Home Theater

Have you ever wondered what all those audio settings are for on your home theater audio/visual (A/V) receiver? This article explains the purpose and difference between every multi-channel audio format from Dolby Laboratories (Dolby), DTS, and Auro Technologies.

Choosing the Best Surround Sound Format

Which surround-sound format is the best? Almost everyone looking for the type of information contained in this article has this question. The reality is; there is no straight-forward answer. It depends on your personal goals and the constraints of your equipment and environment. In a very general sense, there are some subtle distinctions between Dolby, DTS, and Auro audio codec technologies you should take into consideration. Most importantly, it is wise to plan your speaker layout beforehand.

3D Sounds: Atmos, DTS:X, Auro-3D

3D spatial sound is the pinnacle of home theater audio. There are three prevailing reasons why this is true:

1. Consumers' desire for an immersive multimedia experience
2. The technology to create a believable illusion of immersion exists visually and audibly
3. Content creators desire their audience to experience sounds emanating from the direction intended in the context of a 3-dimensional space (x, y, z axis)

Now that we've established the "why," let's examine the "how." At a high-level, there are some technical differences in how the three companies driving this audio revolution approach delivering 3D.

Dolby, DTS, and Auro use significantly different approaches to creating 3D sound environments.

Dolby Atmos, DTS:X, and Auro-3D are competing implementations of this concept. Each utilizes different approaches toward meeting the same goals:

• Dolby
  • All-in-one, single-codec solutions incorporating spatial sound (e.g. some codec + Atmos extension)
  • Keep It Simple: 3D audio objects, compression, and channelized audio rolled into a single application
  • Speaker-level targetting using "discrete" audio channels (channels map to specific speakers)
• DTS
  • Range of solutions: hybrid legacy/audio object (DTS:X), exclusive audio objects (DTS:X Pro), and various modes of psuedo-3D
  • Speaker location agnostic
• Auro
  • No discrete channel solution of its own (its codecs allow only audio objects, exclusively)
  • Leverages competitive codecs when a source is using discrete audio channels (i.e. it calls Dolby or DTS codecs)
  • Outputs audio based on channel, not speaker position

Each spatial audio format has different configuration requirements.

Speaker Placement is Important

3D sound isn't simply a matter of having the right equipment and content capable of powering a sufficient number of speakers. It's also a function of where those speakers are positioned. Every surround sound format is designed for speakers to be placed in specific locations. This is true of any audio format; 2.1, 4, 5.1, 7.1, 9.1, etc.; even "3D" or spatial surround sound formats with 20 or 30 speakers.

Speaker positions in advanced audio demonstration models are not arbitrary.

Standard 5.1 channel speaker configurations have specific guidelines for where the speakers should be placed relative to the central listening position. The diagram below demonstrates one example. This is not the only 5.1 model.

What does this mean for the home audio/home theater enthusiast? It means you should make a point to decide which format you are most interested in before you design the speaker layout in your preferred listening environment.

Vertical position and angle are important too.

Dolby Atmos in particular is the most restrictive when it comes to speaker location. DTS is the least.

The tolerances and scalability of speaker settings is much smaller than most people believe. You cannot simply adjust it via a renderer using object-based technology without creating a deleterious affect on audio quality.

DTS:X Pro

The diagram to the left shows a DTS Pro:X speaker layout in 30.2 configuration.

This is one possible configuration. Like it's less-robust sibling DTS:X, there is no standard speaker placement guideline, in keeping with DTS "agnostic" implementation.

While this flexibility in speaker placement may seem like a boon to the average consumer, when one stops to consider the limitations of this approach it becomes clear that it does not serve the listener in the best capacity from the standpoint of fully accurate audio reproduction.

As mentioned above, agnostic speaker placement undermines the ability of the content artist to create a product that will faithfully reproduce their intentions for the audience. This places the entire DTS line-up of audio-object and spatial sound systems in the weakest position of the three contenders.

Granted, there is some guidance on the part of the engineer who configures the system to begin with (in the listening environment). However, regardless of that fact, DTS' philosophy of a full-on 100% audio object based solution is in fact a detriment to the listening experience, from the standpoint of realism within all environments. Here, Atmos does in fact hold an edge; though neither approach is as robust as Auro's.

Auro-3D was the first 3D sound format brought to the home theater market, and the first to be described as an "immersive" approach.

Most audio content is channel specific. Even in object-based streams, the vast majority of audio is produced via channels.^{[Munz, 2014]}

Maintaining Perspective

If your concerns are detailed enough to look beyond number of simultaneous output channels (speakers) supported, the next question is whether your solution exclusively requires discrete channels, independent audio objects only, or a combination of both.

The answer to those two questions (number of output channels and type of codec) will narrow down your choices substantially when one requires either a lot of output channels and/or legacy channelized audio or spatial (object) audio. The more specific your demands are, the easier it is to identify a solution. If you're in the camp where a lot of codecs will foot the bill, consider other factors such as bitrate/date throughput capability, which impacts the overall fidelity (quality) of the sound. Greater bandwidth = more sonic details.

The very first multi-channel film may surprise you. Pre-dating Dolby, This is Cinerama debuted in United States movie theaters in 1952. Featuring a specially crafted 7-channel soundtrack, the technology proved too expensive and complicated, and never took off on a large scale.¹

Dolby Multi-Channel Audio

The evolution of Dolby multi-channel audio.

Dolby (or more correctly, Dolby Laboratories) is to multi-channel and surround sound audio formats as Coca-Cola is to the soft drink industry. It is the most recognized brand name in multi-channel audio.

* Same codec
** Plus audio objects (i.e. Atmos metadata)
*** Released for the home theater market in 1995

Evolution of Dolby Multi-Channel Audio
	Home Theater	Channels		Bitrate	Codec	Mode	Year
		in	out
Dolby Stereo	No	4	4		Yes		1975
Dolby Surround	Yes	3	3		Yes		1982
Dolby AC-1	Broadcast	3	4	640 kbps	Yes		1985
Dolby Stereo SR	No	4	4		No		1986
Dolby Pro Logic	Yes	2	4		Yes		1987
Dolby AC-2	Yes	3	4		Yes	Lossy	1989
Dolby SR.D (Spectrum Recording Digital)	No	4+2	5.1	320 kbps	Yes	Lossy	1990
Dolby Digital (AC-3)	Yes***	6	5.1	320 kbps	Yes	Lossy	1991
Dolby Digital Surround	Yes	4	5.1		Yes	Lossy	1996
Dolby Digital 5.1 EX	Yes	6	6.1		Yes*	Lossy	1999
Dolby Digital Surround EX	Yes	6	7.1		Yes*	Lossy	1999
Dolby Pro Logic II	Yes	2	5.1		Yes	Lossy	2000
Dolby Digital EX (THX Surround EX)	Yes	6	5.1		Yes	Lossy	2001
Dolby Pro Logic IIx	Yes	2	7.1		Yes	Lossy	2002
Dolby Digital Plus (DD+ | E-AC-3)	Yes	8	15.1	6,144 kbps	Yes	Lossy	2005
Dolby TrueHD	Yes	8	7.1	18,000 kbps	Yes	Lossless	2005
Dolby Digital Live	No	6	5.1	640 kbps	Yes		2005
Dolby Pro Logic IIz	Yes	8	9.1		Yes		2009
Dolby Digital Plus Pro	Yes	8	7.1		Yes		2012
Dolby Surround	Yes	8**	34		Yes		2014
Dolby AC-4	Yes	8	7.1.4		Yes		2019

Table Legend

Dolby Stereo

Released in 1975 and available only in cinemas, Dolby Stereo was the first true "surround sound" audio format and consisted of four (4) independent channels. The terms "Dolby Stereo" and "Dolby MP" were used in reference to movie theater speaker configurations and audio when displaying compatible films. The "MP" portion is an abbreviation for "Motion Picture." Dolby Surround's encoder was called Dolby MP Matrix. Dolby Stereo would not enter the home theater market until 1982 as Dolby Surround.

