Article is courtesy DTVMAX.Com  
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Virtually all of the recent and planned video delivery media, including DVD, DTV, DirecTV (potentially), and Laserdisc, support Dolby Digital audio. Dolby Digital (formerly called AC-3) compresses 5 discrete full bandwidth audio channels and 1 low frequency (e.g., subwoofer) channel down to a relatively low bit rate data channel (e.g., 384 Kbps).   In order to use Dolby Digital, consumers will need an Dolby Digital decoder, either built into a receiver or as a separate surround sound processor, along with six channels of amplification and suitable speakers.  Building upon Dolby Digital baseline THX was established by Lucasfilms primarily as a series of specifications, testing procedures and processes to ensure the quality and consistency of movie soundtrack recording and playback.  THX was first applied to commercial movie theater installations and equipment.  Shortly thereafter a consumer THX certification program was initiated.  Consumer THX certification initally focused only on speakers, receivers, amplifiers, digital surround sound decoders.  The THX certification program was later expanded to include audio cables software (i.e., initially Laserdiscs) and most recently DVD players.  In order for a product to receive a THX certification it must conform to the specifications established by Lucasfilms and successfully pass testing by Lucasfilms to verify the compliance of the product to the THX specification.  A full THX installation dictates use of THX certified speakers, amplifiers, surround sound decoding, cables and speaker placement.  While purchasing a product that has received a THX certification will provide a degree of assurance that it is a quality product, this not not mean a product with THX certification is inferior.  Lucasfilms also trains and THX certifies home theater dealers/ installers.   Recently a less demanding specification was allowed for products certified as 'THX Select'.  In receivers this allows for lower power output levels as would be appropriate for home theaters in smaller rooms.  Products to conform to the more demanding original standards are now receive a 'THX Ultra" certification.
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Prices for budget receivers with built-in Dolby Digital decoders and the required multi-channel amplification have now come down to the mainstream consumer level for home theaters with street prices as low as $300. Alternatively, some of the DVD players have the Dolby Digital decoder built into the player. In any case you will need a receiver, preamplifier or audio processor capable selecting inputs and controlling the volume of all 6 channels of sound. My advise it go ahead and get a new Dolby Digital enabled receiver, preamp or outboard processor and don't bother with finding a DVD player with a built-in decoder. Ideally the front left, center, front right, rear right and rear left speakers should be identical full range units of a conventional direct radiating design (rather than bi-polar as recommended for Dolby Pro-Logic/THX configurations). Also a dedicated subwoofer is recommended for the low frequency effects channel. In practice most will find it acceptable to forego having five large full range speakers and will use center and rear speakers with limited bass extension. The AC-3 decoder lets you shift the low frequency information over to the low frequency effects channel (i.e., subwoofer) for any channel where your speaker does not have extended low frequency response. See the notes on setting up a Dolby Digital AC-3 installation under the Do-It-Yourself Projects topic.

A competing 5.1 channel coding system is produced by DTS Technology. DTS has been very successful in marketing the commercial version of their system to movie theaters. The commercial theatrical version of DTS stores the 5.1 channels of audio on conventional CD ROMs that are synchronized to a control signal that is included on the movie film. The DTS installation for a typical theater cost perhaps half the price of a Dolby Digital installation. DTS 5.1 channel audio is now available on a few laserdiscs (LDs) and DVDs. The normal digital audio channels on the LDs are replaced with the DTS digital bit stream. DTS is included along with Dolby Digital on a few DVDs.  DTS compared to Dolby Digital, for both the theatrical version and the LD versions, has the advantage of using substantially higher data rate. For example, Dolby Digital typically uses a 384 Kbps rate while DTS uses 1.5 Mbps, or about a 4 times greater data rate. In this case DTS compresses the audio less and the result is superior sound. Dolby Digital is in fact quite good while DTS has the potential be just a little better. However, if DTS were to be used at the lower data rates used for Dolby Digital, its sonic performance would likely drop to that equivalent to Dolby Digital. DVD and DTV standards call for Dolby Digital operating at a data rate that can go up to a little higher than used on LDs. DVD does allow for alternative encoding schemes and there are some DVD now available that include both Dolby Digital and DTS audio. A few of the very latest generation of DVD players support DTS, as well as Dolby Digital audio. Consumer DTS decoders, or combined Dolby Digital and DTS decoders are now available from several manufacturers (check out the DTS Technology web site for more information).    Many manufactures of audio/video receivers are now offering decoding for both Dolby Digital and DTS in their products starting as low as $300.  However, the recently introduced audio-only DVDs, in the officially agreed to long-term format, have enough storage capacity to provide a truly lossless compression of 5.1 channels of audio thus providing the best sonic performance possible (i.e., even better than DTS encoded audio). The first DVD players supporting the audio-only DVD discs starting appearing in late 1999. 

