It was the summer of ’69. We’re not talking about the Bryan Adams song here; we’re actually referring to the first time surround sound became available in the home. At the time, it was called Quadraphonic sound, and it first came to home audio buffs by way of reel-to-reel tape. Unfortunately, quadraphonic sound was very short-lived. The technology, which provided discrete sound from four speakers placed in each corner of a room, was confusing — no thanks to electronics companies battling over formats (sound familiar?) — and it ultimately failed in the consumer market.
Related: Home audio glossary
The idea that one could immerse themselves in a three-dimensional sphere of audio bliss was not to be given up on, however. In 1982 Dolby Laboratories introduced Dolby Surround, a technology that piggy-backed a surround sound signal onto a stereo source through a process called matrix encoding. Not long after, Dolby brought us Pro-Logic surround and has since done its part to advance the state of surround sound in the home to the point where as many as eleven speakers can be used to put listeners right smack in the middle of the action, be it a concert or a battle in deep space.
Unfortunately, surround sound, for many, remains a confusing technology. Though most understand the concept of using multiple speakers for theater-like sound, many don’t understand the difference between all the different formats. There’s 5.1, 6.1, 7.1, 9.2, Pro-Logic IIx, Pro Logic IIz, Dolby DSX and more. It’s a lot to wrap your head around.
With this guide to surround-sound formats, we hope to provide a little clarity as to what separates these different surround-sound versions.
For the purposes of this discussion, “matrix” has nothing to do with the popular film series featuring Keanu Reeves and Lawrence Fishburn (aside, perhaps, from the fact that the dated movies still make for a pretty effective surround sound demo). In this case, matrix refers to the encoding of separate sound signals within a stereo source. This approach was the basis for early surround-sound formats like Dolby Surround and Dolby Pro Logic, motivated in part by the fact that there wasn’t enough space for discrete information on early audio-video media, such as the VHS tape.
Surround sound, at its most basic, involves a set of stereo front speakers (left and right) and a set of surround speakers, which are usually placed just to the side and just behind a central listening position. The next step up involves the addition of a center channel: a speaker placed between the front left and right speakers and primarily responsible for reproducing dialogue in movies. Thus, we have five speakers involved. We’ll be adding more speakers later (lots more, actually) but for now we can use this basic five-speaker arrangement as a springboard for getting into all the different surround formats.
Using the matrix process, Dolby’s Pro Logic surround encodes separate signals within the main left and right channels. Dolby was able to allow home audio devices to decode two extra channels of sound from media like VHS tapes, which fed the center channel and surround speakers with audio. Because of the limited space on VHS tapes, matrixed surround signals came with some limitations. The surround channels in basic Pro Logic were not in stereo and had a limited bandwidth. That means that each speaker played the same thing and the sound didn’t involve much bass or treble information.
Dolby Digital 5.1 / AC-3: The benchmark
Remember Laser Discs? Though the medium was first invented in 1978, it wasn’t until 1983 when Pioneer Electronics bought majority interest in the technology that it enjoyed any kind success in North America. One of the advantages of the Laser Disc (LD) is that it provided a lot more storage space than VHS tape. Dolby took advantage of this fact and created AC-3, now known better as Dolby Digital. This format improved on Pro-Logic in that it allowed for stereo surround speakers that could provide higher bandwidth sound. It also facilitated the addition of a low-frequency effects channel, adding the “.1” in 5.1, which is handled by a subwoofer. All of the information in Dolby Digital 5.1 is discrete –no matrixing necessary. With the release of Clear and Present Danger on LD, the first Dolby Digital surround sound began to hit home theaters. Even when DVDs came out in 1997, Dolby Digital was the default surround format. To this day, Dolby Digital 5.1 is considered by many to be the surround sound standard, and is included on most Blu-ray discs.