The first movie released with Dolby rear channel sound effects was the venerable Star Wars in 1977.
The first 4-channel surround sound movie (ever) in theaters was The Robe (1953), pre-dating Dolby Surround by 22 years.¹

There are several slightly different iterations that used different codecs. However, they have a few things in common. The Dolby Stereo format recorded 4-channels of audio on 35mm film using an optical process. When played back, the 4-channel film audio was mathematically converted into 2-channels, and then into 4-channels for playback. The front center channel was a matrix of the front left and right channels.

Dolby Surround

Dolby Surround was Dolby's first foray into the home, ostensibly heralding the arrival of home theater audio.

Dolby Surround is a 3-channel format. At the time, the consumer VCR (Video Cassette Recorder) had yet to be invented. Consumers watched movies via television. Broadcasters were limited to 3 audio channels at the time. Dolby Surround allowed broadcasters to transmit a normal stereo channel audio and couple a third channel with it. Compatible set-top boxes and audio receivers then converted these 3 (three) channels into so-called surround sound. The system worked by matrixing the front left and right channels to create the center channel, and the rear speakers received identical mono signals derived from the 3rd broadcast channel. Dolby also limited the rear channel frequency range between 100 - 7,000 Hz, a technique Dolby would carryover into its first true 4-channel home theater codec: Dolby Pro Logic.

What is 4-channel Sound?
Four (4) channel surround sound consists of separate audio tracks for a left front channel, right front channel, center front channel, and a surround channel. The surround channel in 4-channel surround sound formats is sent to the rear speaker(s). Early formats such as Dolby Stereo produced identical mono signals from two (2) rear speakers. The rear or "surround" sound channel was used to derive the signal sent to each speaker. Later, rear channels would become muxed (matrixed) in order to apply more granular focus to front audio, which accounted for most of the audience's auditory attention.

Dolby AC-1

Released in 1985, Dolby AC-1 was designed to address the needs of surround sound audio in the television broadcast market. The first analog-to-digital encoder, Dolby AC-1 produced a 4-channel surround sound output similar to Dolby Surround. It was essentially the bridge that brought the effects of surround sound in movie theaters into the broadcast market. When High Definition Television (HDTV) audio debuted, Dolby AC-1 was its de-facto standard.

Dolby Stereo SR

Dolby Stereo Spectral Recording - frequently abbreviated as Dolby Stereo SR or Dolby SR is not a codec. Dolby Stereo SR is a hardware, noise reduction solution for analog audio designed to improve dynamic range. It is mentioned in this article for the sake of completeness. While not a codec, at the time it was instrumental in improving and smoothing analog audio recordings in film.

In 1987, Dolby Stereo SR debuted in theaters on two films: Innerspace and Robocop

Dolby Pro Logic

Dolby Pro Logic - also known as Dolby Surround Pro Logic was released in 1987. At the time, it represented the pinnacle of home theater audio. Dolby Pro Logic was a watershed event for home theater. VHS and Betamax were battling for control of the burgeoning home video market and Dolby Pro Logic supplemented those technologies by embedding a surround sound format onto videotape, bringing the most advanced surround sound technology of the time into living room.

Dolby Pro Logic differs substantially from Dolby's prior surround sound formats. Films were still being recorded in 4-channel audio, but Dolby felt the format did not leverage the front speakers to their full potential. As the fronts account for the majority of audio output directed at the listener, Dolby wanted to create a better experience for consumers. Dolby Pro Logic provides this by re-purposing the 4-channel format. Instead of the historic layout, Dolby Pro Logic applied dedicated audio channels for all three (3) front channels (left, right, center). The fourth channel was sent to the rear (or surround) speakers. Another distinction from previous surround sound formats was the fact both rear speakers emit the same mono signal. This makes them good for rear sounds, but they are not capable of independent, spatial positioning queues. Furthermore, the surround (rear) speakers are limited to a maximum frequency of 7 kHz.

Overall, Dolby Pro Logic cemented the purpose of surround speakers as adjunct rather than primary.

Dolby AC-2

Dolby AC-2 was released in 1989. The next generation to AC-1, Dolby AC-2 improved the audio quality of Dolby's matrixed sound concept while simultaneously lowering the bitrate; effectively packing a superior sound reproduction into a smaller data package.

The dawn of the modern era constituting what consumers think of as "surround sound" began in 1990 when Dolby AC-3 was released.

Dolby SR.D

Just a year after the release of AC-2, in 1990 Dolby released its first digital recording method for 35mm film, called Dolby Spectrum Recording Digital or SR.D. The new format combined a 4-channel digital sound track with a 2-channel stereo analog sound track directly beside it on the film strip.²

Birth of the 5.1 Channel Concept
What's the big deal with "5.1" channel surround sound? Why is this THE benchmark?
In 1989, the SMPTE (Society of Motion Picture and Television Engineers) arrived at the conclusion the ideal number of audio channels for a film was six (6): Front Left, Front Right, Front Center, Left Rear (surround), Right Rear (surround), and a seperate track for Low Frequency Emissions (LFE). The SMPTE's research prompted Dolby to create the first true 5.1 sound format: Dolby Digital.

Dolby Digital (AC-3)

Dolby Digital - also known by its code name: AC-3 - was a ground-breaking codec when it was introduced. Offering six (6) discrete digital audio channels, Dolby Digital heralded in the era of 5.1 surround sound for home theater. Dolby Digital is a lossy format capable of very high audio compression (12:1), though a range of 4-6x:1 is more common. While that sounds impressive (and it is), its audio fidelity is limited to a 16-bit depth at 32, 44.1, or 48 kHz. A contant bitrate is required, broadcast at 192, 448, or 640 kbps.³

The very first cinematic film shown in Dolby Digital was Star Trek VI: The Undiscovered Country Star Trek VI in 1991. However, this was an experiment on Dolby's part, and this particular copy was limited to three (3) theaters in the United States.

The first wide release Dolby Digital film was Batman Returns in 1992.⁴

Surprisingly, the format is still widely used today. AC-3 would later became the de-facto surround sound standard for Digital Versatile Discs (DVDs) when they began to appear in the consumer market (1995). That same year, AC-3 was adopted by the ATSC (Advanced Television Systems Committee) as its standard for broadcast digital multi-channel audio (ATSC A/53).

Dolby AC-3's LFE channel frequency range was limited to 20 - 120 Hz. More modern Dolby codecs retain the 120 Hz ceiling for dedicated LFE channels, but allow sounds as low as 3 Hz (even though they are inaudible to humans).

⁵

Dolby Digital was not without drawbacks. While its compression was very good at the time, it was not a miracle worker. Dolby upped the ante with six (6) digital challenges, but the bitrate remained the same as AC-2. You may recall Dolby cut the bitrate of AC-2 in half when compared to AC-1. This meant Dolby's new codec still had only half the bandwdith per channel as technology it had introduced six (6) years earlier, and in fact Dolby's algorithms would not manage to regain the 640 kbps crown until 1999, with the rollout of its pair of Dolby Digital 5.1 codecs.

