Do you know what this technical audio term is all about? If you have dabbled with enough professional audio equipment, you would come across this term at some point. In order to properly connect all your audio devices, whether in your home studio or at a live sound setup, you need to understand this vital concept. So, what is a line level signal?
Hopefully by the end of this article, you would have a strong grasp on this subject that pertains to electrical audio. Find out how the inputs and outputs of various audio equipment affect the transfer of audio signals. Understand the underlying principles of the line level signal, so you will be able to optimise your audio system. Shall we begin? Then, let’s go!
Line level is a term used to identify the specified strength of an electrical audio signal, used to transmit analog sound between both consumer and professional audio devices such as CD players, loudspeakers, audio power amplifiers, and mixing consoles.
Apart from line level signals, there are weaker audio signals that come from microphones and instrument pickups. The stronger signals typically drive devices such as headphones and loudspeakers. An audio signal’s “strength” is not only directly related to the output voltage of the source device, but is also affected by its output impedance and output power capability.
Electrical audio devices (such as sound cards) usually have connectors labelled either line in or line out. Line out provides an audio signal output and line in receives a signal input. For most consumer-based audio devices, these connectors are often unbalanced, with a 3.5 mm TRS mini-jack connector providing ground, left channel, and right channel, or stereo RCA jacks.
On the other hand, professional audio equipment normally uses balanced connections with 6.5 mm (1/4 inch) TRS phone jacks or XLR connectors. At times, you may find some professional equipment using unbalanced connections with (1/4 inch) TS phone jacks.
These are the topics that we will go through in this article:
- Nominal Levels
- Line out
- Line in
- Audio signal path
A nominal signal level is the level where an electronic signal processing device is designed to operate at. A line level describes a nominal signal level as a ratio, indicated in decibels, against a standard reference voltage. The way the nominal level and reference voltage are expressed, depends on the line level being utilised.
Nominal signal levels vary depending on the device, and the only two reference voltages that are considered standards in the audio world are “decibel volts” (dBV) for consumer devices, and “decibels unloaded” (dBu) for professional devices and applications.
The nominal level for consumer audio equipment is usually “−10 dBV”, and for professional equipment is typically “+4 dBu”. Just take note that an audio 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.
Impedance is the measure of opposition that a circuit presents to a current when a voltage is applied. Cables used to connect line outputs and inputs are often extremely short, as compared to the audio signal wavelength in the cable. Hence, external interference factors can be ignored and impedance matching (to optimise signal strength) is not needed.
Line level circuits in professional audio typically employ impedance bridging, in which a low impedance line output drives a high impedance line input. Most line out connections are designed with an output impedance ranging from 100 to 600 Ω, with newer equipment typically having lower values. As for line inputs, they usually have a much higher impedance of up to 10 kΩ or more.
Take note that we are discussing voltage signals (and not current signals), which represents the signal information (voltage) that we need, not power to drive transducers (using current) such as a speaker or microphone. The actual audio information that is transferred between devices is actually the alternating voltage signal itself, making the current irrelevant.
Generally, line outputs are designed to have a source impedance ranging from 100 to 600 ohms. Hence, if you connect a device with a low-impedance input such as a loudspeaker (usually 4 to 8 Ω) to a line out, it will short circuit the output circuit.
The line output with a high impedance (higher resistance to current), is normally not designed to source the current that would be drawn by a device with an input resistance of 4 to 8 ohm (loudspeaker), at normal line out signal voltages. This results in a very weak sound from the speaker (due to not being driven by enough current) and possibly a line out circuit that is damaged.
Headphone outputs are often mistaken for line outputs. Different headphone types and models have varying impedances, ranging from as low as 20 Ω to a few hundred ohms. The lowest within this range will have results similar to a speaker, but the highest may work fine if the impedance of the line output is fairly low, and the headphone’s sensitivity is high.
Engineers in the audio industry intended to have the line out of one device be routed to the line input of another. Line inputs are designed to receive voltage levels within the range produced by line outputs. However, impedances (which only affects current) are intentionally not matched from output to input. Generally, a line input’s impedance is around 10 kΩ.
A “bridging” connection is created (which optimises the signal’s strength) when a line input is driven by a line output (with low impedance of 100 to 600 ohms). Most of the voltage that is generated by the source (line output) is dropped across the load (the input that receives the signal), and minimal current flows due to the high impedance of the load.
Line inputs should not be confused with “Hi-Z” inputs (“Z” representing impedance) which have an impedance of 470 kΩ to over 1 MΩ. These “Hi-Z” (or “instrument”) inputs typically have higher gain than a line input. They are designed to receive signals from instruments such as electric guitar pickups and direct injection boxes (DI Box).
Signals that come from these sources are only able to provide minimal voltage and current. Hence, the high impedance “Hi-Z” input is designed to not load them excessively (by drawing too much current).
Audio Signal Path
Sound waves (for instance, coming from singing) will be recorded by transducers (microphones or pickups) that will produce weak electrical audio signals. Before being processed further, these audio signals must be amplified to line level, as this makes it more suitable for manipulation by other audio devices such as mixing consoles and hardware signal processors (equalizers, reverb etc.).
The initial amplification process is performed by a device called preamplifier or “preamp”, which boosts the signal to line level by adding gain. After the line level signals have been processed, they are usually routed to a power amplifier to be further amplified to levels that can drive headphones or loudspeakers. Ultimately, the signals are converted back to sound waves which can be heard.
Finally, we have come to the end of this article. This article might be a bit confusing if you are not familiar with technical electronic terms. However, I hope that you are able to somehow get a decent grasp of this vital concept.
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