Intel High Definition Audio - Front Panel Connector

Front Panel Connector

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, but the signal assignments are different for AC'97 and HD Audio headers.

The HDA 3.5 mm connectors are different from connectors used in AC'97 specification and general audio equipment. A regular 3.5 mm jack typically has one pin for ground, two pins for stereo signal and two pins for return signal.

In the AC'97 design, the audio output is sent to the jack. If the headphones are not plugged in, the jack directs the audio to the return pins that are connected to the speakers. When a headphone is plugged into the front panel jack, the audio signal goes to the headphones; the return pins are disconnected, so no audio signal goes to the speakers.

With HD Audio, instead of the return signal pins, there is a sense signal that is connected to an isolated switch inside the headphone jack that detects when the headphone plug is inserted. When the plug is inserted, the isolated switch informs the motherboard, and the codec sends audio to the headphones. When the plug is not inserted, the codec sends the audio directly to the speakers (the audio does not go out to the front panel and then loop back to the speakers). A similar isolated switch is used to detect when a microphone has been plugged in.

The different signal assignments can cause trouble when AC'97 front panel dongles are used with HDA motherboards and vice versa. A loud audio passage may make the HDA motherboard with AC'97 dongle believe that headphones and microphones being plugged and unplugged hundreds of times per second. An AC'97 motherboard with an HDA dongle will route the 5 V audio supply (silence) to the speakers instead of the desired audio.