Updated: March 26, 2016

Dolby Sound Formats Explained

A Guide to Dolby Multi-Channel Surround Sound

Dolby Sound Systems are synonymous with surround. For many, Dolby is the king of surround sound, yet very few home users have a real understanding of the different Dolby audio formats available on most home theater receivers.

Help is on the way! We look at the different multi-channel audio formats available from the king of surround - from Dolby Surround to Dolby Virtual Speaker, and the latest high definition audio formats designed to match the picture quality on Blu-ray Disc players.

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Introducing Dolby Multi-channel Audio
for surround sound

Dolby Laboratories introduced the first surround sound format, named Dolby Surround, in 1976. It was an analog cinema sound format for use on 35mm film. Later in the early 1980s, this first multi-channel surround format became available on consumer gear.

Since then, the lineup of Dolby sound formats has grown to a point that it has become increasingly difficult for end customers to follow suit, with loads of surround sound formats each with its very specific requirements to support a unified soundfield. And as you will soon discover in this guide, today's discrete digital surround sound formats are a far cry from the first four-channel matrix-encoded analog Dolby Surround; their ability to reproduce a wide yet enveloping soundfield, intelligible dialog and pin-point localization of sounds, is more superior than that of the first surround sound format.

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Dolby LogoDolby Sound Formats
Section Index

Part 1: Digital Audio Formats for Playback

Dolby Surround/Dolby Pro Logic

Dolby Digital

Dolby Digital Live

Dolby Digital EX / THX Surround EX

Dolby Digital Surround EX

Part 2: Dolby High-Definition Audio Formats

Dolby TrueHD

Dolby Digital Plus

Part 3: Surround Sound Expansion

Dolby Pro Logic II

Dolby Pro Logic IIx

Dolby Pro Logic IIz

Part 4: Dolby Virtual Surround Sound Technology

Dolby Headphone

Dolby Virtual Speaker

Part 1: Dolby Sound - Digital Audio formats for Playback

We start our discussion by looking at the mainstream Dolby sound formats for multi-channel audio playback, namely, Dolby Surround, Dolby Digital, Dolby Digital Live, Dolby Digital EX, and Dolby Digital Surround EX.

Dolby Surround and Dolby Pro Logic
Dolby Surround

Dolby Pro Logic

Dolby Surround technology delivers four channels of audio - Left, Center, Right, and Mono Surround - that are matrix-encoded onto just two audio tracks. These two tracks are then carried on stereo program sources such as TV broadcasts and feature films on VHS.

Synonymous with Dolby Surround is the Dolby Pro Logic decoder; this is required to reconstruct Dolby Surround encoded audio to its original four-channel surround sound; the diagram below explains further the whole decoding process:

Dolby Pro Logic Decoder


Dolby Surround/Pro Logic is a Matrix-based technology. When a Dolby Surround soundtrack is created, four channels of sound are matrix-encoded into a two-channel sound track (ordinary stereo), by using phase shift techniques.

The Pro Logic  decoder extracts the four channels from the stereo sound; systems lacking the decoder will simply play back the audio as standard Stereo.

Dolby Surround is the consumer version of the original multi-channel analog-optical film Dolby sound technology - Dolby Analog and Dolby Spectral Recoding; it is still included today on nearly all 35mm film prints to serve as a backup in cinemas in case of problems with the digital soundtrack.

Dolby Spectral Recording (Dolby SR) is in effect a noise reduction technology used in almost all modern professional audio analog recordings. When used with Dolby Analog, Dolby SR helps improve the dynamic range of the analog recording by as much as 25 dB - between peak level and noise floor.

The introduction of the Dolby Digital multi-channel film sound format has eventually replaced Dolby Surround as the preferred technology to deliver surround sound via DVD-Video, digital television, and games.

Interesting to note that though Dolby Surround was introduced as an analog format, all Dolby Digital decoders include a digitally implemented Dolby Surround Pro Logic decoder for digital stereo signals that carry this matrix-encoded Dolby Surround sound format.

Editor's Note: Our discussion on Dolby Pro Logic decoder based formats continues under Part 3 of this series of articles, where we discuss Dolby Sound Expansion technology.

Dolby Digital and Dolby Digital Live
Dolby Digital

Dolby Digital - formerly known as Dolby AC-3 (AudioCoding-3), delivers the movie experience through a maximum of 5.1 channels of surround sound audio. Since the mid-1990's, this Dolby sound format has become the most popular surround sound format in use today.

Dolby Digital can be considered as the multi-channel audio standard for DVD-Video, HDTV, PC games, digital cable, and direct broadcast satellite (DBS) systems; it is found in practically every A/V receiver and HTIB on the market.

Dolby Digital

(picture courtesy: Dolby Laboratories)

Dolby Digital is, as its name implies an all-digital standard. The sound information contained in each of its six available channels is distinct and independent.

