Articles and whitepapers

Ensuring Compatibility Between Next-Generation High-Definition Disc Players and Your A/V System (2/12/2005)

By Robin Dyer, Dolby Laboratories

Next-generation high-definition (HD) optical disc players will differ in many ways from the functionality and performance of today's standard-definition DVD-Video players. The good news is that whether you own a legacy audio/video (A/V) system equipped with Dolby Digital, or a more current A/V receiver equipped with an HDMI connection or external line-level multichannel inputs, a compatible, high-quality playback path is assured.

The next-generation of optical disc players

As a result of advances in coding systems and disc storage capacities, and the commercial development of blue-laser technologies, next-generation optical formats are poised to deliver high-definition video quality. New audio codecs including Dolby Digital Plus and Dolby TrueHD will also bring high-resolution audio performance to the entertainment experience.

Dolby Digital Plus is an extension of Dolby Digital. Designed to be adaptable to the changing demands of future audio and video delivery, it offers more channels beyond the regular 5.1, yet supports backward compatibility with existing Dolby Digital decoders. Dolby TrueHD uses lossless audio technology to complement the recent developments in high-definition optical disc formats, and provides audio performance equal to the highest-resolution studio masters currently available.

Indeed next-generation optical formats will bring a new level of audio interactivity to the playback experience. For example, home theatre enthusiasts may watch a high-definition video while listening to an audio track mastered on the disc that is simultaneously mixed with the director's commentary streamed from a studio website. With each feature or programme selection from your remote control, you may experience unique sounds created within the player.

The most practical way that the next-generation disc players achieve these new interactive features is by processing all of the related audio elements in the player. This is the same model that has been used for video on DVDs: the main video is decoded, then overlaid with subtitles or menus, and output as a complete video presentation, either as composite or component analogue, or as DVI or HDMI digital baseband signals.

In HD disc players, the audio will be handled in much the same fashion. Soundtracks decoded from the disc, as well as audio elements streamed or downloaded from an Internet connection or generated internally in the player, will be decoded in the player as digital PCM signals. PCM is the format players use to perform all internal audio processing operations, including mixing. In the mixing stage, streaming commentary, button sounds, and other non-disc-audio will be mixed with the native 5.1 or 7.1 soundtrack from the disc. The result will be the complete audio presentation as intended by the content maker.

Next-Generation Six-Channel Optical Player with Dolby Digital Output Encoder

The implications of this decoding within the player are significant. New features can be created for a given title long after the discs have shipped. More importantly, the fact that players will be mixing the audio internally means that it will no longer be possible or necessary to output raw audio bitstreams from the player as is typical with DVD-Video. As a result, consumers can no longer assume that every player will work with every A/V receiver.

Ensuring compatibility

Two methods already exist for reproducing the high-resolution soundtracks of next-generation optical formats through your A/V receiver or audio processor:

Single-cable digital connection

Increasingly, A/V processors and receivers are being equipped with IEEE-1394 (FireWire) or HDMI connections, capable of transporting up to eight channels of 24-bit/96kHz PCM audio content. If your A/V receiver is equipped with this type of next-generation connection, you should look for a similarly furnished next-generation optical media player. By this method of connection, the mixed PCM signal is transported from the HD player to your A/V receiver, where digital signal processing and bass management can be easily effected.

Connection via Current HDMI

Multichannel analogue connection

A next-generation optical player may also include line-level audio outputs sourced from the multichannel mixed PCM signals passed through digital-to-analogue converters. The advent of SACD and DVD-Audio in recent years has led to the incorporation of 5.1 and even 7.1 external inputs on many A/V receivers. If your A/V receiver is equipped with 5.1 or 7.1 external audio inputs, the selection of an optical player equipped with 5.1- or 7.1-channel line-level outputs will provide full-bandwidth reproduction of the audio signal originating from your HD player.

Connection via Multichannel Analogue Inputs

A connection through either of these existing interfaces will let you experience the full potential of the high-resolution audio delivered on next-generation optical formats.