MP3, MKV, M4A, and ALAC are examples of modern file types that still leverage the AC-3 codec.

AC-3 did not enter the home theater market until 1995. At the time it was only available on LaserDisc.

Why has Dolby Digital or Dolby AC-3 withstood the test of time so well? Why is it still popular?

Yes, Dolby Digital is still the de-facto standard for multi-channel sound in MP3 audio files and MPEG-2 multimedia files. MP3 is the most popular digital music file format, and MPEG-2 remains the default for standard video resolution DVDs. These facts haven't changed, and the end result is DD is still very popular; even nearly 30 years after it was released. It owes this honor due to the ubiquity of content encoded with the AC-3 audio codec.

Dolby Digital is the only Dolby codec supporting S/PDIF.⁶

Dolby Digital Surround

Despite what seemed like an eternity since its last revolutionary codec release, behind the scenes Dolby's engineers had been busy attempting to improve on theatrical sound. Sticking with their "digital" theme, and also in 1996, Dolby released a new sound codec designed to breathe life into the older 4-channel audio format. While AC-3's six-channel audio recording capability had been around since 1990, many popular films existed that had been produced prior to the AC-3 era. As theaters had improved the cinematic experience for movie-goers with additional sound systems to support AC-3, they were forced to continue playing these older films using the older 4-channel surround format. Unfortunately, consumers had become accustomed to the improvements wrought by six-channel surround and thus this issue detracted from the theaters' income. Furthermore, in the home theater market consumers were purchasing DVDs of their favorite films, yet many of them were mastered in the old 4-channel surround format or even stereo due to technological limitations of the time. There was no means to re-master the 4-channel audio into a 5.1 Dolby Digital format.

This is the backdrop for the advent of Dolby Digital Surround. Released the same year as Dolby Digital (1996), Dolby Digital Surround was an up-conversion codec that converted 4-channel surround sound into 5.1 (matrixed) surround. Also known as, "5.1 Virtual," the technique allowed cinemas and consumers to directly compare the old with the new.

While this new technology and codec from Dolby proved very popular (especially for the home theater market), Dolby's choice of product names arguably began what has become a tremendous source of confusion for most consumers with regards to what each Dolby's sound format represents and how they differ, given their very similar naming conventions. Just imagine being a non-audiophile consumer in 1997 and trying to understand whether there was any difference between "Dolby Digital" and "Dolby Digital Surround" when they were both advertised as 5.1 surround sound formats.

Dolby Digital 5.1 EX

The release of Dolby Digital 5.1 EX in 1999 cemented Dolby's confusing array of similarly named codecs as it upped the ante relative to alternative surround sound formats beyond the venerable 5.1 standard. DD 5.1 EX up-converts 5.1 sound to 6.1 channels. It adds a Rear Center channel and otherwise functions as 5.1 does. Note however, the 6th channel is matrixed (meaning it is not discrete). In other words, the 6th - center rear - channel is virtual. It is a phantom channel created by the decoder. It is NOT a seven (7) channel (6+1) audio recording format.

As if Dolby hadn't confused matters enough on its own with its re-branding debacle of Dolby Digital (AC-3), the 1992 name change prompted many professional and pro-sumer audio enthusiasts to begin using the term "Dolby Digital 5.1" shortly thereafter to clarify its purpose and distinguish the 1992 codec from the 1990 codec. It is possible this informal re-branding contributed to Dolby's labeling of it's 6.1 channel solution released in 1999 as, "Dolby Digital 5.1 EX" even though there wasn't already an official "Dolby Digital 5.1" codec to begin with. As one can imagine, this simply led to another round of mass confusion and mis-information as Dolby again proved its exceptional knack for creating brand names that didn't align well with their product's actual function.

Dolby Digital 5.1 EX and Digital Surround EX share the same codec.
They take advantage of psycho-acoustics to provide the illusion of more discrete audio channels.

Dolby Digital Surround EX

Also released in 1999, Digital Surround EX is a cousin to Dolby Digital 5.1 EX. Whereas Dolby Digital 5.1 EX up-converts 5.1 to 6.1 channels, Digital Surround EX kicks it up a notch by up-converting 5.1 to 7.1 channels. This is accomplished by matrixing both the rear and side channels. You may think of it as taking a 5.1 channel and creating phantom or virtual rear and side channel audio from the 5.1 signal. It is considerably more complex than DD 5.1 EX, as the decoder must matrix both front and rear channels in order to create the side channel effects.

Dolby Pro Logic II

Dolby Pro Logic II is another up-conversion technology. Its codec creates 6 discrete channels (5.1) from 2-channel stereo.

Dolby Digital EX (THX Surround EX)

Released in 2001, Dolby Digital EX - also known as THX Surround EX - was the first codec to bring Lucas Films' THX surround sound format to the home theater. THX is not a recording technology. It is an environmental playback format. THX dictates specifications on how certain types of audio are to be presented to the user. Thus, from a home theater perspective, it does require a codec that supports THX. Dolby Digital EX was Dolby's first codec to do so for the home audio/home theater market.

Dolby Pro Logic IIx

Two (2) years after Pro Logic II, Dolby released Dolby Pro Logic IIx in 2002. Pro Logic IIx builds on its predecessor's abilities by adding up-conversion capabilities from 5.1 Dolby Digital audio to 6.1 or 7.1 surround sound formats.

Dolby Digital Plus (DD+)

Released in 2005, Dolby's Dolby Digital Plus or DD+ - also known as Dolby Digital Enhanced AC-3 or E-AC-3 - is an advanced, lossy codec capable of supporting up to 16 channels (including one dedicated LFE channel). DD+ is based on the AC-3 format (ATSC A/52), however E-AC-3 is not compatible with the AC-3 format. Dolby Digital Plus' data throughput rate is about 10 times higher than its similarly named Dolby Digital predecessor.

Dolby Digital's typical maximum output is 7.1 (8) channels, but as mentioned above the codec is capable of more. The current 7.1 output figure is due to the limitation of Blu-ray disc recordings and output to eight (8) audio channels (7.1). Many other modern devices are also limited to 8-channel PCM audio output and 8 discrete channels. Thus, as of this writing there is not a high demand for more input channels at this time. Regardless, on the output side of the equation, the DD+ codec is capable of up-converting to its limit of 16 discrete channels (15.1).

It's unlikely Dolby Digital's true potential of 16 discrete channels will ever be realized, as the industry is pivoting away from discrete channels to embrace audio objects. Being channel agnostic, audio objects allow the industry to continue defining new output channel (speaker) positions and effects without having to re-engineer the physical limit of eight (8) input channels applicable to most audio/visual hardware.

Dolby Digital Plus was invented prior to Atmos and other object-oriented sound formats and therefore, it did not originally support them. However, it does now.⁷

In spite of their similar secondary names, Enhanced AC-3 is NOT backward compatible with AC-3.

Netflix was one of the first organizations to adopt the widespread usage of streaming audio content in DD+.⁸

Dolby TrueHD

2005 was a busy year for Dolby. Dolby TrueHD debuted alongside Dolby Digital Plus. The codecs differ substantially from one another. TrueHD is a lossless codec based on Meridian Lossless Packing and yields a much higher maximum bitrate (18 mbps vs. 6.144 mbps).⁹ Though TrueHD can carry up to 14 audio channels (13.1), in practice it is limited to 7.1. At the time, Dolby sought to future-proof its codecs and get ahead of where the market seemed to be going. As with Dolby Digital Plus, the maximum channel capability of Dolby TrueHD was designed to exceed what was physically possible at the time in order to set the stage for the future. In the case of Dolby TrueHD, its high bitrate created an additional challenge as well: storage space and information processing requirements well beyond the capabilities of any audio processor and storage medium at the time. TrueHD is a lossless codec, and even with compression cannot attain better than about a 2:1 ratio (50% compression). This results in a HUGE amount of audio data to be stored and transmitted. Going from 7.1 to 13.1 would almost double the space and bandwidth required, making it still relatively impractical for content producers to include it.

