Digital Audio

Last Updated August 2007

Home theater systems depend heavily on have a quality surround sound system to create the theater experience.  There are two companies the provide the surround sound technologies used for both commercial movie theaters as well as for home theaters.  These companies are Dolby Laboratories Inc. and Digital Theater Systems (DTS).  This page starts off with a background on Dolby and DTS technologies as used for home theaters.

Dolby Digital and Surround Sound

Virtually all of the standard definition and high definition digital video delivery media, including DVD, DTV, Blu-ray Disc (BD), HD-DVD, DirecTV, and Dish Network support Dolby Digital audio. For the direct broadcast satellite systems only certain channels are offered with Dolby Digital surround sound audio while the others are typically offered with conventional 2-channel stereo audio.  Dolby Digital (formerly called AC-3) compresses up to 5 discrete full bandwidth audio channels and 1 low frequency effects (LFE) channel (referred to as the 0.1 channel) down to a relatively low bit rate data channel (e.g., 384 Kbps or higher).   Some of movies include a sixth channel with the addition of rear surround channel that is encoded using a matrix process (i.e., not a discrete channel.  This center rear effects channel is decoded using essentially the same technique as used with Dolby ProLogic to derive the center channel from information included in right and left channels.  This 6.1 channel configuration is referred to as THX EX or Dolby Digital EX or Dolby Digital 6.1.  In order to use Dolby Digital, consumers will need an Dolby Digital decoder, either built into an A/V receiver or as a separate surround sound processor along with 5 or more channels of amplification and suitable speakers.  Building upon the Dolby Digital baseline, THX was established by Lucasfilms in the early 1980's (but is no longer owned by Lucasfilms) primarily as a series of specifications, testing procedures and processes to ensure the qualify 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 initially 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 then DVDs) and most recently DVD players.  In order for a product to receive a THX certification it must conform to the specifications established by THX and successfully pass testing by THX 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 does not mean a product without THX certification is inferior. THX also trains and certifies home theater dealers/ installers.   Subsequenytly recently THX established 3 levels of THX certification for Audio/Video (A/V) receivers.  Products that conform to the original THX standards are now receive a 'THX Ultra" certification.  A less demanding specification is applied for products certified as 'THX Select' indicating products that would be appropriate for home theaters in smaller rooms (e.g., allowing lower power outputs).  Finally a high-end products are now being offered with a THX Ultra2 certification indicating conformance to the THX Ultra specification and includes features intended to best accommodate the reproduction of all music and movie soundtrack audio formats..     

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, Dolby Digital 6.1 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 same scheme used by Dolby Pro-Logic to derive the center front channel from the discrete right and and left front channels.  Most manufactures of Dolby Digital and THX processors and Dolby Digital enabled A/V receivers now support for the new 'EX' feature with 6.1 channels in all of their mid-range and higher units.   Movies that supported the Dolby Digital EX feature in theaters allow for the correct rear center channel decoding on the DVD version or when broadcast via digital TV (DTV or HDTV) or via satellite (e.g., DirecTV) when Dolby Digital audio is offered.

High Definition Dolby Surround - With the introduction of high definition optical discs in 2006 (i.e., Blu-ray Disc and HD-DVD) two additional Dolby surround sound technologies were introduced.  These new Dolby surround formats provide higher audio quality and provision for additional discrete audio channels as compared to conventional Dolby Digital.  These new, higher fidelity, Dolby surround formats are Dolby Digital Plus and Dolby Digital TrueHD, with TrueHD being the highest fidelity Dolby format as it provides lossless compression with the decoded output is bit-for-bit identical to the original master recording.  While both Blu-ray Disc (BD) and HD-DVD standards require conventional Dolby Digital soundtracks be included on their format's disc, both of the improved Dolby audio formats are optional.

DTS and Surround Sound

A competing 5.1/6.1 channel coding system is produced by Digital Theater Systems (DTS). 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 (or 6.1 channels with DTS ES) 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 is now available on a increasing number of DVDs.  DVDs with DTS also include a Dolby Digital soundtrack.  When comparing DTS compared to Dolby Digital for theatrical use, DTS has the advantage of using substantially higher data rate. For example, conventional Dolby Digital typically uses a 384 Kbps rate while DTS uses up to 1.5 Mbps, thus allowing substantially less data compression with the result being superior sound quality.  However, for the audio on DVD's conventional Dolby Digital generally is encoded at a somewhat higher data rate than for theatrical use and DTS is frequently encoded at a  lower data rate than for the theatrical version.  As a result DTS audio tracks on DVDs will generally be only slightly superior, to the Dolby Digital audio track.  Virtually all of the current generation of DVD players support the output of a digital audio data stream in DTS, as well as Dolby Digital formats.  Likewise virtually all of the current generation of A/V receivers support decoding both both Dolby Digital and DTS when connected to a suitable source device (e.g., DVD player) using a digital audio connection.  DTS is an optional audio format for use with DVDs with Dolby Digital is required.