S/PDIF connection

If your A/V receiver or processor has neither multichannel analogue or digital inputs, but is equipped with 5.1-channel Dolby Digital decoding and playback, you will still be able to enjoy 5.1-channel performance from next-generation optical players. Included within 7.1-channel multichannel Dolby Digital Plus and Dolby TrueHD streams is a core 5.1 mix prepared by the content maker that is used when the player is set for 5.1-channel mode. After playback audio signals have been mixed in the player, the PCM signal can be encoded to a Dolby Digital signal and output from the player via optical or coaxial S/PDIF to your connected Dolby Digital A/V receiver or processor.

In many instances, the audio quality you will experience from this connection may be better than what you would experience during playback of standard-definition DVD-Video discs, especially if the native signal on the disc is Dolby TrueHD or high-bit-rate Dolby Digital Plus. This is a direct result of a higher-quality source signal feeding a Dolby Digital encoder running at 640kb/s higher than the maximum bit rate on DVD-Video.

Connection via S/PDIF

Because Dolby Digital encoding support is optional in HD players, you will need to look for a next-generation player equipped with an S/PDIF output and built-in Dolby Digital 5.1-channel encoding technology.

Dolby TrueHD and Dolby Digital Plus in A/V receivers

Eventually, A/V receivers will have direct access to Dolby Digital Plus or Dolby TrueHD bitstreams. Dolby is working with the IEC (International Electrotechnical Commission) and HDMI (High-Definition Multimedia Interface) organisations to update data protocols to enable future versions of these high-bandwidth interfaces to carry these bitstreams.

To decode these bitstreams, the A/V decoder will need to support the updated data protocols, as well as incorporate these new decoding algorithms. In addition, it will be necessary to select HD discs in which the content maker has permitted the core 5.1 or 7.1 audio bitstreams to bypass the player's mixing process and be sent directly to the digital outputs of the player. We expect that certain HD discs will permit this, but they may represent a minority of titles. In the end, the sound quality will be essentially the same as audio that was decoded in the player as PCM and transported through a current-generation HDMI connection to the A/V receiver.

With six or eight channels of 24-bit/96kHz audio transported from these new HD formats, post-processing DSP requirements for an A/V receiver more than double. Rather than devoting the considerable DSP resources to decoding the core audio signals within the A/V processor itself, it may be more fruitful to use the A/V processor's DSP resources to perform high-resolution post-processing such as bass management, room or speaker equalisation, Dolby Pro Logic IIx decoding, or other types of digital signal processing.

Connection via Next-Generation HDMI

Summary

As a result of the quality and capabilities that new digital interfaces provide, hardware manufacturers can offer more highly-optimised system designs that attain the ultimate in performance, while providing the greatest flexibility and efficiency for the consumer.

Compatibility between next-generation high-definition disc players and existing A/V systems can be assured through technologies such as Dolby Digital Plus. Its backwards compatibility means that a Dolby Digital Plus stream can be repackaged by a next-generation player and sent as Dolby Digital via S/PDIF. Consumers can therefore, continue to use the same connection and technology as they currently enjoy between their DVD-player and home theatre system. With Dolby TrueHD, the process will be player dependent. Many will decode the signal and output this as six channels of digital (PCM) audio over HDMI, whilst others will be able to convert the soundtrack into a Dolby Digital stream, to pass over S/PDIF Ð again for backwards compatibility.

Moving forward, a next generation connection type, such as HDMI, will allow the raw Dolby Digital Plus bitstreams to be transmitted. Users can then benefit from the advantages stated above with this technology, such as the potential for more channels. Whether the configuration to a different connection will be manual or automatic remains to be seen, but HDMI, unlike S/PDIF, does allow for bi-directional communication between devices.

Robin Dyer is the Marketing Manager, Consumer Technologies, for Dolby Laboratories, developer of products and technologies that make the entertainment experience more realistic and immersive. For four decades Dolby has been at the forefront of defining high-quality audio and surround sound in cinema, broadcast, home audio systems, cars, DVDs, headphones, games, televisions, and personal computers.

www.dolby.com