Sound in HD entertainment

Next-generation disc media is in the news. HDTV is at our doors, HD-ready displays are on every high street. However, high-definition entertainment is about the sound as well as the picture, reminds TONY SPATH, Dolby Laboratories’ Marketing Director, Technology (UK).

Surround sound wraps you in the moment and makes you part of the action on screen – and with Dolby’s new high-definition formats designed specifically for HD entertainment, it’s even more realistic than before.

In order to best enhance the visual improvements in home entertainment, Dolby Laboratories has introduced two complementary new audio technologies for high-definition entertainment – Dolby TrueHD and Dolby Digital Plus.

Dolby TrueHD

Probably the biggest impact on the audio on the next generation of disc formats will be the promise of high quality and studio audio performance from Dolby TrueHD. If it says “Dolby TrueHD”, the audio will be a bit-for-bit copy of the master, always the best quality available, with up to 7.1 channels of sound on the new HD disc formats.

With its roots in the lossless format that provided 24 bit 96kHz 5.1 audio in DVD audio (MLP), Dolby TrueHD creates smaller size files on disc, allowing more room for commercially important bonus features and additional content, while always providing the same audio stream – bit for bit – as the final master.

Dolby TrueHD can be considered as a “gold standard” for audio. Yet it can code a 16 bit 48kHz 5.1 soundtrack bit-for-bit to a smaller file size than some lossy codecs. So, no HD disc need be without a Dolby TrueHD track, whatever the content is.

Dolby TrueHD presents the industry with an opportunity to differentiate HD disc formats – from what has been available up to this point, as well as from HDTV where the data-rates that Dolby TrueHD operates at are not feasible.

And for music and the audiophile, where for segments of the market 24 bit 96kbit sampling is a sine qua non, Dolby can now deliver that level of quality in 7.1 channels on both HD-DVD and Blu-ray disc.
Dolby TrueHD is part of the standard for both HD-DVD and Blu-ray, mandated in HD-DVD players and an option in Blu-ray players. The following table shows the maximum audio channel configurations and bit rates that each format allows.

While Dolby TrueHD is constrained to 8 channels in these disc formats, it was designed to accommodate 14 channels, which takes in most of the significant speaker placements as defined under SMPTE 448 for future digital cinema speaker layouts.

In addition to more audio channels and a higher bit-rate ceiling, Dolby TrueHD has taken on some of the innovative metadata features from Dolby Digital, such as level normalisation and a late-night listening feature. With the enormous dynamic range possible with 24/96 audio, both these features will prove useful in the “make-it-work-for-me” entertainment experience which people demand today. In particular, the ability to listen to a wide dynamic range soundtrack at family-friendly levels without dynamic range artifacts will be appreciated – especially as it comes at the touch of a button.

As with Dolby Digital today, Dolby TrueHD offers player manufacturers options to segment their product lines. An entry level player might decode the audio stream to stereo and pass the encoded signal to a dedicated home cinema system as an encoded signal carried under HDMI.

A more fully-featured player may implement full decoding to 6 (or 8) surround outputs which are passed as separate cables to the home cinema; this player might also implement HDMI 1.3 which, in addition to carrying an encoded signal for decode in a home cinema, might also carry 6 channels of discrete PCM decoded from Dolby TrueHD in the disc player.

Dolby Digital Plus

Some applications can’t meet the requirements of bit-for-bit audio. Bandwidth may be an issue, as it is on HDTV, or multiple soundtracks need to be carried on disc more efficiently than a lossless audio technology can. This is where Dolby Digital Plus comes in.

Dolby Digital Plus raises the bar on audio quality for applications where bit-rate is an issue and is included in both the next-generation disc formats. Dolby Digital Plus will bring up to 7.1 channels on HDTV and next-generation discs, offer a level of interactivity within the audio (through mixing of encoded streams) and deliver one-cable compatibility with over 50 million Dolby Digital home cinema systems in use today.

In particular, its ability to carry quality stereo, 5.1, and 7.1 audio at very low bit rates, plus its interactivity features make it very attractive in HD broadcast and IPTV. Both disc standards have included Dolby Digital Plus: its ability to carry audio at low bit rates for additional features and commentary tracks, along with hearing- and visually-impaired services makes these extras come close to being “for free” bit-rate wise.

At higher data rates, audio quality is exceptional – comparable or better than currently available on DVD (depending on bit rate used) – and perfect for carrying multiple 5.1 soundtracks (i.e. different languages) – so the maximum number of homes can experience the involvement of surround sound from the same release disc.

With their overlapping bit rates and channel configurations, the combination of technologies – Dolby TrueHD and Dolby Digital Plus – allows real choice for content makers on quality and features. Whatever the bit budget, creators will not need to cut back on picture quality or additional features to include exciting surround sound with extras as well as the main feature.

How to Master Content for Next-Generation Discs

Content creators have the challenge of encoding assets and mastering files for the new formats and need flexible and efficient workflow tools to do this. Dolby has therefore created Dolby Media Producer, a software-based suite of tools which supports all Dolby audio codecs for mastering not only HD-DVD and Blu-ray, but also DVD-Video and DVD-Audio.

Dolby Media Producer is made up of three elements – Dolby Media Encoder, Dolby Media Decoder, and Dolby Media Tools.

