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Line in is the audio jack found on an audio device that can be used to connect to another audio output device or microphone. Line in could be either digital or analog. The core functionality of the line-in jack is to aid in audio recording or manipulate the incoming audio.
Line in is also known as sound in, audio in or mic in.
Most audio devices and computers have at least one line-in and one line-out port. Line-in and line-out jacks work in opposite fashion in all audio devices. In the case of computers, the ports connect directly to the sound processor or sound card of the computer. Moreover, one can reduce the volume and configure other settings at the line-in and line-out level, especially in computers. For most audio devices, the line-in jack is located at the top, back or sides, whereas for computers, they are typically present at the back. However, in the case of laptops, they are mostly absent; instead, headphone and microphone jacks are present, which are either on the front or on the sides of the laptops. The line-in port looks similar to a headphone port, with the difference that the latter is depicted by a headphone logo, whereas the former is usually denoted by a circle with two inward-facing triangles and is usually blue in color.
At the line level, electrical audio signal is present. A microphone, portable music player or any audio device can be connected to the line-in jack. In some cases, line-in jacks can also perform other functions, such as attaching rear speakers when the motherboard has four or six channels and other jacks are absent.