2. Dolby Digital Sound

In 1965, an American physicist and engineer named Ray Dolby founded the Dolby labs in London. His idea was to develop noise reduction systems to improve sound quality for professionals and the general public alike. The Dolby name is now known throughout the world, and the surround sound standards he created are used both in movie theaters and at home. Here's a brief reminder of the two predecessors of the Dolby Digital flagship standard:

Dolby Digital 5.1, also called AC-3 (for Audio Code-3), has six channels - two front, two rear, one center and one for the subwoofer. Unlike Dolby Surround and Prologic, the bandwidth here ranges from 20 Hz to 20 kHz. The "5.1" refers to the five front and rear parts, plus the subwoofer, called LFE (for Low Frequency Effects), represented by the "1". The term AC-3 refers to a coding technology which eliminates sound data the user cannot hear and produces a Dolby Digital sound band coded on six channels. An absolute prerequisite for Dolby Digital sound is a decoder, such as the one on the Creative Labs Inspire 5700, or else a sound card like Audigy or Acoustic Edge, which decodes 5.1 sound and has the correct outputs.

There are also systems with just two speakers and a subwoofer which use Virtual Dolby Digital. These use a front channel mixing process to produce a virtual center channel; the rear channels are emulated by a processor which "virtualizes" surround sound using the HRTF filters via the front speakers.

Dolby Digital uses a fixed sound compression method of about 1:2. In other words, however much sound there is to encode, the compression will always be the same, because the compression algorithm has a constant output. Beyond this, there is a simultaneously positive (disk space) and negative aspect - negative because sound quality decreases as volume increases. But set against this is that the less space used on a DVD, the more space is available for different languages or bonuses, and this can compensate for a loss of quality. In general, AC-3 sound is coded in 18 bits, so the standard output of an AC-3 flow is 384 kbps (6 channels x 18 x 48 kHz). On restitution, the Dolby Digital decoder transmits with a delay of one millisecond on the front channels because the listening position is usually closer to the rear speakers than the front or center ones. This optimizes simultaneous sound reception. With some decoders you can adjust this delay to get the best listening configuration.

The main advantage of this standard is the fact that it is the digital audio surround standard for DVD. In the rules defining the DVD standard, no other kind of digital sound band can be inserted on a DVD unless there is also a Dolby Digital-encoded sound band. So you'll never find a DTS sound band on its own. This leads to the second advantage of the standard - it is virtually universal. The first movie with a Dolby Digital 5.1 sound band was "Batman Returns" in 1992. Since then, practically all DVDs have perpetuated the standard.