Dolby Media Encoder combines powerful mastering features with comprehensive project and file management capabilities. It is a non-real-time encoder that works either on a local computer or over a network to a centralised server, giving access to multiple clients within a facility. The intuitive Mac OS X interface simplifies preparation of encoding jobs by local clients. Complete administrative control includes job status visibility, accessible system and error logs for all encoder activity, and flexible scheduling. The encoder can be accessed via a standard browser from any networked computer within a facility.

Dolby Media Encoder can encode for DVD-Video, DVD-Audio, HD-DVD, and Blu-ray Disc media, in the formats Dolby Digital Plus, Dolby Digital, Dolby TrueHD, and MLP Lossless.

This professional reference Dolby Media decoder provides decoding and monitoring of Dolby Digital Plus, Dolby Digital, Dolby TrueHD, and MLP Lossless formats. Dolby Media Decoder is designed to run locally on a single computer to provide playback and decoding for an individual audio room.

It supports all consumer listening modes including downmixing and dynamic range control, and it decodes the Dolby Pro Logic II, Dolby Pro Logic, and Dolby Digital Surround EX(tm) formats.

The decoder provides transport control via standard 9-pin protocol for checking A/V sync with video during QC of either source or encoded files. The decoder’s audio output supports Core Audio or ASIO formats for use with FireWire input/output devices, or with other external or internal I/O hardware. It also delivers synchronous file playback with video.

In addition, Dolby Headphone processing is included for ease-of-listening in a studio environment.

The Dolby Media Tools can save time and money by allowing users to repair and update previously encoded files without having to re-encode them. Features include metadata editing, file trimming, timecode striping, and file concatenation. Through the concatenation feature, users can append files; for example, a new trailer can be added. SMPTE timecode can be added to a file in which no timecode exists. It can also be restriped to replace jittery or non-continuous timecode. As with the decoder, Dolby Media Tools is optimised to run locally on a single computer.

Dolby offers hands-on support for studios and content owners, enabling them to get the most out of new disc formats in their facilities and on their schedules. Sound engineers can provide systems consultancy, advise on content production, and deliver training programmes for mastering.

Once a studio is up and running, Dolby engineers can run studio and systems check-ups, making sure that incoming Dolby E audio and the metadata for the new formats is being created appropriately. It can also help with file delivery and quality control. Dolby support is available to facilitate the next-generation audio creation process all the way to the authoring stage.

In addition, Dolby engineers can also help to advise on equipment required, particularly for special events or screenings where technical support can also be provided.

Metadata and Downmixing

Dolby Digital Plus and Dolby TrueHD were designed to carry meta-data just as Dolby Digital does. Metadata (information and instructions pertaining to the audio data) is carried in each Dolby Digital, Dolby Digital Plus, Dolby TrueHD frame. It is used to optimise the audio – in particular levels, dynamic range, and number of audio channels – to best suit the audio system in use at that time, be it TV mono, audiophile stereo, 5.1, or top end home cinema 7.1.

Downmixing – the presentation of a soundtrack in less channels than the original mix – bears consideration. Dolby TrueHD exists as a series of encoded streams that carry combinations of channels to the decoder: a Left Right mix, a Center and Surround mix, and a mix for additional surround channels.

These mixes, produced in the encoder, are re-matrixed using mathematical sum and difference in the decoder to downmix to the requisite number of channels for each home. For example a 5.1 downmix would use the first two of these. This approach is only completely successful with pure signals: as Dolby TrueHD is a lossless codec, it fits this criteria.

Lossy codecs, such as Dolby Digital Plus need a different approach. Creating a downmix using mathematical sum and difference operations on signals with coding artifacts have been shown to result in these artifacts being spread around the replay channels rather than confined to the same position in the audio that produced the artifacts. Spreading artifacts around the audio image makes them more audible: confining them to the same position as the source keeps them concealed or “masked”.

Dolby Digital Plus takes a new approach to downmixing to avoid this “unmasking” of artifacts. Separate mixes are created by the encoder which are combined by replacing individual channels in the decoder rather than mathematical sum and difference. In this way, audio integrity is preserved in whatever channel configuration the consumer has at home.

Summary

Next-generation technologies will only succeed if consumers are inspired to acquire discs and players. High-definition picture is a driver, but increasingly the presence of surround sound underlines the difference in the HD experience to consumers. To emphasise this reassuring message about quality surround sound and the best audio experience in entertainment - for the early adopter, the audiophile, and the movie fan alike - Dolby will be undertaking targeted co-marketing and education in the sales channel and early-adopter media to help differentiate next-generation disc formats.

For content creators, Dolby Laboratories helps to deliver a consistent level of audio quality in next-generation entertainment, offering support to create a good understanding of the new audio technologies and tools to provide a practical solution for next-generation encoding needs. The flexibility of lossless and lossy audio solutions means that content creators have more formats to choose from according to their needs; and as workflow is similar to DVD production, transition should be relatively straightforward.

Dolby Digital Plus and Dolby TrueHD aim to deliver audio that complements high-definition picture but does not compromise it by taking excessive bandwidth; they also offer compatibility along with ease of use for the consumer. The future of HD is bright – and now it sounds reassuringly exciting as well.
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