The theatrical release of "Star Wars Episode 1 - Phantom Menace" introduced a new feature to Dolby Digital and THX.  This was the addition of center rear channel and this enhanced 6.1 channel version carries the Dolby Digital Surround EX or THX Surround EX label.  The rear center channel is not actually encoded as a discrete channel, but rather is derived using matrix decoding from the discrete right and left rear channels.   This is the exact same scheme used by Dolby Pro-Logic to derive the center front channel from the discrete right and and left front channels.  Several manufactures of Dolby Digital and THX processors and Dolby Digital enabled receivers plan to introduce support for the new 'EX' feature with 6.1 channels during 2000.   Expect to see this feature first appear on the mid-to-high end products first.  One of the first such receivers expected to reach dealer's shelves is the Yamaha RX-V1.  This $3199 receiver supports 6.1 channels for Dolby Digital Surround ES as well as the competing DTS-ES system.  Denon is offering their new AVR-4800, a $2000 full featured 125 watt per channel THX Ultra certified receiver that includes THX Surround EX.  For home theater owners that have already invested in a quality Dolby Digital 5.1 channel receiver or processor a upgrade path is provided an add-on Dolby Digital Surround EX adapter box.  Two of the first such adapters are the Model CS-3X available from Smart Devices for $299 and the Model ADA 6.1 processor from ADA for $499.  When upgrading your system from 5.1 to 6.1 channels you will need to add a center rear speaker and, except when using a new EX receiver, another amplifier channel.  Movies that supported the EX feature in theaters should also allow for the correct rear center channel decoding on the DVD version or when broadcast via DTV.  The first EX movies to appear on DVD were Austin Power:  The Spy Who Shagged Me and The Haunting.  Some standard Dolby Digital 5.1 channel titles will also benefit from the additional center rear channel output provided by the 'EX' provisions, but this will be hit and miss.

The electronics for audio systems supporting 5.1 (or 6.1 in the future) channel surround sound can be grouped into four general categories for consideration when building a home theater system.  

Budget Receivers:  The heart of a budget home theater audio system is a receiver that support Dolby Digital with at an amplifier output power of at  least 60 watts per channel.  Budget audio receivers generally retail for $300 to $600.  Many of these receivers will also include support for DTS.  Generally in this price range the brands that offer relative high amplifier output (e.g., 100 watts per channel) provide a less refined sound quality that the better quality brands.  To draw an analog in the price range you can have brains or brawn but not both.  The receivers with the more refined sound start appearing at about the $500 price level with power outputs of 60 to 75 watts per channel.  The Onkyo TX-DS575 is probably the hottest budget receiver for home theater systems.  This Onkyo unit has received very favorable reviews from several publications and has both the features and excellent sound quality the are unexpected in a $500 receiver

High-End Receivers:  A high-end receiver will retail for $1500 to $3200.  This class of receiver will generally offer high power output (e.g., 120 to 150 watts per channel), with a comprehensive array of audio and video inputs and outputs.  Such receivers should be of a very high mechanical quality and offer a sound qualify that rivals that of more expensive component audio systems.  In this price range many of the receivers are certified against THX Ultra (the most demanding THX standards).  Among the high end receivers the top tier 1999 models from Denon and Marantz are certainly among the best.  New for 2000 are a few top-of-the-line models that include 6.1 channel Dolby Digital/THX surround.  Show below is the the Marantz top of the line Model SR-18 5.1 channel receiver. 

The ability to recognize the a good sounding receiver from a mediocre is not something everyone is born with.  Rather it may require some education on how to be a critical listener.  If you don't trust your own ability to weed out the good sounding units from mediocre ones then I would suggest you turn to the opinions of the reviewers of some of the more respected publications, such as the home theater oriented publications Stereophile Guide to Home Theater, Home Theater Magazine, Widescreen Review, and the audiophile oriented publications such as Stereophile and Absolute Sound.  While sometimes the experts that review for these publications may not agree with each other on certain points, a favorable review by more than a single publication should be considered a very positive indication of a specific product's merits.

Given all of the variables, only some of which is briefly discussed above, it is obvious that within each of the above price ranges it is not possible to select one receiver or one set of audio components that can be declared as be the 'best'.  Rather it is possible to identify some manufactures that are well respected for producing products that are generally considered among  the best 'sounding' within their price range.  In particular the receivers from the following manufacturers deserve you consideration as they tend to generally be well above average in each price range:  Denon, Harman Kardon, Marantz, and Onkyo.

Yamaha receivers tend to be well above average sounding but frequently just below the very best sounding competition with a given price range.  However a few Yamaha models do compete for best sounding within their price range.  Some additional manufactures offer certain models that are well above average, but also produce several or even many models that fall in the sea of mediocrity.  This category of receiver manufacturers include:  Kenwood, Pioneer, and Sony.  These, and some other mainstream consumer electronic manufacturers frequently use an extra product nomenclature to distinguish their higher quality products from their mass market run-of-the mill products.  For example Pioneer has their 'Elite' product line, Sony has their 'ES' product line and the Kenwood THX certified models tend to be well above average products, but usually not the very best sounding receivers for the money.

While the above recommendations are not claimed to be comprehensive, they should provide you with a starting point to begin you investigation into finding a quality receiver tailored to your home theater requirements.  

For Additional Information:  The Audio Review Web site provides a good overview of what is important to look for receivers for home theaters and also recommends a number of specific makes and models.  Also the Onkyo web site has an informative page focused a receiver's amplifier rating

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