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1. What is Dolby Digital? Dolby Digital is an advanced for of digital audio coding that makes it possible to store and transmit high-quality digital sound far more efficiently that was previously possible. First used in movie theatres in 1992, it is the result of years of research by Dolby Laboratories in developing a signal processing system that exploits the characteristics of human hearing.
2. How does it work? Dolby Digital is a ‘lossy’ encoding system, which means that it removes information from a digital audio data stream that is determined to be unnecessary. In other words it removes the bits of a soundtrack that we cannot hear. However the removal of sound information develops artefacts, little errors that we’d be able to identify as not quite sounding right. To counter this problem the Dolby Digital encoder uses a masking system to hide the noise created by the artefacts and smooth the signal towards the top and bottom of the perception band as defined in the psychoacoustic model Dolby have developed. The system is capable of providing exceptional audio quality with sampling bitrates as low as 384kb/s, but the higher the bitrate, the better the quality. The determination as to which sound is heard from what speaker is determined by the sound producer on his mixing desk, but the information is encoded as metadata – data describing the Dolby Digital data – which instructs the Dolby Digital Decoder in your set top box or home theatre where to send the various sounds. This metadata is included in the Dolby Digital bitstream. Because Dolby Digital removes elements of the soundtrack, it is usually used at the very end of the production process, just before the DVD master is produced, or just before transmission.
3. What programs are available in Dolby Digital? Dolby Digital audio is available on most DVD discs be they for films, TV shows or audio. It is also used by many Digital Television (DTV) stations be they on satellite, cable or even terrestrial channels. You can usually tell when a program is in Dolby Digital by looking for the Dolby Digital logo in the program listings.
4. I don’t have a home theatre system, will I still be able to hear Dolby Digital encoded programs? Yes, so long as you have a Dolby Digital decoder built into to your receiving box, be it a satellite settop box or DVD player. All Dolby Digital decoders have a feature called “downmixing” which assures full compatibility with any sound playback system. The decoder ‘knows’ what equipment you have attached to it and adjusts the sound mix accordingly. The standard sound mix for Dolby Digital is 5.1 which provides discrete channels of sound for the centre, left surround, right surround and subwoofer speakers. Downmixing combines these ‘extra’ channels with the traditional left/right stereo channels to produce a mix that is correct for your equipment. For example to produce a stereo mix from a full 5.1, the decoder will combine the left surround with the left front channel, the right surround with the right front channel, add the centre channel to both left and right front and ignore the subwoofer channel. To produce a mono track the decoder would then combine the left and right front channels together into a single mono signal. The way this downmixing is performed is set up by the original sound mixers when they first mix the program using metadata, a digital data stream that provides information to the decoders about how the sound channels should be handled.
5. What is “5.1 channel” Dolby Digital? Producers can decide during the sound mixing stage to produce Dolby Digital in full surround sound with five separate channels – left front, centre, right front, left surround, right surround – and a sixth channel for those powerful low-frequency effects (called LFE and sent to the subwoofer speaker) that are felt more than heard in movie theatres. Because it uses only about one-tenth of the audible bandwidth of the other channels, LFE is referred to as a .1 channel.
6. Are all Dolby Digital programs in 5.1 surround sound? No, Dolby Digital soundtracks can provide anything from mono to full 5.1 channel surround sound. DVD video discs or films often carry multiple versions of the soundtrack that differ the number of channels they use. For example the movie may use a full 5.1 soundtrack, but the director’s commentary could only use the stereo channels.
7. I’m tired of having to adjust the volume every time the program changes or I switch sources or TV channels. Can Dolby Digital help? Yes. Conventional program sources often force you to re-adjust the volume when you change channels or even between programs. It takes a broadcaster a great deal of effort to set the levels of different programs to ensure this happens as little as possible. However a Dolby Digital soundtrack has a feature called Dialogue Normalisation which allows the end user to set the playback volume and then not have to adjust it again. Dialogue Normalisation automatically adjusts the volume when you change Dolby Digital programs so that the dialogue remains at a consistent level of volume. Studies have shown that perception of volume levels are based upon the loudness of the dialogue. The average loudness of the entire program is programmed into the metadata that describes how the Dolby Digital bitstream is to be decoded. The system does not alter the range of the program, a film for example may have some quiet dialogue moments and some loud action. The action will still be as loud relative to the dialogue level as it ever was, but if a quiet scene follows the dialogue will be at the same level as it was in the action scene. Broadcasters can even apply this system across all of their programs automatically using Dolby’s LM100 Loudness meter which allows them to quickly and easily set the dialogue level for their programs and add that level to the metadata.