TrueHD was ahead of its time. For starters, the HDMI standard at that time did not support the bandwidth TrueHD was capable of (and the current version in 2020 still doesn't, 15 years later). The same was true of Blu-Ray discs until late 2017.
Promoted as an 8-channel audio format (7.1) upon release, TrueHD was designed primarily to support the HD-DVD standard (coupled with the aforementioned effort at readiness for greater multi-channel formats anticipated in the future). Most Blu-ray discs also support TrueHD audio.

With eight (8) channels, TrueHD could more than saturate an HDMI cable's bandwidth; let alone what a Blu-Ray disc could handle. HDMI would be the bottleneck of throughput in home theater until the release of HDMI 2.0 in 2013, when its capabilities finally matched Blu-Ray discs (18 Mbps throughput speed). Even limited to eight (8) channels, at maximum audio definition, Dolby TrueHD easily exceeded this and could overwhelm a system. Even today, the most recent HDMI version (2.1) released in 2017 maxxes out at a data throughput speed of 42.6 Gbps.¹⁰ This is still less than what TrueHD is capable of using just half of its available channels. Furthermore, this doesn't take into consideration the requirements of video bandwidth and other forms of data that could be included in a signal.

Ironically, TrueHD's capabilities are also limited by itself. Its maximum bandwidth per channel is approximately 4,608 kbps uncompressed (24-bit depth x 192 kHz sampling rate = 4,608 kbps). After accounting for its maximum encoded bandwidth of 18,000 kbps, even when compressed, the codec cannot quite handle eight (8) audio channels at maximum fidelity. Thus, there is a trade-off between number of channels versus audio fidelity. Six (6) channels (5.1) can make use of its maximum per-channel data throughput, but a full complement of 7.1 sound (8 channels) cannot. Coupled with the limitations of HDMI and the bandwidth demands of high-definiton video, even six (6) channels of maxxed out audio is not realistic at this time when paired with the best video resolution.

Dolby Digital Live

Rounding out the digital audio cadre released by Dolby in 2005 is Dolby Digital Live. DDL addresses a very specific market. This codec converts PCM from a computer or game console into a 5.1 digital audio stream transmitted via an S/PDIF cable.

Dolby Pro Logic IIz

Dolby Pro Logic IIz debuted in 2009 as the next evolution in the Pro Logic codec series. Dolby Pro Logic IIz up-converts 5.1 or 7.1 to 9.1 by adding matrixed height output channels; thus the "z" (as in z-axis) in its name. Dolby Pro Logic IIz is not a discrete format like Atmos. It uses matrix algorithms to compute the "height" channel audio output. Quite frankly, Dolby should not have chosen the Pro Logic namesake for Dolby Pro Logic IIz. The IIz version of Pro Logic differs demonstrably from the original Dolby Pro Logic II and Dolby Pro Logic IIx formats. Dolby Pro Logic IIz accepts up to eight (8) input channels (8 discrete channels or 7.1) and outputs 9.1 channels. Thus, the information utilized by the codec to programmitically compute the height channels is much clearer, as compared to the other Pro Logic codecs which only utilize 2 channels of input.

Dolby Digital Plus Pro (DD Plus Pro)

Also known as E-AC-3 or Enhanced AC-3, Dolby Digital Plus Pro is a cross-device compatibility codec similar in design and purpose to DTS-UHD™. Although it is advertised as a "home theater" solution (in addition to smart phones and other completely different device types), the home theater community has generally shunned both DD Plus Pro and DTS-UHD, and the fact is no matter what their creators say; they are sub-par codecs when it comes to true home theaters.

Dolby Digital Plus Pro is fully compatible with both both AC-3 (5.1 channels) and E-AC-3 (7.1 channels) encoded audio content.

DD Plus Pro's specifications for broadcast media were codified under ATSC A/52, Revision B (2012). The latest version was published in 2018.

Dolby Surround

when the company released Atmos into the home theater market. Market-tested in a small number of specially outfitted movie theaters beginning in 2012, the concept launched with the movie Brave (2012).¹¹ Atmos wowed audiences, paving the way for its introduction in the home theater market in 2014.

Continuing its tradition of confusing the whole world with its common codec nomenclature, Dolby quietly released Atmos as part of its (poorly named) Dolby Surround codec. If you've been paying attention, yes this is the EXACT same name as Dolby's 1982 codec. You might informally think of the 2014 of Dolby Surround as "Dolby Surround v2.0," but that is more of a tongue-in-cheek expression than a phrase worth repeating among audio enthusiasts, lest one cause greater confusion.

Generally speaking, nearly everyone refers to this codec as the "Atmos" release, however that is technically not correct. You see, it is worth emphasizing Atmos is not a codec.⁶ Atmos is simply Dolby's name for spatial audio object support. Dolby Surround was the first codec to support Atmos, and consists of the TrueHD codec (circa 2005) with Atmos support. Thus, it is understandable why so many people believe this is the "Atmos" codec and mis-characterize it.

So, what is Dolby Surround (2014)??? Dolby more-or-less recycled a couple of its existing codecs and bundled Atmos with them. Dolby Surround supports two primary codec logic branches: Dolby Digital Plus (lossy) and Dolby TrueHD (lossless). This means the exact same functionality is provided from those pre-existing codecs, and then Atmos is layered on top of them. This allows Dolby Surround to handle both types of audio in a single, unified codec. One: Dolby Surround is a discrete channel audio decoder (think traditional or legacy channelized audio). And, two: Audio object support is provided by the Atmos logic. The fact it supports both lossy and lossless compression completes the package.

Audio objects are not output-channel specific. It describes the characteristics of an audio "object" and leaves it up to the decoder to decide where and how to transmit that information, based on the given playback configuration. Atmos is described in more detail in Epic Battles of Home Theater: Audio Objects.

Dolby Surround supports up to 8 channels of input and is capable of outputting up to 34 discrete full range channels.¹² It competes directly against Auro-3D, DTS:X, and DTS:X Pro. The contrast between these competing formats is explained here.

Atmos theoretically allows a single channel containing audio object data to act as input to a Dolby Surround or AC-4 codec. The ability to eliminate the concept of discrete channels while simultaneously improving audio quality and realism is the holy grail of the audio object concept.

Dolby AC-4

Dolby AC-4 is an overhauled multi-channel audio format designed to accommodate the needs of media broadcasters. As such, it is intended to combine improved audio quality in conjunction with reduced data bitrate requirements. You may think of Dolby AC-4 as a project designed to apply a Dolby Digital Plus model experience to users on multiple types of platforms. One of the key components of AC-4 is its ability to negotiate the volume and depth of data between the sending and receiving devices. This makes AC-4 ideal for platforms such as mobile phone carriers, where the capabilities of the end user's device to reproduce high quality audio and the end-point device's potential network bandwidth are both likely limited.

Previous codecs presume both source and playback devices are local and data throughput is not a challenge. They also place the onus of downgrading or tossing extraneous content upon the playback device. This means in traditional audio playback environments, the playback device must be capable of accepting audio data at least as rapidly as it is doled out by the source device. AC-4 changes this dynamic, by making the data transfer a negotiation process that takes the playback device into consideration and makes the playback device's capabilities a priority. This means down-conversion is processed by the sending device. Overall, AC-4's more efficient architecture promotes the ability to play any content on any device, anywhere.

