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.

click to enlarge
Figure 8: Location of the audio codec on a motherboard.

click to enlarge
Figure 9: Realtek ALC888S codec.

click to enlarge
Figure 10: Analog Devices AD1988B codec.

On 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.

click to enlarge
Figure 11: How the on-board audio works.