Understanding Dolby® Digital

Dolby® Digital is no doubt a popular audio technology. It is used with digital versatile discs (DVDs), high definition television (HDTV), and digital cable and satellite transmissions. It also has been selected as the audio standard for digital television (DTV).*

Dolby Digital, formerly known as AC-3, is a surround sound technology that is the standard for DVD-Video. It delivers high-quality digital audio for up to 5.1 discrete channels (Left/Center/Right/Left surround/Right surround/Low-Frequency Effects). The five speaker channels produce a directional and more realistic effect and the Low-Frequency Effects (LFE) channel (usually reproduced through a subwoofer) can often be felt as well as heard.

All “Dolby Digital” is not necessarily Dolby Digital 5.1. The common version contains 5.1 channels, but the format supports Mono and Stereo as well. For example, Dolby Digital 1.0 is mono audio (single track of audio information), while Dolby Digital 2.0 is stereo (which can be encoded as Dolby Surround or Dolby Surround Pro-Logic). Dolby Digital 4.0 has discrete audio signals for the center, left, and right channels, plus a mono audio signal that is common for the surround channels. Dolby Digital 5.0 is like Dolby Digital 5.1, but lacks a discrete audio signal for the low frequency effects channel.

Fig 1: Different channels are activated on different Dolby Digital formats

Fig 2: Suggested Corner Arrangement of Dolby Digital 5.1 Channel Setup

What are the advantages of Dolby Digital audio?

The first advantage that everyone recognizes is, of course, audio quality. However, another great advantages that Dolby Digital is a very efficient compressor, which means that the excellent quality also comes with a substantial saving in file size. Dolby Digital encodes these multi-channel soundtracks at either the 448 kbit/sec or 384 kbit/sec data rates. Smaller audio files mean more high-quality video can be burned on a DVD. Or you can create audio-only DVDs with over 30 hours of Dolby Digital-quality music that will play in any DVD player!

Is Dolby Digital 5.1 and Dolby Surround the same thing?

No. Briefly speaking, Dolby Digital 5.1 provides 6 discrete channels of audio, whereas Dolby Surround is 4 original channels encoded to 2 channels. These can then be played out over a 2-speaker system, or split out again by a decoder to 4 speakers.

Do all DVD-Video discs have Dolby Digital soundtracks?

No. Discs distributed in countries that use the NTSC broadcast TV standard (such as the U.S.) must provide at least one soundtrack in either of two formats: Dolby Digital or PCM. In countries using the PAL TV standard (including much of Europe), the discs must include at least one soundtrack in any one of three formats: Dolby Digital, PCM, or MPEG. Dolby Digital, however, is the only universal multi-channel standard, and every DVD player sold worldwide incorporates Dolby Digital decoding. Therefore the vast majority of discs do include at least one Dolby Digital soundtrack.

Do I need 5 or 6 speakers to play back DVDs with Dolby Digital 5.1?

Not necessarily. All Dolby Digital decoders, whether 5.1-channel or 2-channel, have a feature called "downmixing" that assures full compatibility with any playback system. On 5.1-channel programs, the decoders can create several types of mixes "on the fly." These include a 2-channel, Dolby Surround encoded mix for playback over a home theater system with Dolby Surround Pro Logic® decoding; a 2-channel stereo mix for regular stereo and headphone playback; or a mono mix for playback over a mono TV set.

What Ulead products support Dolby® Digital?

*The European DVB standard does not use Dolby Digital for audio, but instead uses MPEG standard technology for both the audio and video signals.
**(Plug-in required)

Dolby and the double-D symbol are registered trademarks of Dolby Laboratories. AC-3 is a trademark of Dolby Laboratories.