransducer
A transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
, 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.
See also
- Nominal levelNominal levelNominal 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 output and the low-level internally-generated electronic noise they add to the signal...
- Alignment levelAlignment levelThe alignment level in an audio signal chain or on an audio recording is a defined anchor point that represents a reasonable or typical level...
- Soundcard
- AmplifierAmplifierGenerally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...