Image courtesy of Dolby
DTS: The rival
What’s a technology market without a little competition? Up to a point, Dolby more or less dominated the surround-sound landscape. Then, in 1993, DTS came along. DTS, which stands for Digital Theater Systems, is another company providing digital surround sound mixing services for movie production. DTS surround sound was first heard in theaters with Jurassic Park. The technology eventually trickled down to LD and DVD, but was initially available on a very limited selection of discs. DTS utilizes a higher bitrate and, therefore, delivers more audio information. Think of it as similar to the difference between listening to a 256kbps and 320kbps MP3 file. The quality difference is noticeable, but according to some, negligible.
6.1: Kicking it up a notch:
In an effort to enhance surround sound by expanding the “sound stage,” the addition of a sixth speaker to the original 5.1 configuration brought about 6.1. This sixth speaker was to be placed in the center of the back of a room and was subsequently referred to as a back surround or rear surround. This is where a lot of confusion began to swirl around surround sound. People were already used to thinking of and referring to surround speakers (incorrectly) as “rears,” because they were so often seen placed behind a seating area. Recommended speaker placement, however, has always called for surround speakers to be placed to the side and just behind the listening position. The point of this speaker is to give the listener the impression that something is approaching from behind or disappearing to the rear. It pulls off with more success what the side surrounds attempted to do by working together to fool the ear. Calling the sixth speaker a “back surround” or “surround back” speaker, while technically an accurate description, ended up being just plain confusing.
To make things even more confusing Dolby Digital/THX and DTS offered different 6.1 versions. Dolby Digital and THX collaborated to create a version that is referred to as “EX” or “surround EX.” Information for the speaker is matrix encoded into the left and right surround speakers. DTS, on the other hand, offered two different 6.1 versions. DTS-ES Discrete and DTS-ES Matrix performed as their names suggested. With ES Discrete, specific sound information has been programmed onto a DVD or Blu-ray disc, rather than simply extrapolating information from the surround channels.
7.1: The spawn of Blu-ray
As is often the case in the world of electronics, just when you decide to bite the bullet and buy a new piece of equipment, something new comes out that leaves your shiny new gear in the dust. Such was the case when 7.1 audio was introduced in conjunction with the HD-DVD and Blu-ray discs. Just when people started getting used to 6.1, 7.1 came along as the new must-have surround format.
Like 6.1, there are several different versions of 7.1. All of them add in a second back surround speaker. Those surround effects that once went to just one rear surround speaker can now go to two speakers which happen to be in stereo, too. The information is discrete, which means that every speaker is getting its own specific information. We can thank the massive storage potential of Blu-ray for that.
Dolby offers two different 7.1 surround versions. Dolby Digital Plus is the “lossy” version which, still involves data compression and takes up less space on a Blu-ray disc. Dolby TrueHD, on the other hand, is lossless. Since no compression is involved, Dolby TrueHD is intended to be identical to the studio master.
Image courtesy of Dolby
DTS also has two 7.1 versions, which differ in the same manner as Dolby’s versions. DTS-HD is a lossy, compressed 7.1 surround format, whereas DTS-Master HD is lossless and meant to be identical to the studio master.
It is important to note here that 7.1-channel surround mixes are not always included on Blu-ray discs. Movie studios have to opt to mix for 7.1, and don’t always do so. There are other factors involved too. Storage space is chief amongst them. If a bunch of extras are placed on a disc, there may not be space for the additional surround information.
In many cases, a 5.1 mix can be expanded to 7.1 by a matrix process in an A/V receiver. This way, those back surround speakers get used, even if they don’t get discrete information.
9.1: Pro Logic makes a comeback
If you’ve been shopping for a receiver, you may have noticed that many offer one or more different versions of Pro Logic processing. In the modern Pro Logic family, we now have Pro Logic II, Pro Logic IIx and Pro Logic IIz. Let’s take a look at what each of them does.
Pro Logic II is most like its early Pro Logic predecessor in that it can make 5.1 surround sound out of a stereo source. The difference is Pro Logic II provides stereo surround information. This processing mode is commonly used when watching non-HD TV channels with a stereo audio mix.
Pro Logic IIx is one of those processing modes we mentioned that can take a 5.1 surround mix and expand it to 6.1 or 7.1. Pro Logic IIx is subdivided into a movie, music and game mode.
