The Evolution of Surround Sound: Dolby vs DTS

Dolby vs DTS

Believe it or not, there was a time when surround sound was a novelty. Here's a look back at its origin, advances and latest hi-def formats.

Apr. 08, 2008 — by .(JavaScript must be enabled to view this email address)

Movie-going is a way of life for all of us. Except for television, it’s probably the biggest spectator sport in the world. However, even sporting events cannot draw the crowds that movies like “Star Wars,” “Indiana Jones” or the “Terminator” franchise can generate. It’s an experience to be savored and enjoyed in a darkened cinema. Home theater is an extension of the movie-going experience in that it attempts to recreate the same sense of awe and wonder within the confines of your living/family room. While a picture is worth a thousand worlds, sound is more than fifty percent of the equation. The goal of films today is to aurally engulf you and place you right in the middle of the action so that you viscerally feel it as well as see it. Multi-channel sound is now a way of life for us.

Surround sound comes in many varieties and permeations. Surround processes aural information to give you added depth and sense of realism not heard from 2-channel stereo. Surround sound makes the sound so real that it’s in front of you, behind you, and all around you. Surround for the home began in the early 1990s as an outgrowth of the sound heard at the movies. While Dolby Stereo was the first commercial use of multi-channel sound in theaters some thirty years ago (1977) with “Star Wars,” digital surround sound was not introduced to movie-goers until 1993 by Steven Spielberg (and MCA/Universal) in his colossal hit “Jurassic Park.” This was also the first commercial use of DTS (Digital Theater Systems). Today, DTS is now employed on the soundtracks of several thousand feature films, including many recent blockbusters.

How Does Surround Sound Work?
Basically, it takes two-channel stereo sound and splits it into dialogue and primary sound up front and effects sound to the rear. It’s certainly a vast improvement over plain vanilla two-channel stereo. The Pro Logic processor extracts four channels from the two stereo encoded channels and steers or directs them to the appropriate speakers, e.g. dialogue to the center channel and effects to the rear. Under this scheme, the rear surround channel signal is divided over two speakers, which gives it more coverage. It is, however, a mono signal. The rear channel information is derived by the simple formula of L[eft] minus R[right] with added reverb to give it a more natural like sound.

Digital 5.1 Surround
Multi-channel surround sound with 6 independent channels of sound - Digital 5.1 Surround – arrived in 1995. As the name implies, 5.1 is 5 full-frequency, discrete and independent audio channels (front left, center, front right, right surround, and left surround) plus a separate .1 channel, which is a dedicated Low Frequency Effects (designated LFE) channel. LFE directs bass information to your subwoofer. Since it is not a full bandwidth channel (of 20 - 20,000 Hz), the dedicated subwoofer only gets a .1 number. 

One of the first things that you hear with digital 5.1 surround is that the side or rear surround speakers are now in stereo (versus mono with Pro Logic). Sound, music, dialogue and effects are now directed to their proper placement in terms of screen location. As the old saying goes, “you are there!” now takes on a whole new meaning in 5.1. Digital 5.1 surround is among the most realistic surround currently available. It will be delivered via 5 psycho-acoustically-matched speakers and a dedicated subwoofer.

There are only two schemes available that will provide you with true digital 5.1 surround for the home—Dolby Digital and DTS. Each scheme strives to provide the listener with the ultimate home theater experience. The major difference between Dolby Digital and DTS is that DTS is fully discrete. What this means is that specific aural information is directed toward a specific speaker giving the listening experience a more realistic tone. While Dolby Digital was devised as an evolutionary approach, DTS is more revolutionary one as it was specifically designed for multi-channel sound.

Not all material is encoded in Dolby Pro Logic, Dolby Digital, or DTS, however. Therefore, most equipment manufacturers of audio receivers and processors also offer other artificial surround modes such as Hall, Matrix, Simulated, etc. These modes can add realism to material not found elsewhere. As well, they can be employed in the playback of CDs too, sometimes giving them added depth and the feeling that the music is surrounding you. It may not be an unpleasant feeling.

Dolby Digital
Dolby Digital is a multichannel perceptual coding scheme. Initially, Dolby’s new surround system was called AC-3 (for audio coder 3). It was introduced in movie theaters in June 1992 as Dolby Stereo Digital (or Dolby SR). While Dolby Surround is a single-band-limited surround channel with a range of 100 Hz to 7,000 Hz, Dolby Digital, on the other hand, offers a full dynamic range on all five main channels of 3 Hz to 20,000 Hz. Dolby Digital has been available since late 1995, and there are now thousands of DVD titles presently encoded in Dolby Digital. Many films released today are also in Dolby Digital Surround. All DVD players now include both digital audio outputs – either/or coaxial or optical and 6-channel output to pipe the Dolby Digital or DTS audio stream.  Upconverting DVD and Blu-ray players also include HDMI.

