Most portable CD players, MD recorders, and computer sound cards use minijacks for their analog audio inputs and outputs. Portable players usually output sound via a single headphone minijack. Portable recorders usually add 2 additional minijacks: a line input and a microphone input.
PC sound cards typically have at least 1 minijack line output; some also have a separate headphone minijack output. Like portable recorders, most sound cards also feature minijacks for line input and mic input (these permit you to record sound directly to your hard disk drive).
Most minijack connections are stereo; that is, they pass both a left and a right audio channel. However, some minijack connections (such as microphone inputs) pass just a single mono audio channel.
This is the most common form of interconnect between a computer system and speakers or stereo equipment and are the same connectors used on portable headphones. The reason that these are used so frequently is the size. It is possible to place upwards of 6 mini-jacks on a single PC card slot cover.
In addition to its size, mini-jacks are widely used for audio components. Portable audio has using these for many years making a wide range of headphones, external mini-speakers and amplified speakers compatible with the computer.
With a simple cable, it is also possible to convert a mini-jack plug into RCA outputs for home stereo equipment.
Mini-jacks lack dynamic range though. Each mini-jack can only carry the signal for two channels or speakers. This means in the 5.1 surround setup, three mini-jack cables are required to carry the signal for the 6 channels of audio. Most audio solutions can do this without a problem, but sacrifice the audio-in and microphone jacks for output.