Pro Logic IIz allows the addition of two “front height” speakers that are placed above and between the main stereo speakers. This form of matrix processing aims to add more depth and space to a soundtrack by outputting sounds from a whole new location in the room. Since IIz processing can be engaged with a 7.1 soundtrack, the resulting format could be called 9.1.
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Audyssey DSX: A lesson in one-upmanship
Audyssey, a company best known for its auto-calibration software found in many of today’s A/V receivers, has released Audyssey DSX, which also allows for additional speakers beyond the core 5.1 and 7.1 surround formats. With the addition of front width and front height channels on top of a 7.1 system, it is possible to achieve up to 11.1 channels of surround sound.
AVR flavors of surround: The Malt-O-Meal of surround
Some receiver manufacturers, rather than purchase the rights and processors for Dolby or Audyssey’s products, are making their own versions of surround-sound expansion. Since this is a developing segment, we’re not going to get into this topic much further than to say that those interested should check the manufacturer’s website to discover what that company’s approach involves.
What about 7.2, 9.2 or 11.2?
As we mentioned previously, the “.1” in 5.1, 7.1 and all the others refers to the LFE (low frequency effects) channel in a surround soundtrack, which is handled by a subwoofer. Adding “.2” simply means that a receiver has two subwoofer outputs. Both connections put out the same information since, as far as Dolby and DTS are concerned, there is only one subwoofer track. Since A/V receiver manufacturers want to easily market the additional subwoofer output, the notion of using “.2” was adopted.
3D / object-based surround sound
The latest and greatest in surround sound moves beyond the two dimensions that can be represented in traditional setups. By adding either ceiling-mounted or ceiling-facing speakers, height can now be represented as well, leading to an extra number used to represent channels. A 5.1.4 system, for example, would feature the traditional five surround speakers and a subwoofer, but would also feature four additional speakers adding height information to a signal.
The name object-based comes because with this third dimension, the audio mixers working on a film can represent objects in 3D space rather than being limited by a standard channel setup.
This shouldn’t come as a surprise after reading the rest of this article, but the current most-used object-based surround sound technology is Dolby Atmos. Atmos can process up to 128 distinct objects in a given scene (compared to, say, 8 channels for Dolby Digital 7.1), that can be routed to up to 64 speakers. In the past, if there was an explosion on the right of the screen, half of the theater would have the same sound. With Atmos, the sounds in a theater will come from different locations based on where they’re placed by movie studio audio mixers.
Atmos premiered in theaters in 2012 with Pixar’s Brave and began to be available in home theater A/V receivers in 2015. Already, a fair number of movies in both digital format and on Blu-ray discs are available with Dolby Atmos.
Related: Dolby Atmos Review
Just as with other type of surround sound, DTS has its own version with DTS:X, which was unveiled in 2015. While Dolby Atmos limits objects to 128 per scene, DTS:X imposes no such limits, though whether film mixers are finding themselves bumping up against Atmos’ limit isn’t known. DTS:X aims to be more flexible and accessible than Atmos, making use of pre-existing speaker layouts in theaters and supporting 32 different speaker locations at home.
While DTS:X is beginning to be available out of the box with newer A/V receivers, firmware updates are available bringing the technology to many receivers that initially shipped with only Dolby Atmos support. Companies like Lionsgate and Paramount have promised support for DTS:X, but for the time being, it remains less popular than Atmos.
It may not be as well known as Atmos or DTS:X, but Auro-3D has been around for much longer than either one of them. The technology was first announced in 2006 and has been used in theaters since, though it has only recently started to come to home theater systems with higher-end companies like Marantz and Denon offering it as a firmware upgrade — usually a paid upgrade.
Auro-3D doesn’t use the term “object-based” as its competitors do, but it does work in a similar way and offer similar results, adding to the overall immersion factor when watching a film. Auro-3D’s recent foray into viewers’ living rooms isn’t likely to snatch away the 3D surround sound crown from Dolby, but considering it’s already 10 years into its run, chances are it will continue to hang in there.
Update: This article was originally written on July 31, 2013 and has been updated to include the newest in surround sound technologies, like Dolby Atmos and DTS:X. Kris Wouk contributed to this article.