Audio Terms
Home Up Feedback Contents Search

Advanced Audio
THX Explained

 Home Theater Audio Terms

The sound, or audio, portion of any movie is a vital component. Today there are many technologies employed to bring the viewer (and listener) added enjoyment and greater realism.

Dolby Stereo

Developed in Dolby Labs this format provided high quality sound to movies.  This was is a highly successful format used with 35mm prints. Replacing the old method of placing the sound information between the sprocket holes (mono optical) Dolby Stereo had two optical tracks, one for sound information for right-left channel information and the other for center screen information and a brand new forth channel for ambient sound and effects! (Each optical track could encode two channels) A magnetic version was used in 70mm back in 1977 for Star Wars.

Dolby SR (Spectral Recording)

This was first introduced in 1986. Like other Dolby format noise reduction was fantastic providing crisp, clear sound.  The SR system provided a wider frequency response range enabling  louder sounds and lower distortion.  Unlike other formats this format required special equipment in the theater.

DTSDigital.gif (3187 bytes)

DTS Digital Sound   

Developed by Digital Theater Systems Inc, in California it made it's appearance June 11, 1993 with the release of Jurassic Park. Is is designed to provide the full range of sound reproduction and uses a digital sound track not on the film but rather on two separate CD-ROM players. There is am optical time code track that controls the CD-ROMs and keeps them in synch with the film. This format is growing in popularity among DVD owners.

With Home Theater DTS has taken on an additional meaning.It refers to an alternate six channel, discrete soundtrack for a DVD. The six speakers are laid out exactly in the same manner as Dolby 5.1; two front speakers, a center front speaker, two independent rear speakers and a sub woofer. There are some pretty important differences, DTS is not stored as compressed as Dolby 5.1. This means a better throughput, almost double of 5.1 but the cost is it takes up a lot more bandwidth (space) on the DVD disc. This limits the number of extras on a disc with DTS. The current trend seems to be discs with both DTS and Dolby 5.1 on the same disc, often with a second disc for the extras.

After reviewing several DTS DVDs head to head against the same Dolby 5.1 discs I have personally found that the DTS disc seems to provide a much better 'flow' of sound between the six speakers. In other words, the sound field appears almost seamless rather that originating from the six independent speakers.

Some more advanced details can be found here.

SDDS (Sony Dynamic Digital Sound)

Introduced in August 1994, SDDS is a sound-on-film method where the digital sound information is encoded on strips along the side of the film. This provides a large amount of digital storage and does not require the separate CD-ROM and synchronization codes. This is often found as an eight channel system in the theaters. Far right front, near right front, center front, near left front, far left front, subwoofer, left rear and right rear.

Dolby Prologic Virtual Speaker (Prologic 2)

With more and more people using computers and portable DVD players to enjoy DVD and other programming, Dolby Laboratories has come up with an advancement on their standard Prologic. The new system is called Prologic 2 (Prologic II). With this system there is an enhanced feeling of a true surround field utilizing as few as two speakers. There are two modes for Prologic 2 Virtual Speaker, Reference Mode where the distance between the front speakers sets the distance for the two virtual rear speakers. Then there is Wide Mode where the virtual rear speakers will appear to be farther apart creating a fuller sound stage in your listening area. For more technical details please refer to the Dolby white pages on the subject.

dd-logo_prologi.jpg (5407 bytes)

Prologic (Dolby Prologic)

This refers to a method of sound recording and reproduction developed by Dolby Labs to provide full surround sound. This is an active format that encodes the signals for the five or six speakers on two channels and decodes them during playback.  Unlike regular surround sound the active matrix of Prologic provides a wider feel to the surround sound field. Features include:

bulletHigh separation active matrix for enhanced directionality
bulletFour output channels: left, center, right and surround
bulletAutomatic input balance control
bulletTest noise sequencer for all modes
bulletSurround channel time delay adjustable from 15 to 30 milliseconds
bulletCenter Channel control (Normal, Phantom, Wide) (see below)

Basic Dolby Surround Sound

With this format, all speakers are feed information without processing. The features include:

bulletPassive matrix decoding to derive the surround channel
bulletThree output channel; processing left, right and surround
bulletSurround channel processing limited to 7kHz
bulletDolby B noise reduction
bulletSurround channel time delay fixed at 20 milliseconds
bulletManual input balance control
bulletLevel adjustment required to match surround channel to front
bulletMaster volume control for all output channels

Dolby 5.1 (AC-3)

