Surround Sound - Dolby Formats - Part 3: Dolby Surround Sound Expansion
Updated: April 2, 2016

Dolby Surround Sound Expansion

A Guide to Matrix-based Multi-channel Dolby Sound
Expansion using Dolby Pro Logic Technology

Dolby Surround Sound matrix-based decoders include Dolby Pro Logic, Dolby Pro Logic II, Dolby Pro Logic IIx, and the latest Dolby Pro Logic IIz. All are capable of generating an expansive sound stage from any two-channel (stereo) sound track, yet there is a significant difference between these decoders.

In addition, there is more to Pro Logic decoders than just surround sound. In general, these systems tend to produce a relatively soft-focused overall sound that lets musical sounds breathe, while dialogue and sound effects are less harsh. In other words, matrix-based Dolby decoders sound good!

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Introduction to Surround Sound Expansion

Matrixed Dolby Decoders

The best of the older matrixed Dolby decoders is Dolby Pro Logic - which expands a two-channel stereo sound track to four channels played over five speakers, the front left and right, front center, and two speakers for a mono surround; you may also add an optional sub-woofer.

Sound from a Dolby Pro Logic decoder for surround-encoded movies sounds good, but it has little detail in the surround as a result of its single, frequency-limited (cut-off at 7kHz) monaural surround signal. Furthermore, while a Dolby surround sound Pro Logic decoder can be driven by any stereo sound track, CDs and other stereo music content does not really sound like surround.

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This Pro Logic decoder tends to throw too much of the mix into the front center speaker when driven by ordinary stereo content - thus resulting in an unbalanced surround sound field. In other words, you cannot just leave the Pro Logic mode 'on' at all times. This deficiency eventually led to the design of a second Dolby surround sound matrixed-decoder - Dolby Pro Logic II, in an attempt to achieve a cleaner sound that matches better with any stereo content.

Dolby Pro Logic II

Dolby Pro Logic II

Dolby Pro Logic II builds up on the same matrix Dolby surround sound decoding technology as its predecessor - Dolby Pro Logic, with the difference that it adds an extra channel for the surround. In other words, it breaks up the surround channel into a pair of stereo rear-surround channels. Vide diagram below.

Dolby Pro Logic II Decoder

But there are further differences from the original Pro Logic Dolby surround sound process than just the addition of a stereo rear-surround.

Dolby Pro Logic II transforms any stereo signal into five-channel, full-bandwidth surround sound in contrast to the 7 kHz frequency-limited monaural surround of the former. This yields a better surround sound-stage that is ideally suited for home theater systems.

Pro Logic II Dolby surround sound technology also provides more control of the center image and front/rear balance. And it does all this with simpler processing, thus yielding even cleaner sound.

Finally, it is designed for use with both surround-encoded sources as well as with any ordinary stereo sound track. This means that it can yield an expansive surround sound-stage even with ordinary stereo content.

It is found in the Apple® iPod®, in MP3 and CD playback, in VHS movies, in the now extinct laser discs, and in stereo broadcasts.

Dolby Pro Logic II is fully backward compatible with all Dolby surround sound Pro Logic technologies and can be used to provide 5.1-channel playback for the thousands of video cassettes and TV programs encoded in four-channel Dolby Surround, the encoding counterpart to Dolby Pro Logic's decoding technology.

Pro Logic II decoders support three listening modes, with the most important being 'music' and 'movie'. Thus, in the music mode, this Dolby surround sound technology provides for a more spacious feel than in the dialogue oriented movie mode. In addition, its 'dimensional control' provides the user with a sort of front/rear balance control that may turn out useful for certain content, e.g. if your stereo content provides little room ambience, you can literally break up the front focused soundstage to let it breathe towards the rear.

As with all multi-channel audio playback systems, Dolby Pro Logic II surround sound systems require precise speaker placement to produce the best enveloping soundstage.

