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Line level
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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 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.

Nominal levels

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Voltage vs. time of sine waves at reference and line levels, with VRMS, VPK, and VPP 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: decibel volts (dBV) for consumer applications, and decibels unloaded (dBu) for professional applications.

The decibel volt reference voltage is 1 VRMS = 0 dBV.[1] The decibel unloaded reference voltage, 0 dBu, is the AC voltage required to produce 1 mW of power across a 600 Ω impedance (approximately 0.7746 VRMS).[2] 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). Modern audio equipment does not use 600 Ω matched loads, hence dBm unloaded (dBu).

The most common nominal level for professional equipment is +4 dBu (by convention, decibel values are written with an explicit sign symbol). For consumer equipment it is −10 dBV, which is used to reduce manufacturing costs.[3]

Expressed in absolute terms, a signal at −10 dBV is equivalent to a sine wave signal with a peak amplitude (VPK) 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 1.736 volts, or any general signal at approximately 1.228 VRMS.

Peak-to-peak (sometimes abbreviated as p-p) amplitude (VPP) 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 ±0.5 V has a p-p amplitude of 1.0 V.

Line levels and their approximate nominal voltage levels
Use Nominal level Nominal level, VRMS Peak amplitude, VPK Peak-to-peak amplitude, VPP
Professional audio +4 dBu 1.228 1.736 3.472
Consumer audio −10 dBV 0.316 0.447 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 +1.5 V) to the equivalent negative voltage (−1.5 V).[4]

Impedances

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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 out

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            Line-out symbol. PC Guide color   lime green.

Line outputs usually present a source impedance of from 100 to 600 ohms. The voltage can reach 2 volts peak-to-peak with levels referenced to −10 dBV (300 mV) at 10 kΩ. The frequency response of most modern equipment is advertised as at least 20 Hz to 20 kHz, which corresponds to the range of human hearing. 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

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Connecting a low-impedance load such as a loudspeaker (usually 4 to 8 Ω) to a line out will essentially short 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 20 Ω 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.[6] 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

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            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

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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

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Line level

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Line level refers to the standardized electrical signal voltage used for interconnecting analog , such as mixers, processors, amplifiers, and recorders, where it serves as the typical output from preamplifiers and the input for downstream devices before final power amplification. There are two predominant standards for line level: professional line level at +4 dBu, equivalent to approximately 1.23 volts RMS and aligned with 0 on metering scales, which is the norm in broadcast, studio, and live applications for its robustness against over longer cable runs; and consumer line level at -10 dBV, equivalent to approximately 0.316 volts RMS, which is standard in systems like CD players and DVD receivers for compatibility with lower-cost components. The approximately 12 dB difference between these standards (precisely 11.79 dB) arises from distinct reference points—0 dBu at 0.775 volts RMS and 0 dBV at 1 volt RMS—requiring attenuators, boosters, or level shifters to interface mismatched equipment without introducing , overload, or excessive hum. Unlike lower mic level signals (typically 1–100 millivolts) that demand preamplification or higher speaker level signals (tens to hundreds of volts for driving loudspeakers), line level operates in the intermediate range to maintain across professional workflows.

Introduction

Definition

Line level refers to a standardized electrical signal voltage range used to transmit analog audio between audio components, such as mixers, amplifiers, and media players, without the need for additional amplification or gain staging. Typically, line level operates at 0.316 Vrms (-10 dBV), while line level is at 1.228 Vrms (+4 dBu), providing a consistent strength that ensures compatibility across devices. The primary purpose of line level is to facilitate direct interconnection between while preserving , avoiding distortion or noise that could arise from mismatched levels. It distinctly positions itself between weaker microphone-level signals (in the millivolt range) and stronger speaker-level signals (several volts or more), allowing seamless routing in recording, , and playback systems. At its core, a line-level signal represents audio waveforms as sinusoidal variations across the human of 20 Hz to 20 kHz, with nominal peak-to-peak voltages approximating 0.9 V for applications and 3.5 V for ones, based on a full-scale . This standardization originated from early 20th-century standards developed by Bell Laboratories, which were adapted for broadcast and recording audio applications during and to meet the growing demands of electrical sound systems.