This is a new version of Prologic with a totally new filtration system.  The former name was AC-3 but now it is more commonly known as Dolby 5.1. The sound to each of the six speakers is totally independent from the others. This provides a far greater degree of control and separation. Currently, Dolby 5.1 is being released in DVD format. This requires either an adapter or an amplifier capable of reproducing Dolby 5.1. One example that recently came to my attention is a helicopter scene in the movie Under Siege. You can actually hear the blades of the helicopter moving over you around the room! The difference lies in the fact that Dolby 5.1 has two rear channels rather than the mono surround sound channels employed by other surround sound formats. Right now there is a growing number of DVD discs that support Dolby 5.1, often along sixth regular two channel Dolby. This system that compresses digital audio through auditory masking over all channels and dynamically allocating bandwidth from a "common pool".

This format places six channels of sound optically on the film. In addition to the four tracks in the SR method Digital  separates the channels so that each of the four original channels is independent of the others. A fifth channel for rear surround and sixth for sub-bass was also added. This lead to some minor improvements in Dolby Surround. This is at the moment the most popular format for  home theater/DVD use.

Dolby Digital also decides how the bits are distributed among the various channels from a common bit pool. This technique allows channels with greater frequency content to demand more data than sparsely occupied channels, for example, or strong sounds in one channel to provide masking for noise in other channels.

Dolby Digital's sophisticated masking model and shared bit pool arrangement are key factors in its extraordinary spectrum efficiency. Furthermore, where other coding systems have to use considerable (and precious) data to carry instructions for their decoders, or to carry the same audio in separate channels for compatibility reasons, Dolby Digital can use proportionally more of the transmitted data to represent essential audio, which means inherently higher sound quality.

Please Note: DTS uses sub-band ADPCM compression while Dolby Digital is based on hybrid forward/backward-adaptive bit allocation.

Dolby EX 6.1

This is the latest sound format form Lucas Labs. The first movie with it was Star War: The Phantom Menace but many more are constantly being released in this format. It is similar to Dolby 5.1 but includes a new rear center channel. This creates a more realistic sound field and permits the sound to appear to zoom behind the listener. This requires seven discreet channels of sound to be encoded onto the film. One drawback can be seen from the theater owners viewpoint, it takes a rather major upgrade in the speaker system of the theater to present a Dolby EX film. Fortunately, in large cities, there are theaters that are willing to make the investment. The DTS camp has something very similar with DTS ES. This is also a seven channel format.

Along with this new sound, Lucas is starting to move into the arena of fully digital audio and video. Lucas Productions is working on a central site to distribute a film in digital format. Under this means of distribution, the movie theater connects to the distribution site and plays the movie from the central server. This means that each theater will have the same source material, no film will be used to be damaged and the owner of the movie will have greater control over distribution. Hopefully, this will one day be included to encompass home theater use. Lucas hope to have a totally digital release with the second or third installment of the new Star Wars trilogy. 

DTS ES

Unlike Dolby EX where the rear center channel is matrix or derived from the two rear channels, DTS ES can be either matrixed or discrete. This means that for DVD player and receivers that can process a separate, seventh channel you get better separation for the rear center channel. Whether the disc is encoded for discrete rear center or not a matrix coding is always provided so for those 'older' DTS systems the rear center channel is not lost.

THXbord.gif (1966 bytes)

THX

At the moment the best set up for any theater or home system. This system was developed by LucasFilms and is growing in popularity. Get a hold of any of the Star Wars movies (especially the Special Edition) to hear and see what this is about. THX is a fully digital format that presents crisp, clear sound. THX started out in 1980 when George Lucas hired Tominson Holman to investigate movie soundtracks and improved them.    This system requires the theaters to have THX equipment installed. This is a bit of an expense but considering what kind of profits Lucas films make, over 1,000 theaters are now THX enabled. The first movie released in this format was Return of the Jedi in 1983. Since then, many have joined the ranks and many more have been redone in this fully digital format.  Most home systems that can handle Prologic can reproduce THX in an excellent manner. As far as home theater systems go,

THX refers to certification a theater, tape or DVD receives from THX as to the quality of the reproduction and that the media's reproduction meets the high THX standards. A movie such as Twister is record on DVD in Dolby 5.1 (AC-3) but is also THX certified.  Named after George Lucas' first movie ( and senior project in film school) THX-1138 but also named for the co-creator Tominson Holman eXperiment.

For more information on THX go to the LINKS page and follow the link to the THX site.

 

Thanks to everyone visiting this site.

Send email to doug@hometheaterinfo.com with questions or comments about this web site.
Copyright 1999-2017 Home Theater Info