For speaker placement details associated with multi-channel surround sound, please refer to our Home Theater Speaker Placement article.

Dolby Pro Logic IIx and IIz: The latest generation of Dolby Surround Sound Expansion

Dolby Pro Logic IIx

Dolby Pro Logic IIx is the DTS Neo:6 counterpart; it builds up further on the Pro Logic II Dolby surround sound decoder by transforming any stereo or 5.1-channel signal into six or seven-channel full-range surround sound. In other words, this is the extended format for Dolby Pro Logic II using advanced matrix-based technology.

The actual number of playback channels dependents on your speaker playback setup rather than the Pro Logic IIx technology itself. This Dolby surround sound technology is featured in iPods, MP3 and CD playback, DVD-Video, VHS movies, and in stereo and surround broadcasts.

With Pro Logic IIx, any movie, CD, TV program or video game can be enjoyed through this enhanced Dolby surround sound technology, thus creating a seamless, natural surround soundfield that immerses you in the entertainment experience.

Dolby surround sound Pro Logic IIx technology supports three listening modes 'Movie', 'Music' and 'Game'; these allow the listener to tailor the audio to meet the different needs of the content. Thus, in Game mode, special effects signals are routed to the surround channels for fuller, dramatic impact.

Additionally, when selecting the Music mode, this Dolby surround sound format features three additional user controls:

Dolby Center Width adjusts the balance of the main vocals in the center and front channels.

Dolby Panorama creates a seamless, wraparound surround effect by leaking some information from the front to the rear.

Dolby Dimension lets you set a deeper or shallower surround soundfield.

Dolby Pro Logic IIz - Picture courtesy of Dolby

Front 'height' speakers in
Dolby Pro Logic IIz setup

[Picture courtesy: Dolby Labs]

Dolby Pro Logic IIz: In 2009, Dolby introduced a new concept in surround speaker placement with its Dolby surround sound Pro Logic IIz technology, namely that of 'Height' speakers for the fronts.

Height speakers are placed above at an elevation of 45o with respect to the main seat and at a ±45o angle relative to the center speaker with respect to the main seat.

This means that the 'Height' speakers reside outside the main front left and right speakers since the main fronts form an angle of 22o in movie playback and 30o in music listening relative to the center speaker with respect to the main seat.

Dolby Pro Logic IIz is not at present encoded in playback material except for a few games, but is generated through digital signal processing by identifying and decoding spatial cues that occur naturally in all standard multi-channel playback content - stereo, 5.1- and 7.1-channel material. The aim is to create ambient sound and certain amorphous effects such as rain or wind and directs them to the front height speakers. In theory, it should produce the vertical placement of those sounds - by creating a more vertical spacious soundfield. In practice however, it all depends on the playback material since not all movies will benefit from the generated sound information.

No DTS equivalent but...

There is no DTS equivalent to Dolby Pro Logic IIz but a new emerging technology that is very much similar in concept to this Dolby surround sound format is Audyssey DSX 7.1. The latter includes two extra front speakers which can be configured either as 'Height' speakers in a similar speaker setup to that adopted in a Pro Logic IIz Dolby surround sound configuration, or as 'Wide' speakers.

Wide speakers are placed on the same level as the main fronts but at an angle of ±60° relative to the front center speaker with respect to the main seat. Research shows that in general, the presence of the two Wide fronts leads to a more enveloping sound than the addition of the two Back Surround speakers found in a standard 7.1-channel speaker setup.

It is interesting to note here that Audyssey DSX 7.1 is just but a subset of the full speaker configuration supported by this new Audyssey surround sound technology. A full Audyssey DSX configuration supports up to 11.1 speaker channels, the normal 7.1 channels plus the extra four fronts - two Height speakers and two Wide speakers.

Editor's Note: For information on exact speaker placement requirements associated with both Dolby Pro Logic IIz and Audyssey DSX formats, please refer to our 'Home Theater Speaker Placement Guide'.

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