It is described as a '5.1-channel' Dolby sound system because it carries five full-bandwidth channels having a frequency range from 3 Hz to 20 kHz for Front Left and Right, Center, and the two Surrounds, plus one 'Low Frequency Effects' (LFE) channel devoted to frequencies from 3 to 120 Hz.

While the sound quality associated with Dolby Digital is not up to the same standard as the high resolution formats of DVD-Audio, SACD and the latest ultra-HD audio formats from both Dolby and DTS, yet its efficient coding structure is still capable of delivering high-quality discrete multi-channel audio that is more than adequate for home movie applications. It is thanks to this efficient coding that Dolby Digital has become the 'quasi' de facto surround sound standard for the DVD world — it is capable of delivering quality audio without compromising on available DVD storage space for quality video content.

Dolby Digital is a flexible surround sound format. More specifically, it is an audio encoding scheme that supports up to 5.1 channels, but not all Dolby Digital soundtracks have 5.1 channels of audio; those that are, carry the designation 'Dolby Digital 5.1.  For example, within Region 1 (U.S. and Canada), Dolby Digital 2.0 soundtrack (stereo, or Dolby Surround Pro Logic) is compulsory on all DVDs. 

Dolby Digital Live:  This is a Dolby sound encoder that is generally found in PC and console-based video game applications. It is capable of converting any stereo or multi-channel audio signal to a Dolby Digital format bitstream — on the fly — for transport and playback through a home theater system. This yields for more realistic surround sound effects during interactive video game-play. Transport of the encoded bitstream through Dolby Digital Live is carried out over a single digital connection (fiber optic link), equipped with an S/PDIF connector.

Dolby Digital EX and Dolby Digital Surround EX
Dolby Digital EX

Dolby Digital EX creates a more intense surround sound experience by adding a sixth full-range channel - referred to as Center Back - for enhanced localization of surround effects and a more spacious soundstage.

It is optimized for Dolby Digital Surround EX™ content, however interest in this Dolby sound format is low and availability of EX-encoded DVDs is scarce.

This extra 'Center Back' channel is in reality matrixed over the two left and right surrounds, rather than discrete.  This is somewhat different from the design philosophy adopted in its extended DTS counterpart where this extra channel is discrete. This also explains why DTS ES can provide a more precise location for the rear-effects soundstage than the Dolby Digital EX format.

This extended surround Dolby sound format was originally developed by Lucasfilm THX and Dolby Laboratories under the brand name THX Surround EX. It is the home theater version of the 'Dolby Digital Surround EX'. Later, Dolby Laboratories began to license the THX Surround EX format under its own name as Dolby Digital EX for consumer home equipment. In other words, THX Surround EX and Dolby Digital EX are equivalent.

To enjoy Dolby Digital EX, you need a home theater receiver or preamp/processor with Dolby Digital EX decoding and a speaker setup that supports 6.1 or 7.1 audio playback. In the case of a 7.1 speaker setup, the two back surround channels in this Dolby Sound format will be actually playing the same identical sound (ref. to picture below).

Dolby Digital EX 6.1 Dolby Digital EX 7.1

Dolby Digital EX 6.1
Speaker Configuration

Dolby Digital EX 7.1
Speaker Configuration

Note: A 6.1 or 7.1 speaker playback setup implies that to implement this extended surround format in a home theater room, your prime listening position should not be located near a wall.

(pictures courtesy: Dolby Laboratories)

Dolby Digital EX encoded content is fully backward compatible with the standard 5.1-channel Dolby Digital playback systems in that no real sonic information is lost, though the added realism provided by the sixth channel would be missing from the overall listening experience.

Current Dolby Digital Surround EX soundtracks contain a digital flag that can automatically activate the EX decoding in a Home Theater receiver. For titles released prior to late 2001, however, you need to turn on the EX decoding manually.

As already indicated, Digital EX encoded content is scarce but this extended surround Dolby sound format can be used to improve the sound of regular Dolby Digital encoded DVDs.

Dolby Digital Surround EX

Dolby Digital Surround EX is the movie theater version of the Dolby Digital EX; rather, the Digital EX was developed by Dolby as the home theater version of the Digital Surround EX for cinema surround sound).

As expressed under the Digital EX format, this is a 6.1 surround Dolby sound format and the extra back surround channel helps to create a fuller, more realistic sound for increased dramatic effect in the theater.

On the film itself, the additional back surround information contained in the Surround EX soundtrack is matrix-encoded onto the regular left and right surround channels of conventional Dolby Digital 5.1 soundtracks. This means that even if a cinema is not equipped with the extra Surround EX speakers in the rear, the information intended for it through this Dolby sound format, will still be played through the traditional left and right surround-channels.

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