When you or an application wants to play back a recorded sound, the CPU fetches a compressed digital sound file like a .WAV or .MP3 from your hard drive or CD-ROM. This sound file is in binary form and is then sent by the CPU to a digital signal processor (DSP). The digital signal compressors job is to relieve the CPU of the chores involved in processing sound, it also decompresses the digital sound file with the help of the ROM chip which contains the instructions for handling the digital sound file.
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Once the sound file has been decompressed it is then sent to a digital to analogue converter, which converts the digital sound code into an analogue wave form which we can hear. The sound card can handle more than one wave signal at a time allowing the usage of stereo sound. The analogue waves are then sent via cables to your speakers or headphones. Once at the speakers the analogue signal is amplifies the signal, thus recreating the sound.
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Sound cards can also record sound. If a microphone is plugged into the appropriate jack, and you speak you send an analogue signal to the sound card, this analogue signal is then converted into a digital code by an analogue to digital converter (ADC). The digital code then passes to the digital signal processor which compresses the digital signal with the help of the ROM, this is so that the sound file takes up less space on you hard drive. The compressed data file is then sent to the CPU which in turn sends the data to a hard drive to be stored as either a .WAV or .MP3. To then playback the sound file the process explained above takes place.
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