Dolby Digital’s perceptual coding seeks to eliminate the data we cannot hear, while maintaining all the information that we can hear. Its purpose is to get more information into the available spectrum. Perceptual coding has been designed to decode multichannel digital audio. It divides the audio spectrum of each channel into narrow frequency bands that correlate closely to the frequency selectivity of human hearing allowing coding noise to be very sharply filtered taking advantage of the psycho-acoustic phenomenon known as auditory masking. Coding noise stays close in frequency to the audio signal being coded. This effectively masks the noise. AC-3 uses a “shared bit-pool” arrangement plus human auditory masking to make use of transmitted data as efficiently as possible. Bits are distributed to meet the needs of the frequency spectrum. By using a model of the audio-masking scheme, bits are distributed among the various channels according to need. Basically AC-3 allows proportionally more of the transmitted data to represent audio, which, according to Dolby Labs, yields higher sound quality. In turn, it allows multichannel surround sound to be encoded at a lower bit rate than required by just one channel on a CD. Today, all receiver brands now include a Dolby Digital decoder built into every A/V Receiver. There are 28 versions of Dolby surround. 

For HD on television, and all movies on disc now include a Dolby Digital soundtrack as do most TV shows like C.S.I.: Crime Scene Investigation, C.S.I. – Miami, LOST, ER, Desperate Housewives, Gray’s Anatomy, Battlestar Galactica, or The Tudors.

DTS Coherent Acoustics Coding (CAC) maps discrete 6-channel, 24-bit encoded data onto the 16-bit PCM digital audio stream, which is found on either a DVD or compact disc. The DTS CAC signal is passed via the digital output (either coaxial or optical) present on many DVD and CD players. Any optical medium playback device with a digital output can pass along the DTS signal to a DTS processor and/or A/V Receiver amplifier. Virtually all DVD players will playback both Dolby Digital and DTS. In turn, depending on the encoding, DVD discs have the capacity to playback both an AC-3 (Dolby Digital) and DTS soundtrack.

While Dolby Digital uses a perceptual coding scheme to reduce the bit rate, DTS uses compression technology and the CAC algorithm to do pretty much same thing. The compression ratio is 3.75:1 of a 24-bit PCM digital audio stream with an 8 times oversampling rate. It has a typical data rate of 1,411 kb/s (much higher than Dolby Digital). It performs transparently by coding 24-bit data at a bit-rate lower than 16-bit linear PCM. The Coherent Acoustic Coding algorithm is a scaleable digital coding methodology, which operates on a multirate filterbank. It has been designed to filter the audio signal into frequency bands, which match the critical perceptual bands of the human ear. Within each frequency band the signals are re-quantified at a variable resolution. This is determined by the available bit-rate and an analysis of the long/short periodicity of the audio signal in each frequency band. According to DTS, this allows an efficient sharing of the limited number of quantisation bits without any transient pre-echo distortion. Further, by coding the spectral analysis to extend and include all channels, the re-quantisation routines are fed from a common bit-pool. As well, DTS feels that this optimizes the coding performance and audio quality of each individual channel in a multichannel format delivering a full-bandwidth for each. Essentially, this allows 6 channels of transparent quality 24-bit recorded material at 48 KHz with less digital compression. While Dolby Digital uses a different approach of providing multi-channel sound with low bit-rates, the results are somewhat similar. Dolby Digital is an evolutionary approach building upon what has come before. DTS, on the other hand, is a revolutionary approach that was designed specifically for multi-channel sound from the beginning.

DTS has also spawned several variations including: DTS-ES 6.1 (both Matrix and Discrete), DTS NEO:6, DTS-96/24, and DTS Interactive.

Beyond 5.1
So, as good as Dolby Digital and DTS are today, there are newer surround sound schemes that “push the envelope” even further in providing the listener with additional realism and a 360-degree experience recreating much more of a theater-going experience in either 6.1 or 7.1 digital surround configurations. These new flavors include: Dolby Digital EX, DTS-ES, Dolby Digital Plus, Dolby TrueHD, DTS-HD, and DTS-HD Master Audio.

In most Home Theater set-ups, smaller speakers can be used for the fronts (Left, Center, Right) and dipole speakers for the rear. Dipoles emulate the soundfield of an array of speakers. The use of a subwoofer allows you to be able to use smaller speakers, which fit better, in most living/family rooms environments. Back surround speakers in 6.1 or 7.1 set-ups can be one or two more dipole speakers for a complete 360-degree experience.

