The new digital sound. What we are going to hear.

To talk about HD sound, we are going to focus on the latest formats to appear on the market, which we believe, given their relevance and the levels of quality they achieve, are of the greatest interest to our readers. We are talking about Dolby True HD and DTS HD Master Audio (hereinafter DTS HD MA).

Let's start by reviewing how both systems work.

In essence, Dolby True HD is a digital sound reproduction format developed by Dolby that allows up to eight audio channels in a 7.1 layout, although it actually has a capacity of up to 14. It uses a variant of lossless coding technology, MLP (Meridian Lossless Packing), offering bit-for-bit identical reproduction of the original studio master, hence the name True. It has a flexible frame structure, Variable Bit Rate (VBR), encoding at 24 bits/96 kHz, giving a transmission rate of 18 Mbits/s, although it can support 24 bits/192 kHz (for stereo applications). The coding ratio is approximately 4:1. This increases the capacity for metadata. It uses the HDMI 1.3 interface to transmit the signal between player and amplifier.

As for DTS HD MA, it also uses a variant of MLP coding, Coherent Acoustics plus HD Bit-for-bit Extensions, with a variable transmission rate from 1.5 up to 24 Mbits/s. The initials MA stand for Master Audio, meaning identical to the original audio recorded in the studio, much like Dolby's True. Thanks to metadata, it can include up to seven different speaker configuration layers, depending on what has been recorded. It also offers 7.1 audio channels at 24 bits/96 kHz. It uses the same interface as Dolby True HD.

Let's now look at the main differences between the two formats. Both systems are based on the same type of coding, MLP, but with their own variants depending on the manufacturer. Given its higher number of transmission bits, DTS-HD MA appears to offer higher quality; however, it is the Dolby True HD system that has prevailed in most of the market.

Let's now look at the connection options these formats offer us.

In the first case, the Blu-ray player decodes to Multichannel Digital PCM and transmits that signal over an HDMI 1.1 or higher connection. Logically, both the player and the A/V receiver need to have an HDMI connection for this, and it must be the same on both devices.

The second option is for the HD source to convert the multichannel PCM signal obtained previously from digital to analog and transmit it to the amplifier via the RCA outputs. Both the source and the amplifier need to have multichannel analog connections.

Finally, we can have the player transmit the high-resolution audio signal directly over the HDMI connection and let the amplifier take care of decoding it. In this case, both the player and the receiver need to have HDMI 1.3 connections, since this version is the only one that supports the transmission of both Dolby True HD and DTS HD MA data, given the large amount of information involved.

Let's now talk about metadata, whose importance in these formats is crucial, and whose relevance has not yet been explained with sufficient precision.

Basically, it is a set of predefined bits containing instructions for the decoder. It is data that does not include audio, but does include instructions on, for example, dialogue normalization or dynamic range control.

In the past, producers working with multichannel audio had problems with soundtracks whose volume was well above or below the rest of the broadcast. This was common when surround content was broadcast from TV studios with mono or stereo sound outputs. With Dolby Digital, engineers can mix audio content independently for various playback systems and levels. This makes it possible to create a more consistent signal and thus ensure the presence of the most important content. Metadata is added during the production or post-production phase.

In this way, it provides a number of key parameters that are used specifically to adapt the sound of the broadcast to the viewer's specific listening environment. These are known as dialogue level (also called dialogue normalization or dialnorm), dynamic range control and downmixing. Although technically not an individual metadata parameter, the latter is carried out in the viewer's decoder by means of specific parameters, as with the other two. Particular care must be taken when monitoring and selecting the metadata parameters that do this. The sound engineer is ultimately responsible for optimizing the multichannel mix for the best possible reproduction in a perfect listening environment. However, it must also be checked for less well-equipped environments. Conversely, enjoying a television program should not be limited to consumers with full home theater systems. Dolby Digital and metadata combine to provide a simple, easy-to-use method that guarantees top-quality reproduction regardless of the number of channels, the level of ambient noise or the quality of the consumer's equipment. Let's now look at the different metadata parameters one by one:

Dialogue level

It provides a normalization value to the home decoder. This lowers the volume to a preset level, which is adjusted depending on the programs and content. Its presence is crucial for use in decoders, and it provides three main functions:

- During audio production for DTV, the mixer matches the audio recording level to that of the on-screen performance. In television post-production, this level allows different kinds of content (commercials, news, sitcoms, etc.) to be interspersed while maintaining the same volume in viewers' homes.

