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TECH PAGE: HD Audio - EMusician

TECH PAGE: HD Audio

As the world's largest microprocessor manufacturer, Intel has long been concerned with computer audio capabilities. In 1997, the company introduced an audio architecture called Audio Codec '97 (AC'97). It offered standardized hardware and software specifications to ensure
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As the world's largest microprocessor manufacturer, Intel has long been concerned with computer audio capabilities. In 1997, the company introduced an audio architecture called Audio Codec '97 (AC'97). It offered standardized hardware and software specifications to ensure that the quality of a computer's internal audio system was up to snuff.

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Intel''s Audio ­Studio application demonstrates the capabilities of HD Audio.

Of course, technology moves ever onward, and what was up to snuff eight years ago seems woefully inadequate today. Intel recently unveiled its next-generation audio spec, High Definition (HD) Audio, which will supplant AC'97 as the audio standard for all new Intel-based Windows PCs. (It is also available for Linux platforms; developers can download the source code at www.alsa-project.org.)

As you would expect, the resolution of HD Audio is higher all around than that of AC'97. For example, the old spec allows sampling rates of up to 96 kHz for 2-channel audio or 48 kHz for 6-channel operation; HD Audio can output up to six channels at 192 kHz. And the new spec's maximum bit depth is 32 bits as opposed to 20 bits in AC'97. HD Audio can also accommodate all of the latest audio codecs, including Dolby Pro Logic IIx, which simulates a 7.1-channel surround soundfield from a 2-channel source. In fact, Intel has been working closely with Dolby to certify the audio quality of the new spec.

New in HD Audio is the ability to process multiple, simultaneous, independent streams of audio — up to 15 inputs and 15 outputs — each with up to 16 channels of audio data. Each output stream has a maximum bandwidth of 48 Mbps, while each input stream can handle 24 Mbps. These bandwidths determine the maximum number of simultaneous audio channels in the stream, which depends on the sampling rate and bit depth. That's a big jump beyond a single stream of no more than six channels in AC'97, and it's a serious boon for electronic musicians who need simultaneous input and output streams to overdub new parts while monitoring recorded tracks.

Of course, recent-model PCs can handle simultaneous input and output streams, but up until now, that has required an add-in sound card. With HD Audio, multiple streams will be a standard feature of all new Intel-powered PCs. Granted, the I/O capabilities on most stock PCs are likely to be less sophisticated than serious musicians need (for example, unbalanced minijacks and rudimentary A/D and D/A conversion), so some form of add-in card might still be necessary for optimum recording.

The default Windows driver for HD Audio is Microsoft's new Unified Audio Architecture (UAA), which provides support for USB and 1394 audio devices as well as the internal PCI bus. UAA is scheduled to ship with the next generation of Windows, code-named Longhorn, which, rumor has it, will be available in 2006. The driver is designed to consume a minimum amount of CPU time while streaming, and it will follow the planned Longhorn real-time coding guidelines for glitch-free audio. UAA will support a minimum of 2-channel recording and playback of 24bit/96 kHz PCM as well as 6-channel playback if the underlying hardware allows it.

Interestingly, HD Audio was not designed to be backward-compatible with AC'97. As a result, HD Audio must be considered a completely new specification with new hardware requirements. To that end, Intel has introduced the 915 and 925 chipset families to implement the new standard. These chipsets are available now, and Microsoft has released patches to existing versions of Windows to support them. All in all, HD Audio promises much higher-quality audio performance from all new Intel-powered Windows PCs, which is good news for recording musicians and consumers alike.