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Surround Formats

Surround sound didn’t happen overnight. It grew out of stereo and evolved into today’s state of the art discrete digital multichannel formats. Before building your dream system, you’ll need to know the differences.

Dolby Pro-Logic

Squeezing all those extra channels onto a piece of film or recording tape isn’t easy. In fact, before the advent of digital sound, it was impossible. To accommodate surround information, Dolby Laboratories developed Pro-Logic, which piggybacked the center and rear channels onto the analog stereo tracks. The Dolby Pro-Logic surround system is found on nearly all movies made in the past twenty years, but it suffers two major limitations. First, the rear channel is monaural: whether you use one, two or twenty speakers at the back of your theater, they all receive exactly the same signal. What’s more, that signal has extremely limited frequency range, with no high treble or deep bass. Second, there is little separation between any two adjacent channels, which significantly compromises the surround effect. For watching videotapes (which cannot accommodate digital multi-channel formats), Pro-Logic is all you need. If, however, you’d like to hear your DVDs, Laserdiscs or Mini-Dish satellite broadcasts at their very best, you’ll also want digital surround sound capability that is described in the next section.

Dolby Digital

The most popular of the digital surround formats, Dolby Digital (a.k.a. AC-3) features five full frequency range channels-left, center and right front, plus left and right rear-and a sixth channel for Low Frequency Effects (bass). Digital surround sound is often referred to as 5.1; 5 main channels plus the Low Frequency Effects (LFE) channel. The advantages of digital 5.1 surround are more exciting surround effects and imaging with life-like impact and dynamic range. The difference in sound quality between Dolby Digital surround and Pro Logic surround is similar to the difference between CD and LP records.

Digital surround decoding systems such as Dolby Digital require digital sources such as Laser Disc, DVD disc or the latest generation of Digital Satellite Systems (DSS). Most DVDs contain a 5.1 channel Dolby Digital sound track and all DVD players can play Dolby Digital discs.

DTS: Digital Theater Systems

DTS (an acronym for Digital Theater Systems) is an emerging digital 5.1 system that is competitive to Dolby Digital. In order to play a DTS disc you must have a DTS surround processor and a Laser Disc or DVD player with a digital output. Not all DVD players can play a DTS disc. If you are interested in playing DTS discs make sure that you get a DVD player that can pass a DTS digital signal.

THX–It’s Not a Format

THX is a set of equipment specifications, compatible with all surround formats, intended to standardize the performance of any theater system. It does not compete in any way with Dolby Digital, Pro-Logic or any other surround format. Software (movies) certified by THX may be played on any Dolby surround system and conversely, THX hardware can play all Dolby surround movie software whether or not it is THX certified.

There are actually three parts to the THX standard: one covers the mastering of home videos, specifically laserdiscs and DVDs. All THX certified software will work with any player, in any system. Part two is known as the Theater Certification Process, wherein a commercial movie theater can achieve THX status by complying with a series of hardware and setup guidelines. Part three deals with the gear consumers purchase for their home theaters, and is known as Home THX.

Home THX components are not necessarily better or worse than other products: they have simply earned THX approval by conforming to all THX mandated specifications. Once the product has been submitted to Lucasfilm, LTD (the company that developed and licenses the THX standard) and passed a battery of tests, it can wear a THX Approved badge. In order to be a true THX system, all the audio components in that system--processor, amplifier, speakers and subwoofer--should be THX approved. But that does not mean that THX and non THX components cannot be used together, they can.

So, should you buy THX approved gear? That depends. For inexpensive, entry level gear, THX approval guarantees a minimum standard of quality: a THX-approved receiver or amplifier will drive a variety of loudspeakers and, conversely, THX approved speakers will work with most amps. So will most non THX components! It would be foolish to ignore a product simply because it has or hasn’t been THX certified. 

Digital Signal Processing (DSP)

Many surround receivers have DSP circuits that allow your surround system to mimic the acoustics of various locales such as concert hall, church, jazz club, etc. The sound quality of these surround modes ranges from pretty good to simply awful. If this type of surround processing is important to you, listen before you buy. If not, ignore it. All units permit the DSP to be turned off.

This article was last modified Apr 17, 2014

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