Articles and whitepapers

Audio Over HDMI (1/12/2005)

By Geoff Meads, Arcam

Analogue is out and digital is in. Home entertainment is now dependent on digital connectivity, and high-resolution formats are arriving thick and fast. So how do we move all this digital data around? Faster and more intelligent interfaces - that's how - and if Hollywood has anything to do with it, all future connections will be totally secure.

Enter the high-definition multimedia interface (HDMI). A universal interface for high-resolution audio/video, HDMI takes much from its video-only forerunner DVI. Adding HDCP (High-Definition Content Protection) and enhanced device identification, HDMI was born to be the 'Golden Child' connection for video. It is HDMI's audio capabilities however, that will surely define it as a universal solution for home entertainment systems.

For consumer adoption, an interface must be simple to use, available on many products and be capable of high quality. AV dealers and installers look for reliability and practicality, whilst software producers look for encrypted copyright-protected systems.

HDMI encompasses all of these, plus room for future development. Headline specifications are impressive, but it is HDMI's flexibility that proves most impressive. On the video side, it supports up to 1080p video (1080 lines progressive). For audio, the story is developing all the time, but already multichannel high-resolution sound is available.

The practicalities of HDMI

HDMI uses a 19-core cable with a high-density connector that is a little larger than a USB plug. The connector is convenient, carrying all required signals from source to destination in one go. For the installer, the connector is small - around 1" in width - and designed for pulling through a 2"x4" beam. Unlike DVI, there are no computer-style thumbscrews at each side of the connector to get caught inside walls.

An HDMI lead

HDMI is a high-bandwidth point-to-point connection that supports two-way communication, and ensures security by constantly handshaking between 'source' and 'sink' components using the HDCP protocol. Should the link be broken, data transmission stops, meaning only correctly approved and licensed equipment can be used successfully. Illegal products that do not transmit the required digital 'keys' will not be authenticated, and data transmission will fail.

Compatibility between devices is facilitated using equipment identification signalling based on the E-EDID (Enhanced Extended Display Identification Data) standard. This first quizzes the receiving device about its abilities, and then only allows signals to be sent that the receiving device can read.

Audio compatibility

HDMI's audio capabilities vary, depending on which HDMI standard is employed. While these are developing all the time, at the time of writing, the situation is as follows:

V1.0 - two-channel PCM audio up to 48kHz/24-bit including standard Dolby 5.1 and DTS 5.1 bitstreams.
V1.1 - up to 8-channel PCM audio up to 192kHz/24-bit.
V1.2 - as V1.1, plus DSD audio (for SACD) at 1.824Gb/s.
V1.3 - (proposed) as above, plus Dolby Digital+ and DTS-HD enhanced audio.

The above list shows that HDMI's audio applications are far-reaching. Multichannel audio can be moved from one product to another in its full-bandwidth digital glory. This area has been poorly served in the past, with only the iLink standard being used in a proprietary form by a few manufacturers for DVD-Audio and SACD. HDMI's totally secure transmission will also mean that decoding of surround information need not necessarily have to take place within a receiver/processor.

An obvious benefit here might be that a new type of source component such as a disc player using new audio codecs, could output pre-decoded audio in full resolution over HDMI to an audio control centre such as a receiver. This would mean greater longevity for the receiver/processor without sacrificing quality or compatibility. No matter where decoding takes place, HDMI should offer plenty of options for the secure audio transmission needed for developing DVD-HD and Blu-Ray formats, and perhaps beyond.

The Arcam DV29 DVD player with HDMI V1.1 output including multichannel audio

Audio performance matters

The performance of any digital audio system depends on accurate clocking. Any errors in clock accuracy are measured as jitter - the higher the jitter the less accurate the signal and, typically, the poorer the resulting sound quality. Using current implementations, HDMI audio appears to have a relatively poor jitter performance meaning that, although mainstream consumers will find the resulting sound quality rather good, it is unlikely to satisfy audiophiles. A number of manufacturers intend to cure this issue by developing better clock recovery techniques, so in time, the problem should be resolved.

The very high video data rates supported by HDMI also means that the cable length that can be used is limited. Distances of 20-30m are possible for standard-definition video (480/576 lines), whereas a 10-15m limit is common when passing high-definition pictures such as 720p and 1080i. Tight bends in the cable can also prove problematic, causing minute timing differences between the 19 cores of the cable, and since HDMI operates at tremendous speed - well into the Gigabits per second realm - these timing differences can be terminal. Such problems could be solved if longer cable lengths were possible, and several companies are already offering repeater boxes and fibre optic cable converters for HDMI, so cable length is unlikely to be a problem long-term.

The performance benefits for video and the sheer convenience of HDMI far outweigh any difficulties. High-definition video can be an addictive experience for customers, and HDMI, due to its copy protection regime, may prove the only legal way to connect from DVD, set top box or other source, to display.

Conclusion

The benefits of HDMI are clear, and its support by hundreds of manufacturers and software providers suggests a positive future. The growth of home cinema and high-definition television are driving the widespread adoption of the HDMI interface, and for some content, the security it offers has made it a mandatory inclusion in the signal chain. Next-generation gaming devices are almost certain to use HDMI for their high-definition outputs, and PCs, especially media centre types, will almost certainly follow, meaning HDMI will soon become part of our everyday lives.

HDMI's audio capabilities are constantly developing and, short-term issues aside, the security and bandwidth it provides are certain to ensure that its use becomes commonplace. In recent years, sound quality within home entertainment systems has often come a poor second to picture size, with large portions of the budget being diverted to the latest projectors and flat panel screens. The fact that so much development effort has taken place to allow very high audio performance over the HDMI interface suggests that this may change, and that customers and studios are demanding more from their audio systems, be it from TV, DVD, video games or even good old music.

Geoff Meads is Brand Manager for Arcam, manufacturer of high-performance home audio and video equipment.

www.arcam.co.uk