DTS Multi-Channel Audio

DTS - now the company's actual name - was at one time an abbreviation for Digital Theater Systems.

DTS was late to the multi-channel codec party, joining the foray in 1993 when it burst on the theatrical scene by way of the movie, Jurassic Park (and helped in no small part by Steven Spielberg, who in addition to being the director of Jurassic Park, was also an early investor in DTS).^13,14 Since then, DTS has typically lagged behind Dolby's pace of innovation, though in recent years the gap has narrowed considerably and DTS has even trumped Dolby's capabilities on a few occassions. Generally speaking, DTS has made up for its shortcomings. The company has a track record of releasing technological solutions that not only closely mirror the functionality of its main competitor's products, but often improve upon the concepts in way that makes its version just a little bit different, and at times a little bit more flexible or capable.

This does not imply DTS' audio codecs are superior to Dolby's (though at times this question is the subject of intense debate); rather, Dolby and DTS solutions present slightly different methods of solving the same problem. There are some areas where one or the other is arguably better if you are concerned with sonic depth or fidelity, bitrate, data throughput, number of channels, or some other objective metric. DTS tends to have a more robust repertoire of options for music, while Dolby has more offerings for cinemas and home theater environments. DTS also addresses a few more specialized scenarios. Its codecs tend to be more capable when it comes to accomodating unusual speaker combinations, while Dolby is more ubiquitous in terms of content. DTS is also stronger in the area of cross-compatibility with other formats, while Dolby tends to be more restrictive. Any way you look at it, neither completely dominates the home theater space. They both have their pros and cons.

Theatrical vs. Home Theater Codecs

Technically, "DTS" as a family of codecs refers to the theater (cinema) version of the company's audio codecs, while the home theater versions are actually, DTS Coherent Acoustics or DCA codecs. Naturally, the average person could care less, and the acronym DTS has come to mean (colloquially) codecs produced by the company, regardless of whether they are in the theater or home environment.¹⁵

^* Both DTS ES versions use the same codec
^** Audio only

Evolution of DTS Multi-Channel Audio
	Home Theater?	Channels		Bitrate	Codec	Mode	Year
		in	out
DTS Digital Surround	Yes	6	5.1	1,510 kbps	Yes	Lossy	1993
DTS 96/24	Yes**	5.1	5.1		Yes		1997
DTS Neo:6	Yes	2.1 | 3.1 5.1 | 7.1	3.1 | 4.1 5.1 | 6.1 | 7.1		No		1999
DTS ES Matrix	Yes	5.1	6.1 | 7.1		Yes*		2000
DTS ES Discrete 6.1	Yes	6.1	6.1		Yes*		2000
DTS Express	No	2 5.1 | 7.1	2 | 5.1 | 7.1	512 kbps	Yes		2002
DTS-HD Master Audio	Yes	5.1 6.1 | 7.1	2.1 | 5.1 | 7.1	24,500 kbps	Yes	Either	2005
DTS Neo:X	Yes	2.0 5.1 | 7.1	6.1 | 7.1 9.1 | 11.1		No		2012
DTS-HD High Resolution	Yes	5.1	7.1	6,144 kbps	Yes	Lossy	2013
DTS-UHD	No	n/a	2 - 32		Yes		2014
DTS:X	Yes	5.1 | 7.1	7.1.4		Yes		2015
DTS Neural:X	Yes	2.1	7.1.4		No		2016
DTS Virtual:X	Yes	2 | 5.1 7.1.4	7.1.4		No		2017
DTS:X Pro	Yes	n/a	31.1		Yes	Either	2019

DTS Digital Surround

DTS Digital Surround was DTS' first foray into home theater multi-channel codecs. Sporting a 20-bit, 1,510 kbps throughput bit stream, this lossy audio compression format takes up more space on a DVD than Dolby's comparable format: Dolby Digital (AC-3). In fact, DTS Digital Surround's bandwidth requirement is almost five (5) times that of AC-3! Is it worth it to encode an audio track for the same number of channels, but with 4.72x the depth? It was 1993, and the DVD hadn't been invented yet. The pinnacle of audio/visual high fidelity media at the time was the LaserDisc. It would be another two (2) years until the same technology was moderated to a smaller form factor, more compressed storage, and the DVD was born.^16,17 While impressive in its audio quality (DTS at the time easily blew away Dolby's comparable format), DTS Digital Surround was a marketing disaster.

The first movie produced with DTS was 1993's Jurassic Park.

Many movie producers and directors had to ask themselves the same question I posited above: Is it worth it? Most said, no. In fact, its huge space requirement was such a turn-off that only four (4) known movie titles were released on LaserDisc that included a DTS soundtrack: the 1993 classic, Jurassic Park (1993) was the first in 1997, followed by Apollo 13 (1995), The Twelve Months (a re-make of a an animated 1956 film), and DTS Experience (1998); a promotional release from DTS. The former two are the only DTS-enabled LaserDiscs ever released in the United States, and they arrived at the pinnacle of LaserDisc's popularity, and just before its rapid decline (in the United States).¹⁸ The latter two were released in Japan.¹⁹ Ironically, Jurassic Park was also the first feature film release in movie theaters with a DTS soundtrack.²⁰

If you're old enough to remember LaserDiscs, you likely recall that although they offered unparalleled audio and visual resolution at the time, the technology remained prohibitively expensive and there were few commercial releases. When the DVD was born in 1995, DTS was virtually unheard of; while Dolby was enjoying a surge of popularity in the home. Alas, DTS ran into the same challenge in gaining a footprint on DVDs as it had on LaserDiscs, and was notably absent from the consumer market until its DVD debut on the 1998 animated production, The Legend of Mulan.²¹

DTS 96/24

Four years after DTS released its Digital Surround audio format for movies with its ground-breaking level of detail, in 1997 DTS was staring at a very low adoption rate. The fact was very few movie producers were willing to sacrifice precious DVD space for a feature most consumers weren't aware of and couldn't appreciate. It was time for DTS to tweak its strategy or risk completely missing the boat.

DTS chose to re-group and apply its marketing efforts to a niche where it stood a greater chance of being successful: the audiophile market. DTS 96/24 is an "upscaled" version of DTS Digital Surround that allows 5.1 surround sound on audio DVDs. Prior to DTS 96/24, audio DVDs were only available in stereo and listeners had to resort to artificial upscaling to simulate surround sound effects.

DTS Neo:6

The next innovation from DTS was DTS Neo:6. Having enjoyed some success with DTS 96/24 (albeit to a limited audience), DTS decided to continue its foray into filling gaps in the user experience that Dolby largely ignored. Not a codec, DTS Neo:6 is a post-processing surround sound processor. It up-converts 2.1, 3.1, or 5.1 audio to 3.1, 4.1, 5.1, 6.1, or 7.1 and is capable of down-converting 5.1 and 7.1 audio to lesser formats. Of course, it is not without some limitations. For example, in Neo:6's 7.1 output implementation, the rear speakers share a single (mono) channel.

DTS Neo:6 isn't a miracle worker. It is designed primarily to up-convert and down-convert three (3) to eight (8) channels, including an LFE channel. These days, its usefulness is relatively rare. It is best used in non-standard environments where surround sound is desired, a standard speaker layout does not exist, and the input source is a standard. Neo:6 was replaced by DTS Neo:X.