Dolby Digital EX
Dolby Digital EX takes the Dolby Digital 5.1-channel setup one step further with an additional center surround channel (reproduced through one or two speakers) for extra dimensional detail and an enveloping surround sound effect. Feature films originally released in Dolby Digital Surround EX (the cinema version) carry the encoded extra surround channel in their subsequent DVD releases, as well as onto 5.1-channel digital satellite and TV broadcasts. If your home theater system has a receiver or preamp/processor with Dolby Digital EX decoding and speakers to support 6.1 or 7.1 playback, you can hear Surround EX soundtracks as they were meant to be heard, with the increased realism created by the extra surround channel. As in the cinema, with regular 5.1-channel Dolby Digital playback no sonic information is lost (although you’ll miss out on the heightened realism). Current Dolby Digital Surround EX soundtracks contain a digital flag that can automatically activate the EX decoding in a receiver or preamp/processor.

DTS-ES 6.1
DTS-ES 6.1, which stands for Extended Surround, builds on the DTS aural experience by utilizing 6.1 discrete channels of audio information with the help of a back surround channel matrix, which is encoded into the left and right surround channels. While other matrix 6.1 surrounds are input and decoded in the analog domain, DTS-ES is entirely decoded in the digital domain with 24-bit precision and full octave equalization. What this means is that DTS-ES 6.1 allows for a fully discrete back surround channel as opposed to the matrix-encoded back surround channel of conventional Surround EX-encoded soundtracks. DTS-ES translates back channel information into precise sound effect positioning and imaging with no sound bleed into the left and right surround channels, or for a collapsing of left and right surround information toward the back surround channel. There are two types of DTS-ES: Matrix and Discrete. The Matrix version derives a matrixed back channel from standard 5.1-encoded software. Discrete, on the other hand, is, as the name implies a discrete sixth channel of aural information encoded on the disc for a fully-discrete back channel.

Dolby Digital Plus
According to Dolby, Dolby Digital Plus is audio that completes the high-definition picture. Dolby Digital Plus is the next-generation audio codec technology for all high-definition programming and media. It combines the efficiency to meet future broadcast demands with the power and flexibility to realize the full audio potential of the upcoming high-definition experience. Built upon Dolby Digital, Dolby Digital Plus was designed for the delivery formats of the future, but remains fully compatible with all current A/V receivers. With Dolby Digital Plus, you get even higher quality audio, more channels, and greater flexibility. It features: Multichannel sound with discrete channel output, channel and program extensions can carry multichannel audio programs of up to 7.1 channels and support multiple programs in a single encoded bitstream. Plus outputs a Dolby Digital bitstream for playback on existing Dolby Digital systems, supports data rates as high as 6 Mbps, bit rate performance of at least 3 Mbps on HD DVD and up to 1.7 Mbps on Blu-ray Disc, and accurately reproduces what the director and producer intended, interactive mixing and streaming capability in advanced systems, and supported by HDMI (Version 1.2a and above).

Dolby Digital Plus was specifically designed for high definition discs such as HD DVD and Blu-ray Disc. It employs an altogether new technique to address the downmix compatibility issue, and is the only perceptual coder thus far to do so. In its “core plus extension” structure, the Dolby Digital Plus core is a complete 5.1-channel mix; the extension contains the new channels, plus any channels that have been modified between the 5.1 and 7.1 renditions. The high coding efficiency of Dolby Digital Plus—coupled with the large capacity of HD disc formats—means there is no material penalty in employing this method. Dolby Digital Plus can deliver 7.1channel soundtracks with superb quality at bit rates of 1 Mbps or less. No longer restricted to 448 kbps, Dolby Digital Plus—and Dolby Digital—will bring enhanced quality from a highly sophisticated perceptual audio coder while concurrently enabling content providers to include multiple surround audio streams on the disc without impacting the data needed for high-quality video or added feature content.

Dolby TrueHD
Dolby TrueHD is the pinnacle of Dolby’s next-generation lossless technology developed for high-definition disc-based media. Dolby TrueHD delivers tantalizing sound that is bit-for-bit identical to the studio master, unlocking the true high-definition entertainment experience on next-generation discs. When coupled with high-definition video, Dolby TrueHD offers an unprecedented home theater experience that lets you enjoy sound as stunning as the high-definition picture. It features: 100 percent lossless coding technology, up to 18 Mbps bit rate, supports up to eight full-range channels of 24-bit/96 kHz audio supported by High-Definition Media Interface (HDMI), supports extensive metadata including dialogue normalization and dynamic range control.

