Dolby Digital / AC3

Dolby Digital
  • Introduction to the Dolby Digital format
  • Dolby Digital EX
  • Dolby Headphone

Introduction to the Dolby Digital format

Launched in 1987, the Dolby Digital format is a digital audio encoding standard developed by Dolby Labs.

Unlike Dolby ProLogic systems, Dolby Digital audio tracks are independent (the term "discrete" is also sometimes used). For consumer electronics, the Dolby Digital format relies on a compression algorithm called AC3 (Audio Coding 3), which can compress audio streams by a factor of 10 to 12 with a 16-bit sampling rate of 48 kHz, for an overall bitrate of 384 kbps. For this reason, the Dolby Digital format is sometimes called Dolby AC3. For professionals, instead of an AC3 decoder, Dolby Digital uses a system called SR-D.

Dolby Digital is likely the most commonly used system in home theatre systems. To be able to use media recorded in Dolby Digital format (DVD video or DVD audio, for example), you need a device with an integrated AC3 decoder. These devices are normally identified by the presence of this logo:

Dolby Digital

The Dolby Digital format allows for surround sound, as it uses six independent audio channels:

  • A central speaker, generally placed atop or above the screen, for reproducing dialogue.
  • Two audio tracks for the front speakers, for accentuating the context of the sound coming from the central speaker.
  • Two channels for the rear speakers, used for reproducing noise and the sound environment, in order to create ambiance.
  • A channel for low frequencies (a subwoofer) for amplifying special effects (like explosions and earthquakes).

The Dolby Digital format's bandwidth falls between 20 Hz and 20 kHz. Dolby Digital audiotape can be encoded in either of the following formats:

  • in stereo
  • n 5.1. It should be noted that Dolby Digital is often incorrectly confused with the generic term "5.1".

Dolby Digital EX

Dolby Digital EX has an additional channel (the term "6.1" is sometimes used) created with two rear channels in order to hide the "sound gap" which exists behind the listener. Dolby Digital EX devices traditionally have the following logo:

Dolby Digital EX

However, a Dolby Digital EX can be read just as easily by a traditional Dolby Digital 5.1 decoder.

Dolby Headphone

The Dolby Headphone standard is used to privately enjoy surround sound with a traditional set of headphones, without bothering nearby people, using a stereo or multichannel source.

Dolby Headphone is based on a process called "virtualization," which uses HRTF (Head Related Transfer Functions) to simulate 3D sound by taking into account the distortion of sound as it passes through the different parts of the listener's head, and combining this information with the acoustics of a virtual listening room.

Devices (like sound cards, home theatre systems, videogame consoles, etc) compatible with Dolby Headphone bear this symbol:

Dolby Headphone
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