DTS ES Matrix

The "ES" in DTS ES Matrix stands for, "Extended Surround" and is perhaps together with "Matrix" in the name is a bit of a play on words. DTS ES Matrix is a dual 6.1/7.1 output codec that alternates between two different methods of projecting rear (surround) audio. Technically, a 6.1 output codec. If the incoming signal is a traditional 5.1 layout, DTS ES Matrix adds an extra output channel and directs it to a single rear speaker directly behind the center listening position. It does this by matixing the two (2) surround (rear) outputs of a 5.1 input and splitting them between the surround speakers (2) and the center rear speaker. If the incoming signal is a 7.1 format, DTS ES Matrix takes the outward rear two (2) surround channels (left rear and right rear), and merges them into a single rear speaker directly behind the center listening position.

Thus, this mode will either up-convert or down-convert on-the-fly, depending on the source input type. The important point here is this listening mode only works properly when the listening environment is a 6.1 speaker configuration.

DTS ES Discrete 6.1

DTS ES Discrete 6.1 is very similar to DTS ES Matrix. The difference is the Discrete version decodes a soundtrack encoded natively in the 6.1 format. Thus, in a 6.1 listening environment, it reproduces the sound stage exactly as intended by the content creator. DTS ES Matrix on-the-other-hand, makes an educated guess in up or down converting another format to fit the listener's environment.

DTS ES Discrete 6.1 was the first audio codec with seven (7) truly discrete channels.

Dolby vs. DTS Nitpick
You may have noticed that Dolby also has a 6.1 output codec. It's called Dolby Digital EX. So, what's the difference? Is there one? Yes.

Dolby Digital EX is essentially equivalent to DTS-ES Matrix. Their difference lies lies in the fact Dolby does not have a "discrete" (or true) 6.1 format codec, as does DTS. This means you will never find a work of art encoded with a Dolby 6.1 format. It does not exist.
Now, whether or not that is actually important to anyone, is beyond this article. The point is that if you DO care particurly about a native 6.1 sound format, DTS is the only offering that meets the requirement.

DTS Express

DTS Express is a scalable DTS codec that appears to be aimed at low-bandwidth devices. It has a maximum throughput of 512 kbps and minimum 48 kbps, yet it can handle up to 5.1 channel audio with 24-bit depth (although this is at a sub-par 48 kHz). DTS Express doesn't belong in a home theater environment unless you're planning on creating an alternate stream of some sort. Even then, is the device you're streaming do really going to capably support a 5.1 channel input? Probably not. And even if it does, it's even less likely it can create any viable output in 5.1 that anyone would want to listen to unless they have terrible hearing.

DTS Express may seem to many people like it's a half-assed version of DTS, but in reality it's intended to be a flexible method of providing a DTS sound codec under circumstances where bandwidth-hungry codecs would overwhelm the receiving application. Unfortunately, this does not prevent it from being used inappropriately at times, such as on a Blu-ray disc for example. A Blu-ray publisher could claim DTS support, although any sound engineer who knows what they are doing also knows it is a half truth if the only iteration of DTS on the media is DTS Express.

DTS-HD Master Audio

DTS-HD Master Audio is where the DTS home theater oriented codecs really start to take hold. With a maximum bandwidth of 24,500 kbps, DTS-HD Master Audio is capable of providing an incredibly rich sound environment in a 7.1 channel format. Furthermore, it allows pure, uncompressed audio or the option to compress it, paired with 24-bit depth and up to a 192 kHz sampling rate (though limited to 5.1 channels). When transmitting a discrete 7.1 channel signal, the sampling rate tops out at (a still respectable) 96 kHz.

DTS Neo:X

DTS Neo:X is not a codec. It is a post-processing surround sound format, and replaced Neo:6. DTS Neo:X may be applied to any soundtrack after it has been decoded (e.g. DTS, Dolby Digital).

Problem it solves: Up-mixes and down-mixes incoming channels to match the listening environment.

Neo:X improves on the Neo:6 format by extending its capabilities to 11.1 speaker layouts. Its matrix conversion modes are:

• Up-mix 2.0, 5.1, 7.1 audio to 6.1, 7.1, 9.1, or 11.1
• Down-mix 11.1 to 5.1 or 7.1.

Though it has only 2 channels of discrete height, DTS Neo:X is capable of outputting up to 11.2 or 11 discrete channels + 2 subwoofer or LFE outputs.

What makes DTS Neo:X special is it's ability to artificially create height channels in the listening environment. Thus, one may think of it as a pseudo-Atmos of sorts. It produces at Atmos-like experience from soundtracks that don't include height-based speaker channels. Just keep in mind DTS Neo:X is not a true height-based audio codec. It is designed to simulate 3D audio from traditional channel inputs. DTS did not introduce a true 3D audio codec until three (3) years later with the advent of DTS:X.

The very first Blu-ray disc released with an DTS Neo:X optimized soundtrack was The Expendables 2 (2012).

DTS-HD High Resolution

DTS-HD High Resolution (also abbreviated as DTS-HD HR) is similar to DTS-HD Master Audio and Dolby Digital Plus (DD+). DTS-HD HR offers a 24-bit depth and a 96 kHz sampling rate and supports MPEG-DASH.

DTS-HD HR is capable of high sampling rates (up to 6,144 kbps total throughput). Since it is a lossy codec, the actual amount of data transmitted is even higher (3:1).³ While its bitrate is not as high as DTS-HD Master Audio, its maximum throughput is much higher than most codecs. It uses a contant bit-rate output, making it better suited to solutions that aren't bothered by high bandwidth output bitstreams, as one would expect to find in most home theater environments. DTS-HD HR introduced a concept DTS refers to as extensions.

DTS-UHD

DTS-UHD was the first object-based audio codec designed exclusively for home theater systems. While that may sound exciting, this format is ignored by most home theater enthusiasts. DTS-UHD is a "codec on a single chip" solution targetted at consumer device manufacturers, and specifically those providing broadcast television reception. The reality is DTS-UHD is primarily a convenience for broadcast media distribution as it allows audio to be transmitted as compressed audio objects.²²

DTS:X

DTS:X is an independent codec consisting of DTS-HD Master Audio + audio objects (like Atmos). It is capable of managing up to 32 speakers (like Auro-3D), and runs on top of the DTS-HD MA decoder (it's core). DTS:X is DTS' equivalent to some functionality found in Dolby Surround. The difference here between Dolby's approach and DTS' approach is Dolby combines it's TrueHD lossless codec, plus Dolby Atmos, plus up/down mixing into a single codec. Meanwhile, DTS splits these features into DTS:X and DTS Neural:X.

The first DTS:X movie (Blu-ray) was Ex Machina (2015)²³

DTS Neural:X

Sometimes referred to as "pseudo DTS:X" or "fake Atmos," DTS Neural:X was released in 2016, superceding the Neo post-processors (Neo:X/Neo:6).

Problem it solves: Your content does not support audio objects, but your speaker system does.

DTS Neural:X up-mixes and re-maps legacy bit-streams and/or raw PCM to almost any speaker configuration (up to 12 channels). It's primary function lies in its ability to simulate height surround sound effects when this information is absent from the source media. Part of the family of DTS pseudo-immersive emulators; DTS Neural:X is not a true 3D (spatial) codec. It is a post-processing surround sound format. You may think of it as the DTS equivalent of Dolby Surround's up-mixing capabilities.

DTS Neural:X is designed to up-mix non-3D (spatial) channel-based audio to listening environments with spatial/height speakers. Think of taking all of your old DVDs in 5.1 and up-mixing them to take advantage of ceiling speakers (ala Atmos). This is what DTS Neural:X does.

