However,, most audio hardware manufacturers did not implement the full specification, especially 32-bit sampling resolution. Motherboards typically do not have any more than eight onboard output channels and four input channels. Users requiring more audio I/Os will typically opt for a sound card or an external audio interface, as these devices also provide additional features that are more oriented towards professional audio applications.
Like AC'97, HD Audio is a specification that defines the architecture, link frame format, and programming interfaces used by the controller on the PCI bus and by the codec on the other side of the link. Implementations of the host controller are available from at least Intel, Nvidia, and AMD. Codecs which can be used with such controllers are available from many companies, including Realtek, Conexant, Integrated Device Technology , VIA, Wolfson Microelectronics, and formerly C-Media. Realtek currently supplies most of HD Audio codec chips used on motherboards and standalone cards. Nearly all chipsets for x86 processors and x86 SoCs, designed for consumer and embedded PC markets, introduced since 2005, implements integrated HD Audio controller. The most recent exemption in consumer chipset market is AMD TRX40, which, for first time since the early 2000s, does not itself integrate any kind of audio controller. Instead, motherboard manufacturers places USB audio controller chip, in pair with usual HD Audio codec chip, to provide onboard audio functionality. The Realtek ALC4050H controller and ALC1220 codec is the most common pair used in sTRX4 motherboards.
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 pin assignments for the AC'97 and HD Audio connectors are: The HDA 3.5 mm subminiature audio connectors differ from connectors used in the AC'97 specification and general audio equipment. A regular 3.5 mm audio jack typically has 5 pins: one pin for ground, two pins for stereo signal and two pins for return signal. When there is no plug in the jack, the two stereo signals are connected to their return pins. When a plug is inserted, the stereo signals contact the respective channels on the plug and are disconnected from the jack's return pins. The HDA 3.5 mm jack does not have the two return audio signals; instead it has an isolated switch that senses the presence of a plug in the jack. In the AC'97 design, the audio output is always 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. For the microphone jack, the return signal pins ground the microphone input when the microphone is not plugged in. Many motherboards with AC'97 audio need two jumpers to short the pins in case no front panel audio module is connected, so audio passes to the speakers. In the HDA design, when the plug is not inserted, the codec sends the audio directly to the speakers. When the plug is inserted, the isolated switch inside the jack informs the motherboard, and the codec sends audio to the headphones. A similar isolated switch is used to detect when a microphone has been plugged in. HD Audio can sense the presence of an audio dongle. A 10 kΩ pull-up resistor is attached to pin 4. When the HDA dongle is plugged in, it pulls pin 4 to ground with a 1 kΩ resistor. The motherboard can determine if the dongle is plugged in by examining the logic level on pin 4. If the motherboard does not detect the presence of an HDA dongle, then it should ignore SENSE1_RETURN and SENSE2_RETURN signals. In AC'97, pins 2 and 4 are audio ground, but pin 4 is often disconnected. Consequently, an HDA motherboard would not detect the presence of an AC'97 dongle with a disconnected pin 4. Intel warns that HDA dongles should be used with HDA motherboards: The different signal assignments can cause trouble when AC'97 front-panel dongles are used with HDA motherboards and vice versa. An AC'97 dongle returns audio on pins 6 and 10 rather than a digital plug sensing signals. Consequently, a loud audio passage may make the HDA motherboard with AC'97 dongle believe that headphones and microphones are being plugged and unplugged hundreds of times per second. An AC'97 motherboard with an HDA dongle will route the AC'97 5 V audio supply to the speakers instead of the desired left and right audio signals. To avoid this, some motherboards allow choosing between HDA and AC'97 front panels in the BIOS. Even though the actual audio hardware is HD Audio, it will allow using the AC'97 front panel that way. Likewise, some modern enclosures have both an "AC'97" and an "HDA" plug at the end of the front-panel audio cable.
