Figure 1: DVD-Video disc soundtrack format icons
DVD-Video doesn't just offer a better picture. It also provides vastly improved sound-and a lot more of it. Up to eight soundtracks, including different-language versions of a movie, can be included on the same disc. What's more, the soundtracks can carry anything from mono sound up to 5.1-channel surround sound, and use different digital audio coding technologies.
Soundtrack Formats
Most DVD-Video discs feature a mono, two-channel or 5.1-channel Dolby Digital soundtrack. Many two-channel soundtracks, particularly on discs of movies, are Dolby Surround encoded for four-channel playback over systems equipped with Dolby Surround Pro Logic decoding. Older movies, of course, often have mono soundtracks.
Sound Modes
Many DVD-Video releases display icons that show the number of audio channels in a soundtrack and the channel configuration. An example of these sound mode icons is shown in Figure 1.
Some 5.1-channel decoder units and A/V receivers have playback mode displays based on these same icons. Some discs provide a variety of soundtracks using different sound modes. A movie disc, for example, might use 5.1-channel Dolby Digital for its English track, and Dolby Surround for other languages.
Digital Audio Types
A DVD-Video soundtrack can be of one of several audio types, based on different digital audio coding systems-and soundtracks of different audio types can be included on same disc.
Dolby Digital is categorized as a "mandatory" audio type, meaning it can be the only one used on a disc. Those that are categorized as "optional" can be used only when at least one soundtrack of a "mandatory" type is on the disc as well. Figure 2 lists the various audio types used on DVD-Video discs.
Because all DVD-Video players are equipped with Dolby Digital decoding as standard, most DVD-Video titles have only a mono, two-channel or 5.1-channel Dolby Digital soundtrack as previously noted. Packaging for these titles usually displays the 
logo.
Additional indicators or logos are displayed if soundtracks of more than one audio type are present. The Dolby Digital track on such discs may be identified by the 
icon combined with a sound mode icon.
Figure 3 shows an example of a disc with four soundtracks utilizing different sound modes and digital audio types.
Playback
Most DVD-Video players provide two-channel analog outputs plus a digital audio output.
As shown in Figure 4, the two-channel analog output is for connection to a TV, a two-channel stereo system, or a home theatre system with Dolby Surround Pro Logic decoding.
The player will automatically create a Dolby Surround encoded, two-channel version of 5.1-channel Dolby Digital soundtracks. For mono playback, some players can also create a mono version at the listener's option.
As shown in Figure 5, the digital output is for connection to an external unit, such as an A/V receiver, with 5.1-channel Dolby Digital decoding.
Some players, as shown in Figure 6, have built-in 5.1-channel decoders for connection to a "5.1- channel ready" receiver. External 5.1-channel decoders, however, provide the greatest flexibility, and often superior sound as well.
About Surround Sound
Dolby Surround delivers four-channel surround sound via regular stereo VHS video tapes, TV and FM broadcasts, laser discs, DVD-Video and DVD-ROM discs, video games, and CDs. Four channels (left, center, right, and surround) are encoded into two-channel soundtracks, then recovered on playback by means of a Dolby Surround Pro Logic decoder. (Figure 7.)
Dolby Digital can deliver 5.1-channel sound from DVD-Video and DVD-ROM discs, laser discs, and digital TV broadcasts, satellite transmissions, and cable systems. It differs from Dolby Surround by providing two separate surround channels for greater realism, and an additional ".1" or "LFE" channel for low-frequency (bass) special sound effects. (Figure 8.)
The LFE channel is sometimes incorrectly identified as the "subwoofer" channel. While a subwoofer can be helpful in reproducing low bass in general, it is not necessary to have one if one or more of the speakers in the playback system have extended bass response. Without a subwoofer, the LFE channel's bass effects can be directed to those speakers with extended bass. Furthermore, not all multichannel soundtracks have the ".1" channel; it is used primarily for explosions, rumbles, and the like on movie soundtracks, and is not present on all programs.