- Consumers' requirements for enjoying multichannel audio vary enormously. For example, not everyone who watches DTV listens to it in a multichannel environment. Bear in mind that each viewer will set what he or she considers reasonable for their ears within their own system. In addition to matching levels between different programs and media, dialogue level offers the benefit of acting as a kind of protection against digital clipping, lowering the overall volume by a very small percentage and providing something similar to a "master volume".

- An appropriate dialnorm value provides the "null band" in the dynamic range profile, where the audio level is neither raised nor lowered. Without dialnorm, reduced dynamic ranges might not work correctly.

The term dialogue normalization was developed by the film industry. Movies are mixed with this parameter at a specific level, and all other audio elements are subordinate to the dialogue. Moreover, in the world of cinema, dialogue is reproduced through the center channel, giving the sound engineer a very simple way of measuring this level. He simply measures the output of the center channel and makes sure that the conversation coming from this channel is not masked by music or sound effects.

This does not mean that dialnorm is only a valid parameter for dialogue. In purely musical productions, this parameter can be described as the program's average main volume.

In its simplest form, dialogue level is like turning down the volume on a home system. However, simply adjusting the volume on a stereo system does not give us the other advantages that dialogue level provides, such as dynamic range reduction for the listening conditions, adjusted and musically dynamic compression, and the virtual headroom extension that takes priority over the digital-to-analog circuitry.

The dialogue level in a correctly installed Dolby Digital decoder should not be modified. It does not involve any compression or expansion of the program material; it simply lowers the audio volume to a standardized level.

Dynamic range control

Dynamic range control (also known as dynamic range compression or midnight mode) gives the viewer the flexibility to listen to an audio program with reduced dynamic range. Compression allows viewers to watch TV without disturbing the neighbors. This control is optional and can be switched off on many Dolby Digital decoders.

Dynamic range control in the Dolby Digital data stream consists of two parameters or profiles: RF mode and Line mode. These two parameters do not alter the sound content encoded in the audio stream. They are used to adjust the extremes of the broadcast material in the listening environment for rooms where it is preferable or necessary to hear the program with reduced dynamic range.

RF mode is designed for peak-limiting situations, where the decoded program is output through an RF input on a television, as if through the antenna output of a set-top box. The RF profile is also used in decoders, ensuring that the dynamic range compression in an action movie will not wake the neighbors (hence the name midnight mode).

Line mode provides a lighter type of compression and also allows the user to adjust the boost of the lowest levels and the cut of the highest levels in the parameters of a home decoder, so that the viewer can tailor audio playback to his or her specific listening environment.

At low volumes, hissing sounds or whispered dialogue are harder to hear. However, if the viewer raises the volume, the loudest sounds (explosions, gunshots) become too loud for comfortable listening.

When dynamic range profiles are included in the decoder, it raises the level of the quietest passages of the program while lowering that of the loudest ones, allowing the user to enjoy the movie without constantly reaching for the volume control. Once again, this ability to scale the amount of compression only applies to the Line mode profile, and depends on the level of customization available to the user in his or her decoder.

Downmixing is a feature of Dolby Digital that allows a multichannel program to be reproduced through fewer loudspeakers. Put much more simply, it allows us to enjoy a digital TV broadcast without requiring a 5.1 multichannel installation.

Just as a stereo mix is monitored in mono to maintain compatibility, a multichannel audio mix requires the engineer to check it against fewer channels to ensure compatibility in downmixing situations. In this case, metadata allows Dolby Digital to be an "equal opportunity" technology, so that all viewers who receive a Dolby Digital data stream can enjoy the best possible audio reproduction, regardless of the number of channels in their system.

Digital terrestrial, cable or satellite television equipment will normally provide a mono signal at the RF/antenna output, an analog stereo line-level signal and a coaxial and optical digital output for passing the Dolby Digital stream to a decoder.

The analog stereo signals coming from these units are normally an internal downmix of the original multichannel Dolby Digital audio signal. This analog output can be one of two types of stereo signal. One is the Lt/Rt compatible stereo downmix, available for Dolby Surround Pro Logic. The other is a simple Lo/Ro stereo downmix, available for recording on a hi-fi stereo system or for listening on headphones.

During downmixing, the settings of the dynamic range control parameters are limited, and this is no different when using the stereo signals present at the RF/antenna outputs.

We know that we have gone one step further than usual in the already technical information we offer. The importance of metadata, which has received little attention until now, obliged us to do so. According to respected sound engineers, its importance in the development of the new digital audio formats is paramount. What's more, it is the foundation of everything that may come in the future, and, as always, we will keep you informed.