What is an AC‘97 Codec？

　　The AC’97 codec is a complete audio subsystem with multiple stereo input channels， multiple stereo output channels， a higher current headphone output channel， A/D converters （ADCs）， D/A converters（DACs）， and mixers， with gain controls for each path. The simplified block diagram （see Figure 1） has multiple analog inputs going to two places， a selector for the input to the ADCs and attenuators. The attenuators go into a mixer that feeds the audio outputs. The AC’97 digital link gives data to a stereo DAC that also goes into the same mixer through an attenuator. This allows one to mix real-time analog signals from a microphone with prerecorded digital music （karaoke）。 The output of the mixer goes to three analog outputs， each with its own attenuator. The headphone output has more drive than the other two outputs，and is capable of delivering 50 mW. into 32 Ω.