Sound card (audio card, audio interface) is computer sound module for input/output of audio signal (in analog or digital format).
Read below short and informative articles about PCI, PCIe, USB, FireWire cards, about analog and digital connectors.
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Audio card convert:
1) an analog signal at linear input to digital signal coming to computer.
2) a digital signal stored or created into computer to analog for listening at loudspeakers or headphones.
- for playback multimedia audio (games, skype, music, movie);
- input voice and analog audio to computer (skype, music editors);
- for creation of music (input and output sound created in music editors - a.k.a. DAW - Digital Audio Workstation);
- connecting hardware MIDI-instruments.
Connection sound cards to computer:
Parts of sound card:
1 Analog filter before analog-digital converter (ADC) - for suppression of conversion artefacts. Ultrasound components of signal (lying over 1/2 of sample rate) are shifted to audible range. It perceived as noise or whistle;
2 Analog-digital converter transform analog signal (microphone, guitar, synthesizer analog inputs) to a digital form;
3 Controller of computer interface - passes digital signal to/from computer buses PCI, PCIe, USB, FireWire;
4 Digital-analog converter (DAC) - transform digital signal from computer to output analog signal;
5 Analog filter after DAC suppress conversion (digital signal to analog) artefacts.
6 Input and output balanced circuits - for suppression of interference (for some cards only).
Sound card may have processing module for changing sampling frequencies, apply effects, etc. "on fly".
About audio card PCI and PCIe here...