Line (electrical engineering)

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In electrical engineering, a line is, more generally, any circuit (or loop) of an electrical system. This electric circuit loop (or electrical network), consists of electrical elements (or components) connected directly by conductor terminals to other devices in series.

Specifically, consumer electronic devices concerned with audio often have a connector labeled "line in" and/or one labeled "line out". "Line out" provides an audio signal, "line in" receives one.

The signal out of "line out" remains at a constant level, regardless of the current setting of the volume control. So 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 from it.

For example, while recording from the line output of a stereo FM receiver, you can turn up your favorite song, then turn the volume back down after it ends, while you continue recording. When you play back the recording, if you leave the volume control alone, all the songs will be at the same loudness; you will not hear the volume increase where you turned up your favorite song (assuming that the radio station was broadcasting all the songs at the same volume level to begin with). In contrast, if you recorded by connecting to the speaker (or headphone) connector, in playback you would hear the change where you turned up the song, and if you didn't want that song to be louder, you would have to turn it /down/ every time you got to that point in the recording, then turn it back up (every time) after the song ended.

According to the ??? standard, the impedance of the "line out" is ??? (around 100) ohms, and the voltage levels are at ??? millivolts. This impedance level is much higher than the usual 8 ohms of a speaker, so a speaker connected to "line out" essentially short-circuits this connector. Even if the impedances would match (yielding the theroetical maximum power transfer of 50%), the power supplied through "line out" is not enough to drive a speaker, anyway.

On the other hand, the final power amplifier stage of a typical audio device often introduces distortion. But "line out" is derived from some point before that final amplification takes place. So "line out" signals tend to be of higher quality than those from a speaker (or headphone) connector.

"Line in" expects the kind of signal "line out" provides. So you can typically connect the "line out" connector of one device with the "line in" of another. However, if you do this with a straight cable, and both devices are AC powered, you may run into ground - loop - hum.

A typical "Line in" inputs is actually a high impedance input with an impedance of around 10,000 ohms. When a "Line out" signal output, with its impedance of around 100 ohms, is connected to a high impedance "Line in" input like this one, the result is that most of the voltage (over 99%) appears across the input resistance, and almost none of the voltage is dropped across the output impedance. This is the desired effect. Since the impedances are far from matched, very little power is transferred, but the goal is not power transfer, it is voltage transfer. These are voltage signals (as opposed to current signals*) and it is the signal information that is desired, not power to drive a transducer (e.g. speaker) or (transmitting) antenna.

[* "Voltage signal" and "current signal" are terms that identify which quantity, voltage or current, varies to signify the information being carried by the signal. In most cases, voltage and current are related quantities, so defining one of them determines the other. (In order to vary with control--to modulate--both voltage and current independently, you would need a controlled variable impedance.)]