How On-Board Audio Works

The Audio Codec

The south bridge chip or the audio controller can’t deal with analog audio. They need a small chip called audio codec (short for coder/decoder) to make the proper digital-to-analog (DAC) and analog-to-digital conversions (ADC). Digital-to-analog conversion is made when the computer is sending sounds to the speakers, while analog-to-digital conversion is made when you are feeding the computer with an external analog audio source (for example, when you connect a tape deck or a turntable to the PC to convert music into MP3 or CD).

Physically speaking the audio codec is a very small chip measuring ¼ sq. in. (7 mm2) and usually located on the rear border of the motherboard (see Figure 8). The two most popular manufacturers of this chip is Realtek (RTC), whose chips are named starting with the letters ALC, and Analog Devices (ADI, also known as “SoundMax”), whose chips are named starting with the letters AD. On Figures 9 and 10 we show you examples of codecs from these two manufacturers.

codecFigure 8: Location of the audio codec on a motherboard.

Realtek ALC888S codecFigure 9: Realtek ALC888S codec.

Analog Devices AD1988B codecFigure 10: Analog Devices AD1988B codec.

In Figure 11 we show you a small diagram explaining the relationship of the south bridge chip, the codec and the audio connectors found on the motherboard.

on-board audioFigure 11: How the on-board audio works.

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Author: Gabriel Torres

Gabriel Torres is a Brazilian best-selling ICT expert, with 24 books published. He started his online career in 1996, when he launched Clube do Hardware, which is one of the oldest and largest websites about technology in Brazil. He created Hardware Secrets in 1999 to expand his knowledge outside his home country.

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