A Guide to Multi-Channel Surround Sound

Dolby Sound Formats

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 HD-DVD and Blu-ray Disc players.

Introducing Dolby Multi-channel Audio Formats for Surround Sound

Dolby Laboratories introduced the first surround sound format - Dolby Surround - in 1976, as 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 the end customer to follow suit.

As you will soon discover in this guide, to-day'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 far superior than ever.

Dolby Sound Formats - Index

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

Dolby High-definition Audio Formats

• Dolby TrueHD

• Dolby Digital Plus

Surround Sound Expansion

• Dolby Pro Logic II

• Dolby Pro Logic IIx

Dolby Virtual Surround Sound Technology

• Dolby Headphone

• Dolby Virtual Speaker

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 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. Refer to diagram for details.

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 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 increasing the range - 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 analogue 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.

	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 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.

(picture courtesy of Dolby Laboratories)

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 movie applications.

It is thanks to its 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.

Note: 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, Dolby Digital 2.0 soundtracks (stereo or Dolby Surround Pro Logic) are compulsory on all Region 1 (U.S. and Canada) DVDs. 

Dolby Digital Live Found in PC and console-based video game applications, this Dolby sound encoder 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 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 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 Speaker Configuration	Dolby Digital EX 7.1 Speaker Configuration
(pictures courtesy of Dolby Laboratories)
Note that 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.

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 is 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 is the movie theater version of the Dolby Digital EX, (rather, the Digital EX was developed as the home theater version of the Digital Surround EX cinema theater 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.

 Next: Dolby Digital High Definition Audio

 ...back to Dolby Sound Formats Sec. Index

 Copyright © - Practical-Home-Theater-Guide.com  All rights reserved.