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Posted 30 March 2011 | News & Blog   

The first widely used forms of surround sound were released in the 1980s during the boom of home video tapes on VHS.

Introduced by Dolby Labs, Dolby Surround was an acoustic algorithm that added an extra speaker behind the audience. All that was required was a stereo audio signal which most VHS tapes could produce when played through a stereo VCR and a receiver with Dolby Surround. The signal processor takes certain sounds from the two-channel audio signal and replays them with a delay through the rear channel. This is not a discreet channel, but simply a matrix of the stereo channels. The matrix technique would be used again anytime multi-channel audio is produced from a stereo source.

Dolby Pro-Logic was a slight improvement on Dolby Surround. Dolby Pro-Logic added a center channel designed to play the mid ranges that would playback the voices in dialogue sequences of films. The rear channel was extended to include a second speaker, but no significant additions to the content were added. Pro-Logic was still a matrix of the stereo signal.

Dolby Digital 5.1, originally known as AC-3, was the first discreet fully digital surround process. Things get really interesting with Dolby Digital and its ability to work six separate channels of discreet sound. Since each channel or speaker is discreet rather than comprised of a matrix of the other channels, each channel can play back an independent sound. The addition of a subwoofer or Low Frequency Effects channel makes the subwoofer an important player in any soundtrack. The LFE channel is indicated by the .1, meaning that it plays back one tenth of the audio information of the other full range channels.

DTS is a company that began creating an alternative to Dolby Labs digital effect processing since 1993. DTS soundtracks now appear on popular DVDs everywhere. They have been renowned for creating some of the finest soundtracks ever played back in home theater systems to date. Notable soundtracks done by DTS include Gladiator, Saving Private Ryan and Moulin Rouge. With a higher sampling rate than Dolby Digital and slightly more emphasis on effects, DTS has made themselves the audiophile choice for surround film enthusiasts.

Extended Formats: The extended surround formats include Dolby Digital EX and DTS ES; seven channel surround sound methods. They’re really not much different from the 5.1 versions, but these add additional rear channel effects from back speakers that are to be placed behind the audience pointed at the audience’s back rather than to the sides. These extra channels of sound are not discreet however and only a matrix of the surround channel information. This is why the extended formats are not considered true 7.1 or 8.1 surround methods; these are still only 5.1 digital audio formats. The only advantage to using the extended formats might be if you’re using a very large room and need to fill in the surround channels with more kick.

Legend has it that the first extended surround format was thought up by George Lucas and co-developed by THX and Dolby Digital to produce Dolby Digital EX or THX-EX, which are interchangeable terms for the same format. Lucas was dissatisfied with the fullness of the surround channels and wanted more sound behind him. Most people probably aren’t listening to movies in a Home Theater system the size of Lucas’s, but if you think you might be, then by all means the extended formats probably have something tangible to offer.

Dolby Pro-Logic II and DTS Neo6: Two channel audio is alive and well in the digital music realm. As long as there are two channel sources feeding stereo signals into multi-channel receivers, there will be encoding formats designed to take advantage of all that extra hardware with which they’re endowed. Dolby PL2 and Neo6 are just Dolby Labs and DTS’s means of addressing this concern by offering processing to your two channel material that allows them to recruit the rear channels. They’re strictly matrix methods like Dolby Pro-Logic, but a bit more flexible. They’ll add a bit of dimension by taking from certain elements of the two channel information and playing it back in the rear channels.

These stereo audio signals were recorded and designed to play back in one of two channels, left or right. No processing was necessary at the pre-amplifier stage as separation was assigned during the recording. Today you have discreet multi-channel encoding so that processes at the playback are required to decode exactly what channel which sounds should come from, up to as many as eight.

It’s infinitely more complex for Home Theater surround applications, but for music good old fashioned stereo is still king. An overwhelming majority of music even when recoded digitally is recorded in two channel stereo. Granted there are processes designed to liven up a stereo recording, but these effects are largely superfluous and don’t add anything necessary to the sound quality.

The newest of these effects is Dolby Pro-Logic II which has become a standard on Home Theater receivers. Dolby PLII allows the listener to tailor the sound to add a bit of depth. Dolby PLII recruits the rear speakers to play back certain frequencies in a time delay with the rest of the speakers to add the illusion of an acoustic field of depth. Digital signal processors (DSP) settings are meant to mimic equalizer presets- acoustic settings on many receivers that allow you to set your sound to “Church” or “Hall.” Usually DSP settings add a tremendous amount of echo.

Audio purists shun these effects as cheesy, robbing the recoding of fidelity. They would tell you that you should try to play back the recording in the way that it was meant to be heard. But most people find effects like Dolby PLII or DSP pre-sets a fun toy used for amusement while playing with buttons. At best they might compensate for some environmental shortcoming in your audio room.

Popular Home Theater processes include:

Dolby Digital
Dolby Digital EX
Dolby Pro-Logic
Dolby Pro-Logic II
Dolby Surround
High end surround processors like Sunfire’s Theater Grand units look like full featured receivers, but they have no built in amplification. These are designed for true audiophiles who want to provide amplification and processing only from dedicated boxes engineered to perform these functions to the best of their ability. By using dedicated boxes, manufacturers of signal processors are free to build however they choose. The constraints of saving room for amplifications and all the errant side effects that might entail are not constraints.

Most Home Theater receivers that do it all in one box offer the flexibility of pre-ins and main-outs. This means the unit can be used as a dedicated processor without using its amps at all or conversely, it can be used as a multi-channel amp without using any of its signal processing capabilities. Using pre-ins/main-outs on a receiver might be a way to upgrade a system to a separates, perhaps by buying a dedicated signal processor or amp one at a time and using the receiver to fill in space.

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