Computer motherboards often provide a connector to bring microphone and headphone signals to the computer's front panel. Intel provides a specification for that header; the signal assignments are different for AC'97 and HD Audio headers.[9] Also the connectors are different from connectors used in AC'97 specification and general audio equipment. A regular jack 3.5 mm plug typically has one pin for ground, two pins for stereo signal and two pins for return signal. Return signals connect the signals going to the speakers rear connector with the front connector so when there is no headphone connected to the front connector, the sound signal is sent to the speakers. When a headphone is plugged in the front connector, the signal going to the speakers is broken and sent to the headphone. With HD Audio, instead of the return signal pins, there is a sense signal that is connected to a switch inside the female plug that detects when a male plug is inserted. As a consequence to this change, a regular front panel connector can be used with a HD Audio motherboard if the BIOS provides a way to specify the mode of the audio connector but the speaker output will not be muted when a headphone is connected as is the case with a genuine AC'97 motherboard. The same happens when a HD audio front panel is connected to a genuine AC'97 motherboard.