8. Can my Dolby Digital home theatre system decode the new digital TV broadcasts? Dolby Digital has been adopted as standard for all ATSC and DVB broadcasts across the US and Europe. All of these broadcasts are compatible with existing Dolby Digital decoders. Like other Dolby Digital program material, the audio format of DTV broadcasts varies from mono to full 5.1 channel surround sound. Because many broadcasters are still moving to 5.1 productions their older programs are in a variety of sound formats, and many programs such as the news or the weather may never appear in surround sound, many DTV programs are being broadcast in 2 channel Dolby Digital. The Dolby Digital decoder simply switches from 5.1 channel to 2 channel (or mono) when instructed to by the programs metadata and the end user will not hear nay sound from those speakers that are not used by the program. DTV sets and tuners also vary with respect to the number of audio channels they provide. Some have two-channel Dolby Digital decoders and will generally use a Euro AV SCART plug to connect to your TV set and provide a Stereo sound mix. Others will have full Dolby Digital 5.1 surround sound decoders ready to connect to either an external amplifier and speakers or have the amp and speakers included in the unit. Those units without a full Dolby digital decoder will pass though the un-decoded Dolby Digital to an external Dolby Digital decoder unit, usually as part of a home theatre system. These units, which usually provide a stereo output as well, are typically marked with the Dolby Digital Bitstream Out logo. These Bitstream Out decoders allow you to add a Dolby Digital home theatre system at any time and without fear of obsolescence or equipment duplication.
9. What is metadata? The key to the adaptability of Dolby Digital is metadata, "data about the audio data." Metadata is created during the production of DTV programming and is carried in the Dolby Digital bitstream. Metadata parameters maintain the "vision" of the audio created by a program's producer, make life easier for broadcasters, and give viewers the best audio DTV has to offer, whether viewers own mono, stereo, or 5.1-channel audio systems. Three key Dolby Digital metadata parameters are dialogue normalization (also known as dialnorm), dynamic range control (DRC), and downmixing.
10. What is Dolby Digital Plus? Will this make all my Dolby Digital equipment obsolete? Dolby Digital Plus is an audio medium for broadcast that offers increased bandwidth efficiency through improved coding and compression allowing for more sound channels or the same 5.1 sound channels in less space freeing up more of the broadcasters available bandwidth for visual enhancements such as HD. This gives Dolby Digital Plus the power and flexibility needed to support a wider range of current and emerging market applications, such as HD TV, IPTV and HD-DVD/Blu-Ray. To make use of Dolby Digital Plus requires the end user to have a Dolby Digital Plus decoder in their set-top box or HD-DVD player. The Dolby Digital Plus decoder provides an output that is completely compatible with existing Dolby Digital equipment and provides the same thrilling 5.1 soundtracks and automatic downmixing and dialogue normalisation that Dolby Digital provides.
11. How do I produce multichannel audio content? Many genres of programming are already being produced in HD with 5.1 audio, such as live awards shows and sports events, premium dramas, music, and movies. Production of original 5.1-channel audio for HD is straightforward and can be performed using a wide variety of mixing consoles and workstations. A key consideration when producing 5.1 material is the setting of audio metadata. Metadata is additional data about the audio that is carried with the audio signals, ultimately allowing the sound to be presented in the best possible way in the home. For example, it controls how a 5.1 soundtrack is downmixed for presentation on systems with fewer speakers, and also controls dynamic range compression characteristics for small speakers or for late night listening. Setting metadata during the mix gives you complete control over how the mix will sound in the home, whether it is being reproduced in mono, stereo, or over the full 5.1 channels. As it is likely that the 5.1 mix will be the only audio stream that is transmitted for HD services, you can use the metadata to ensure that anyone listening to the stereo or mono downmix gets precisely the mix you want them to have. Whether your program is live or post-produced, the easiest way to deliver HD video and 5.1 audio to the broadcast center is using Dolby® E. Dolby E allows up to eight discrete channels of audio, and associated metadata, to be carried on one AES channel pair of a tape, server, or link (see figure 1). This makes synchronization of material much simpler and means that there is no potential for misplacing associated tapes. The HD picture always has associated audio, and audio always has associated metadata. Dolby E is also widely used for carrying multichannel audio for 5.1 broadcasts on SD services.
