A Guide to Multi-Channel Dolby Sound

Dolby Surround Sound Expansion

Dolby Surround Sound matrix-based decoders include Dolby Pro Logic, Dolby Pro Logic II, and Dolby Pro Logic IIx. All three 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! 

Surround Sound Expansion

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, front left and right, front center, and two speakers for a mono surround, plus 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.

Further more, while a Dolby surround sound Pro Logic decoder can be driven by any stereo sound track, yet CDs and other stereo music content does not really sound like surround. 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 matrixed-decoder, Dolby Pro Logic II, in an attempt to achieve a cleaner sound that meshes better with any stereo content.

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.

But there are further differences from the original Pro Logic Dolby surround sound process. 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.

Dolby Pro Logic II also provides more control of the center image and front/rear balance. And it does all this with a 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 provide an expansive surround sound-stage even for ordinary stereo content.

It is found in the Apple^® iPod^®, in MP3 and CD playback, in VHS movies, in 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 videocassettes 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, it provides a more spacious feel than in the dialogue oriented movie mode. Further more, 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, Pro LogicII Dolby 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 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 LogicII.

The actual number of playback channels dependents on your speaker playback setup rather than the Pro Logic IIx technology itself. This technology is featured in iPods, MP3 and CD playback, DVD-Video, VHS movies, laser discs, 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 LogicIIx 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, while in Music mode, this surround 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.

More on Dolby Surround Sound Systems will follow soon.	Last updated on: 20th Dec. 2006
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