High Definition DTS Surround -  As with Dolby, DTS in 2006 introduced new higher quality surround audio formats for use with high definition optical discs (i.e., Blu-ray Disc and HD-DVD).  DTS has developed these two new formats under the overall name DTS-HD.  These are "DTS-HD High Resolution Audio" and "DTS-HD Master Audio" (with DTS-HD MA being the highest fidelity DTS format as it provides lossless compression). 

Surround Sound Speakers Layout

The typical arrangement for the main and surround speakers in a 5.1 or a 6.1 channel home theater sound system is shown below.  The rear effects surround channel is only used with 6.1 channels setups such as those supporting THX EX, Dolby Digital EX and DTS ES or the new advanced Dolby and DTS high definition formats.  Also some home theater configurations will use two rear effects surround speakers instead of the single rear speaker depicted in the figure below.  With the additional rear surround speaker this arrangement is referred to as a 7.1 channel system.  Note that conventional Dolby Digital and DTS formats (such as found on DVDs) include only a signal channel of rear information. However, the new high definition Dolby and DTS audio formats offered on some Blu-ray Disc and HD-DVD titles do include 7.1 discrete audio channels.  Some surround sound processors advertised as 7.1 channel, such as built into may mid-to-higher-end A/V receivers, offer surround modes where signal processing is used to artificially provide some separation between two rear effects channels when the source only includes 5.1 or 6.1 discrete channels.   Use of  conventional direct radiating speakers, rather than those with a bi-polar radiation pattern, as recommended for Dolby Pro-Logic/THX configurations, are preferred by many home theater builders at least for the 3 front speakers. Also a dedicated subwoofer is strongly recommended to handle the low frequency effects channel. In practice most home theater owners will find it acceptable to forego having five or more large full range speakers and will use small speakers with limited bass extension in combination with a subwoofer. The Dolby Digital and DTS decoders (e.g., built into your AVR) will let 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. Also note there is no standard location the placement of the subwoofer (LFE channel).  In some higher-end home theater installations two subwoofers are used to reduce 'standing waves' in order to provide more uniform bass throughout the home theater room.  Placement of the subwoofer near the corners of the room will reinforce the bass output.  Corner placement of the subwoofer may be appropriate in certain rooms in combination with certain subwoofers.  However, corner placement can lead to 'boomy' bass that is undesirable.  Finding the optimum placement for the subwoofer frequently calls for experimentation with different locations within the specific room.

Audio/Video Receivers (AVRs)

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 with street prices starting under $200.  Ideally the front left, center, front right, side/rear right and left speakers [and center rear effects speaker(s) if building a 6.1 channel system] should be identical full range units.  

Virtually all recent AVRs include the decoding for conventional DTS and Dolby Digital formats as well as support for extracting surround sound information from conventional stereo audio sources using Dolby Pro Logic.  Most currently AVRs also have support for Dolby Pro Logic II decoding.  This is an improved version of the more than two decade old Dolby Pro Logic system for decoding of stereo audio sources that have the surround information for the center front and the rear surround channels matrix encoded (mixed into) the standard two stereo audio channels. Pro Logic II offers at noticeable improvement in channel separation for the matrix encoded surround sound information.  Many AVRs also support a competing DTS Neo decoding scheme for processing of stereo sources to extract surround sound information.

AVR receivers typically offer video and audio inputs and provide switching to a video output that can be connected to you TV.  For high definition support this started out a providing two or more component video inputs and one component video output.  More recently the mid-to-higher-end AVRs have added HDMI digital inputs and one or more HDMI output.  Just about all HDTV sources (e.g., cable TV boxes, satellite receivers, Blu-ray Disc and HD-DVD) now have HDMI outputs.  

A updated version of HDMI is being introduced in 2007 that proves support for the latest high definition audio formats as well as for possible future enhancements to the HD video sources.  Most of the mainstream manufacturers of Audio/Video Receivers (AVRs) are bringing out new models in 2007 that feature HDMI version 1.3a (digital video and audio) inputs and decoding for the advanced audio codecs that are available as options on Blu-ray Disc (BD) and HD-DVD discs.  Earlier this year Onkyo became the first manufacturer to begin shipping AVRs with HDMI ver. 1.3a inputs and support for the new Dolby and DTS audio codecs, while several of the other manufacturers are just now getting their new models into production.  The table below is a summary of currently available and announced AVRs that include HDMI ver. 1.3a inputs and advanced audio decoding (i.e., all models listed are expected to support the advanced Dolby True-HD and DTS-HD audio formats).

*       Amplifier output power per channel is usually rated with only 2 of the 7 channels driven, but the higher quality, and higher priced AVRs may be able to provide the rated power with all 7 channels driven (see the manufacturer's detailed specifications for more information on how their power ratings are obtained).

**     The Manufacturer's Suggested List Price is given for most models, however where the manufacturer has not provided a MSRP an estimated retail price is listed.

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.  

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