From its inception, Dolby Digital was not bound to any prior channel extension methodology, and could therefore benefit from the subsequent developments of other multichannel codecs. One example of a channel extension technique is the method by which MLP Lossless, Dolby TrueHD, and MPEG-2 LII deliver compatible downmixes for soundtracks with expanded channels. In these codecs, a 7.1-channel soundtrack is first downmixed to create a 5.1 mix, which is supplemented by a two-channel extension (which we’ll call “extension B”). The 5.1 mix is then further downmixed to a two-channel stereo mix, and another supplemental stream is created that carries the 3.1-channel “extension A.” So the 7.1-channel program is delivered in three separate components: a two-channel mix, the 3.1-channel extension A, and the two-channel extension B. The total payload is still 7.1 channels, with preconfigured subsets to create two-, 5.1-, and 7.1-channel presentations. Re-matrixing is itself not a problem, and because it works perfectly with MLP Lossless coding on the now defunct DVD-Audio, the method was carried over into Dolby TrueHD. 

The core technology of Dolby TrueHD is MLP Lossless. As a result, MLP has been in use longer and more widely than any other lossless audio technology, proving itself to consumers and industry experts alike. Dolby TrueHD supports double the maximum bit rate (18 Mbps), double the possible number of channels (14.0), more options for stereo support (including delivering a totally separate stereo mix rather than a downmix), the addition of metadata as used in Dolby Digital—including dialogue normalization and dynamic range control—as well as support for all the new channels introduced in SMPTE 428M.  Dolby TrueHD delivers the lossless audio experience demanded by home theater enthusiasts for high-definition video content.

DTS-HD is obviously DTS’ answer to Dolby Digital Plus. It can encode and decode audio at super high bit rates. In digital recording, the faster the transfer of digital information (data rate), the better the sound quality. DTS-HD has the ability to deliver very high data rates that can capture virtually all of the original audio information (DTS-HD High Resolution Audio) or ALL of the information (DTS-HD Master Audio). What this means is surround sound with unprecedented accuracy and realism.

DTS-HD delivers wider frequency response and greater dynamic range for both movies and music. Higher sampling rates and greater bit depth mean more realistic sounds (clearer and accurate) and greater dynamic punch. Music and dialogue will sound more clear and accurate, and the dynamic range of action sound effects will be dramatically improved. DTS was the only format that delivered 96k/24 bit sound to standard DVD Video discs, and it continues to lead the way in audio performance with Blu-ray and HD-DVD discs.  DTS-HD is capable of 7.1 channels that immerse the listener in a perfect movie or music experience. The additional channels radiate uniform sound coverage throughout your living room, virtually eliminating any “holes” in the surround sound field. It delivers a fully immersive surround experience for everyone in the room - and not just for the person sitting in the “sweet spot.” Many 2008 A/V receivers now include a DTS-HD decoder built-in At the same time, several Blu-ray Disc players may also include a DTS-HD decoder built-in. The new player can connect to your existing receiver with a S/PDIF or HDMI digital input, or 7.1 channel analog inputs.

DTS HD Master Audio
DTS-HD Master Audio is capable of delivering audio that is a bit-for-bit identical to the studio master, and is on par with Dolby True HD. With DTS-HD Master Audio the listener will reportedly experience movies and music exactly as the artist intended. DTS-HD Master Audio delivers surround audio that is indistinguishable from the original soundtrack or music recording. The new high definition optical discs have far more capacity than standard DVDs, which has allowed DTS to develop a surround sound format to deliver surround sound at super high bit rates - up to 24.5 Mbps on Blu-ray discs and 18.0 Mbps on HD-DVDs that are vastly superior to standard DVDs. This bit stream is so “fast” or the transfer rate is so “high” that it can deliver Lossless Audio, a “bit-for-bit” recreation of the original recording. The result is 7.1 channels of audio that are identical to the original studio master. With DTS-HD Master Audio you will experience movies and music exactly as the artist intended.

“Close Encounters of the Third Kind – 30th Anniversary Ultimate Edition” is one of the first Blu-ray Discs that include both Dolby True HD and DTS HD Master Audio soundtracks. The aural experience derived on this special disc set is nothing short of amazing giving you the feeling that you really are atop the Devil’s Tower with the government officials. The floor literally shakes as the Mothership touches down. This is what home theater is all about!

Which is better? Like beauty, it’s in the eye (or ear) of the beholder. Personally, I prefer DTS, and chose it on any disc that I watch or listen to. I believe that I hear more subtly and nuance in the sound presentation. However, others may prefer Dolby Digital decoding, which sounds pretty awesome also. If you’re watching “Lost” on your local ABC-HD channel, for example, the Dolby Digital soundtrack certainly adds depth and presence to the experience. Don’t you just love home theater!!! Pass me the raisonettes.

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