The sound card is the audio input/output of a computer. Its main task is the conversion of audio signal into raw audio data. To achieve it, a soundcard has two internal chips: an "Analog to Digital Converter" (ADC), where audio inputs are processed, and a "Digital to Analog Converter", processing audio outputs. There are different kinds of soundcards, but all of them complete the same functions.

As we said, different types but same function. The basic soundcard has one input and one output, while complex ones have more. The standard code for soundcard input/output is:

Pink			Analog microphone input.
Blue			Analog line level input.
Green			Analog line level output for the main stereo signal (front speakers).
Black			Analog line level output for rear speakers.
Orange			S/PDIF: digital output (sometimes used as an analog line output for center speaker)

Microphones generally generate a very low powered signal, and once connected to a sound card they need more amplification than something like a CD player or MP3 player (which generally have their own built-in amplifiers). So a Mic-in on a soundcard has a dedicated amplifier, wheres as the line-in does not.

You can use the line-in for your microphone but you will need to supply your own amplifier to increase the output of the microphone to a level that is acceptable to the line-in. This can be a good idea if you have a better amplifier available than the one which is built into your soundcard. If you plug a CD-player or a similar device into your microphone input, there is a resulting risk of overloading and burning out your soundcard!

The "Line-in" input requires a higher input signal level than does the "Mic" input. In other words, the Mic input includes an amplifier that the "Line-in" input does not have. In order for you to use the "Line-in" input you must provide your own amplifier in order to increase the output level of your microphone to a level compatible with the "Line-in" input. If your amplifier is less noisy or contributes less distortion than the one included on the "Mic" input you will, indeed, have a higher quality signal. If your amplifier is noisier or less linear, you will end up with a lower quality signal.

LINE INPUT	MICROPHONE INPUT
MIXING DESK (ANALOG MIXER)	MICROPHONE
CD PLAYER
MINIDISK
TAPE DECK
MP3 PLAYER
TRANSISTOR RADIO