Neural:X upmixes 1.0, 2.1, and 5.1 channel inputs to 5.1, 7.1, 9.1, 11.1, or 7.1.4 speaker configurations. And while it cannot be applied to Dolby sources, it can do something no other codec can: process incoming mono signals (1.0) and remix them for 7, 9 or 11 speakers, including ceiling or upfiring speakers.²⁴

DTS Neo:X, Neural:X, and Virtual:X are not true 3D codecs. DTS:X and DTS:X Pro are.

DTS Virtual:X

DTS Virtual:X is part of the family of DTS pseudo-immersive codecs. This mode is designed to simulate or create a virtual 3D listening experience when your equipment does not natively support it.²⁵ It creates "phantom" surround and/or height speakers using psychoacoustic processing of existing soundtracks (e.g. something you might see used with TV sound bars).

Problem it solves: You want a "3D" audio experience, but your speaker system doesn't have any height speakers (e.g. ceiling).

DTS Virtual:X was originally designed for sound bars and confined or challenging listening environments. It will function with as few as three (3) speakers, in a single front, dual surround (rear and to the side) configuration. Notably, DTS Virtual:X will not work if you DO have height (spatial) speakers in your home theater room.

DTS is widely regarding as having an edge in this genre, though it is a rather small niche market and the format is relatively unknown.

DTS:X Pro

DTS:X Pro is a true audio object-based codec. Unlike Dolby's Atmos, DTS:X Pro is a complete, stand-alone audio codec capable of driving up to 32 speakers. That's not much to say about it other than that, as it will likely be quite a few years before anything close to 32 speakers is the norm in home theaters.

DTS claims DTS:X Pro is the official audio standard for IMAX® Enhanced; a new licensing and certification program for home entertainment equipment.²⁶

Auro

Auro is a company. Auro-3D is an audio codec. Neither are well-known, though they predate Atmos and DTS:X by nearly a decade.

Auro-3D

Auro-3D uses object-based technology, but takes a different approach compared to Dolby Atmos and DTS-UHD/DTS:X/DTS:X Pro, though it is more similar to Atmos than DTS' line of codecs. Dolby and Auro-3D have very specific speaker layouts in order to have an end-to-end solution to maximize the preservation of the artistic intent, while the DTS audio object codecs are "agnostic" with regards to speaker placement. You can read more about this in the section at the top of this article, Choosing the Best Surround Sound Format>.

The first (Blu-ray) movie released in Auro-3D 11.1 was Red Tails (2012).
The first (Blu-ray) audio CD release with Auro-3D 11.1 was Souvenir (2012).²⁷

Technical Tidbits

This section explains some related technical aspects of audio codecs at a high level.

Its purpose is to briefly explain some terms you may have heard of, but are relatively insignficant. They are mentioned for the sake of completeness. The home theater enthusiast has little, if any, use for them.

Dolby, DTS, and Auro: Why They Don't Sound the Same

Have you ever wondered how there is room within the audio and home theater landscapes for competing formats such as Dolby, DTS, and Auro? The fact is, they use slightly different psychological methods to produce a "surround" effect. At a high level, they break down like this:

• Dolby uses diffusion
• DTS uses targeted sounds (channel specific), except for DTS:X Pro
• Auro-3D and DTS:X Pro use audio objects ONLY

These organizations' approach to audio encoding also differs with respect to the type of algorithms they use to encode and decode audio information. DTS is based on the Adaptive Differential Pulse-Code Modulation (ADPCM) audio data compression algorithm. In contrast, Dolby Digital (AC-3) is based on the Modified Discrete Cosine Transform (MDCT) compression algorithm. Auro-3D is based on a proprietary algorithm, invented by its founder, Wilfried Van Baelen.²⁸

Dolby Atmos

Dolby Atmos is not a codec. It is functionality built in to Dolby TrueHD.

The DVD Paradox

There's only so much storage space on a DVD (let alone a Blu-ray or HD DVD), and there's no way to increase that space available. Do you prioritize content variety, video resolution, or audio fidelity? Do you want to fit special effects or bonus features onto your commercial DVD release? Or do you want an alternate sound track that caters to a niche audience?

This explains why many DVDs never included a DTS sound track. For starters, Dolby was (and still is) more commonly used. Dolby's 5.1 home theater codec preceded DTS by nearly two (2) years, and the fact DTS took up so much more space exacerbated the divide and made including Dolby's 5.1 tracks and the absence of a corresponding DTS soundtrack nearly a given. Why include another 5.1 soundtrack that took up a significant portion of your DVD space, that virtually no one was familiar with? Many movie producers and/or directors said no. This is why there aren't a lot of DVD movies with DTS soundtracks.

Extensions: Feature Expansion and Backward Compatibility

The primary DTS codec is the DTS Coherent Acoustics codec. Most modern codecs contain a primary stream and a secondary stream. The primary stream is called the "core" and the secondary stream. Dolby calls secondary streams "extensions," while DTS refers to them as "residual" streams. Not all codecs have an extension. In fact, the concept has only been around since 2012. For example, the DTS-HD Master Audio codec has a 'core' stream and a 'residual' stream that converts it from lossy to lossless.

What we think of as codecs may contain one or more extensions/residual streams. From a conceptual standpoint, it's normally easier to think of them as functions, because that is what they offer. For instance, Dolby incorporates Atmos as an extension into its Dolby Digital Plus, TrueHD, and AC-4 codecs. Atmos is not the primary or core function of these codecs. It is an extension of them.

Within the context of audio codecs, backward compatibility is the ability of a codec to process an audio stream type created by a codec from the same codec family it doesn't recognize (e.g. a different codec from Dolby or DTS, for example). Typically, this happens when an audio/visual receiver encounters an audio stream it does not understand because the codec was created after the playback device was manufactured. Since audio codecs are hard-coded into the firmware on A/V receivers, backward compatibility allows some older equipment to decode a portion of a newer type of audio stream and still provide a basic level of audio playback capability. The purpose of the core is partly to "future proof" legacy systems.

Extensions are used to augment the core by incorporating additional functionality. An encoder will apply a portion of its data using the extension during the encoding process. When a decoder does not understand the data in an extension, the decoder ignores the extension data and only the core data is utilized. Thus, backward compatibility is achieved.

Although both Dolby and DTS both provide backward compatibility via what they refer to as a "core and extension" philosophy, their implementations are slightly different.

Dolby Extensions

Dolby began using extensions in 2014, with the advent of Dolby Atmos. Incoming audio signals are first routed through the extension. If the decoder extension understands the data, it processes the portion it understands, and hands over any remaining data to the core. If the decoder does not understand the audio data initially, it just gets routed to the core for processing.

Dolby uses different codecs for encoding and decoding, thus owning a decoder you cannot reverse engineer it to figure out how to use it as an encoder.

DTS Extensions

DTS' implementation of the core and extension model works a little differently. DTS always processes the core, and if additional information exists, it attempts to process that with the current extension. The audio data is then merged to arrive at the final output. The primary data stream (the core) is augmented with the secondary stream (the extension).

DTS has used the same core in every DTS codec since its introduction of the concept in 2013 with DTS-HD High Resolution.

Oddball DTS Formats

Some odd surround sound processing formats created by DTS, with questionable value.

DTS Neo:PC

DTS Neo:PC upconverts 2-channel audio to simulated 7.1.

DTS Surround Sensation

DTS Surround Sensation - also known as DTS Virtual - is basically 5.1 for headphones.