Historical Context

The concept of line level originated in the telephone systems pioneered by Bell Laboratories during the 1920s and 1930s, where standardized signal strengths were defined for long-distance transmission lines to reduce noise accumulation and ensure consistent audio quality over extended distances. These early practices emphasized power matching across 600-ohm balanced lines, laying the groundwork for reliable signal propagation that would influence broader audio engineering. By the 1940s, these principles were adapted for broadcasting and recording applications, with broadcast audio standards adopting levels like +8 dBm for professional transmission over similar impedances, which provided sufficient headroom for dynamic content while minimizing distortion in radio facilities. This evolved into the +4 dBu nominal level for studio use, reflecting a shift toward voltage-based referencing that better suited emerging console designs and tape recording workflows. Post-World War II developments in the introduced a divergence between and audio ecosystems, as home hi-fi systems prioritized cost-effective, lower-power designs leading to the -10 dBV standard for line levels, while studios retained +4 dBu to support longer cable runs and higher rejection in balanced interconnects. The saw further advancement through modular audio consoles, such as Neve's early split designs, which standardized line-level routing between channels and subgroups to enable scalable . In the 1970s, the Electronic Industries Association (EIA) codified these practices in standards like RS-219 for audio broadcast facilities, formalizing line level specifications to promote across equipment. The shift to in the 1980s, marked by the standard's publication in 1985, preserved analog line levels as foundational references, aligning professional +4 dBu with -20 in digital domains to bridge legacy analog systems with PCM-based transmission.

Signal Standards

Nominal Levels

Line signals operate at two primary standardized voltage levels: consumer and professional, each tailored to specific equipment and environments. Consumer line level is defined as a nominal -10 dBV, corresponding to 0.316 volts RMS, and is the standard for home audio devices including CD players, televisions, and VCRs. This level supports unbalanced connections and prioritizes cost-effective components for domestic use. Typical equipment reaches a maximum output of approximately +8 dBV (2 V RMS) before clipping occurs, providing about 18 dB of headroom above nominal. Professional line level uses a nominal +4 dBu, equivalent to 1.228 volts RMS, and is employed in studio environments with gear such as mixing consoles and outboard processors. It accommodates balanced connections for noise rejection over longer runs and offers a maximum of +24 dBu, yielding 20 dB of headroom to handle dynamic peaks without . The voltage difference between these standards is approximately 11.8 dB, with professional signals being hotter, which often requires level-matching adapters or attenuators in hybrid consumer-professional setups to prevent signal overload or excessive noise. The consumer standard emerged in the 1970s with the rise of semi-professional and recording equipment to support cost-effective components. The professional standard was formalized by the (EBU) and Society of Motion Picture and Television Engineers (SMPTE) in the 1970s to meet broadcast requirements for consistency and headroom. Representative examples include consumer line level signals transmitted via RCA cables in hi-fi systems and professional levels routed through XLR connectors in recording consoles.

Measurement Units

Line level signals in audio systems are quantified using (dB) scales, which are logarithmic to reflect the human ear's perception of and to simplify handling wide dynamic ranges. These units express voltage or power relative to a reference value, typically in (RMS) terms for average signal strength, though peak measurements are also considered to assess maximum excursions and prevent . The dBV unit measures voltage relative to 1 V RMS, where 0 dBV equals 1 V RMS. Consumer-grade line levels often use -10 dBV, corresponding to approximately 0.316 V RMS, calculated as V=10(−10)/20V = 10^{(-10)/20}. This scale provides a straightforward reference for modern unbalanced interfaces without legacy impedance constraints. In contrast, dBu references voltage to 0.775 V RMS, a value derived from early 20th-century standards where 1 milliwatt (0 dBm) dissipated across a 600 Ω load produced this voltage via V=P⋅R=0.001⋅600≈0.775V = \sqrt{P \cdot R} = \sqrt{0.001 \cdot 600} \approx 0.775
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