DTS Interactive

DTS Interactive is a real-time DTS streamer that converts discrete multi-channel audio (multiple mono channels) into a simulated 5.1 experience.

DTS Headphone:X

DTS Headphone:X simulates 12-channel sound on stereo headphones through the use of clever psychoacoustic techniques. Wow! Does that truly make any sense at all? Can anyone actually detect 12 different speaker channels that close to your ears???

Sony Dynamic Digital Sound (SDDS)

Sony Dynamic Digital Sound (SDDS) is a cinematic surround sound format introduced into movie theaters in 1993. SDDS was designed for 35mm film. The first SDDS film release was 1993's The Last Action Hero.

A SDDS codec has never been created for the home theater market.

SDDS differs substantially from other surround sound formats. It is an optical system like Dolby Digital, with 7.1 channels arranged in a unique pattern—five front channels (left, left center, center, right center, right), left and right surrounds, and the low frequency effects (subwoofer) channel. This multiple-front speaker system purportedly produces better dialogue imaging, compared with Dolby and DTS codecs, which use between one (1) and three (3) front speakers.

SDDS is actually stored in a 12-channel format, but is applied as eight (8) channels in a unique 7.1 surround sound configuration. The format includes four (4) "backup" channels (accounting for the total of 12 channels). During its design, Sony intended intended for these backup channels to allow recovery of limited audio data in the event the portion of a film strip containing the primary channels was ever damaged. The format incorporated other redundant features that together made it very robust in preventing the total loss of audio data should a film become damaged or cut.

During the 1990's, Sony owned both a multimedia production studio and a chain of cinemas. During that time, the company released a number of movie titles with SDDS soundtracks exclusively. Sony never released the SDDS format to the home theater market, though it's unlikely it would have been particularly popular given the fact its layout differs radically from every other surround sound format. SDDS was released on very few films, and was sunset in 2007.²⁹

MPEG-DASH

MPEG-DASH is effectively a replacement to DTS Express, MPEG-DASH is an adaptive streaming technique enabling multimedia content over an HTTP connection. In a nutshell, MPEG-DASH breaks down files into small, compartmentalized portions of playback content and transmits them over an HTTP (Web) connection. The "DASH" portion of MPEG-DASH means, "Dynamic Adaptive Streaming over HTTP."³⁰

Dolby Vision

Dolby Vision is a significant departure from Dolby's historic core competency and appears to be an attempt from Dolby to flex expand its influence more broadly into the multimedia market. Dolby Vision is not and audio codec; it is a video container type (it contains video codecs). It uses "profiles" to embed metadata about a stream. These profiles allow the incorporation of a pre-defined group of video codecs. Dolby Vision supports an astounding maximum bitrate of 800 Mbps.³¹

Dolby Vision does not contain any audio data.

What the Heck is S/PDIF?

S/PDIF stands for Sony/Philips Digital Interconnect Format. It is a very limited digital audio connection format:⁶

• Available in two (2) forms: optical or coaxial
• Only supports 2-channels
• Supports only uncompressed PCM audio and Dolby Digital audio
• Bandwidth limited to ~6,000 kbps
• Does not support copy protection methods (e.g. High-bandwidth Digital Content Protection or HDCP)

References

Cited sources and reference texts.

End Notes

¹ Miller, Michael. (24 September 2004). The History of Surround Sound. informIT.

² Dolby SR.D™. (n.d.). The Dilettante's Dictionary

³ Wright, Matt. (21 May 2008). Mastering HD PC Audio, Part 1. Tom's Guide.

⁴ 50 Years of Innovation: Dolby History. (2018). Dolby Laboratories, Incorporated.

⁵ Dolby Digital. (5 May 2020). Wikimedia Foundation.

⁶ Mehta, Sripal. (13 April 2017). Dolby Audio over HDMI part 1: Codecs. Dolby Laboratories, Incorporated.

⁷ Dolby Digital Plus. (2018). Dolby Laboratories, Incorporated.

⁸ Fleischman, Mark. (20 October 2010). Netflix Adopts DD+ for Streaming.

⁹ Dolby TrueHD uses MLP (Meridian Lossless Packing), a lossless audio compression technique created by Meridian Audio, LTD and licensed by Dolby Labs.

¹⁰ HDMI. (12 May 2020). Wikimedia Foundation.

¹¹ Theatrical Releases in Dolby Vision and Dolby Atmos. (9 May 2019). Dolby Laboratories, Incorporated.

¹² . (August 2014). Dolby Laboratories. p. 15.

¹³ Jurassic Park (1993). (n.d.). Internet Movie DataBase (IMDB).

¹⁴ Bond, Paul. (25 September 2007). DTS' future in home technology, Kirchner says. The Hollywood Reporter.

¹⁵ Patschke, D., Kefauver, A. P. (2007). Fundamentals of Digital Audio. United Kingdom: A-R Editions, Incorporated.

¹⁶ LaserDisc. (16 May 2020). Wikimedia Foundation.

¹⁷ Bennett, Hugh. (April 2004). Understanding Recordable & Rewriteable DVD. First Edition. Optical Storage Technology Association.

¹⁸ Rise and Fall of the LaserDisc. (24 July 2013). Metro's Info-Nation.

¹⁹ LaserDisc Database.

²⁰ moviebuff82. (13 June 2008). Happy 15th anniversary, “Jurassic Park” and DTS!!! Cinema Treasures (online forum).

²¹ Silva, Robert. (13 December 2019). What DTS means in Home Theater Audio. Lifewire.

²² DTS Preparing To Launch DTS-UHD™ Object-Based Audio Format For Home Theatre. (14 August 2014). Widescreen Review.

²³ DellaSala, Gene. (27 May 2015). Ex Machina First Blu-ray Title Released in DTS:X Immersive Surround. Audioholics.

²⁴ Jacquel, Tristan. (2 May 2018). DTS Neural:X and Dolby Surround: the new post-processing technologies from Dolby and DTS. Le Blog Son-Video.com.

²⁵ What is DTS Virtual:X and can I use it? (n.d.) Denon.

²⁶ DTS:X® Pro technology puts you there. (2020). DTS, Incorporated.

²⁷ BLU-RAYS IN AURO-3D. (n.d.). Auro Technologies website.

²⁸ Ted. (18 November 2015). What You Don’t Know About Auro-3D May Surprise You. Strata-gee.

²⁹ Sony Dynamic Digital Sound. (20 February 2020). Wikimedia Foundation.

³⁰ MPEG-DASH: An Overview. (2016). Encoding.com.

³¹ Dolby Vision Profiles and Levels Version 1.3.2. (16 September 2019). Dolby Laboratories, Incorporated.

Bibliography

A GUIDE TO HOME THEATER AUDIO CODECS. Blu-ray forum.

Dolby Digital Plus Audio Coding. (2008).

Dolby Pro Logic. Home Theater Review.

DTS Announces DTS-ES Discrete 6.1 Surround Format. (25 June 2000). Sound & Vision.

Mellor, David. (1 January 2004). A Dolby Digital Primer (part 1). The Audio Masterclass Newsletter.

Movie Sound Landmarks. (3 April 2004). The SpannerWorks.

Munz, Steve. (16 September 2014). Auro-3D® Immersive Sound Interview with Wilfried Van Baelen.

Silva, Robert. (5 July 2019). DTS Neo:X - What It Is and How It Works. Lifewire.

What is the difference between Dolby ProLogic, Dolby ProLogic II, Dolby Digital (AC3, 5.1) and DTS audio surround sound technology? (23 July 2019). Sony.