When stereo sound came out in the '50s, it must have blown everyone away. After all, compared to the flat monophonic standard of the day, stereo presented recorded music that sounded more like a live performance. Instead of hearing a weak reproduction from a single speaker, you could close your eyes and envision your favorite band twanging, banging and yodeling right in front of you. That proper staging and imaging all shockingly came from two speakers. Despite the incredible abilities of stereo sound, it usually lacks one thing: the ambient information that would surround you during a live performance. It can sculpt a highly realistic stage in front of you, but it doesn't build anything around or behind you. That's where surround sound steps in.
Foundations: Stereo And Dolby Surround
Today different companies offer a variety of surround sound technologies. Despite the confusing barrage of formats, they all utilize the same basic concepts and techniques. Before we even blurt out the words algorithm or encoder, let's check our foundations: how sound is recorded. For traditional stereophonic sound, recording engineers can use two microphones or more. For a four-mic setup, engineers would mix the recorded front left, center, front right and rear surround into the 2-channel recordings that we nowadays take for granted. These stereo recordings improved so much upon mono that audio nuts should have been too aurally satiated to look for something better. Nonetheless, some surround sound technologies (e.g., quadraphonic sound, for which Dolby Pro Logic II creator Jim Fosgate developed the Fosgate Tate 101 quad decoder) were developed before their time. Then, in the '70s Dolby Laboratories came up with a practical way to bring ambient noise to a movie theater, Dolby Stereo.
Like some stereo recordings, Dolby Stereo takes four recorded channels and mixes them into an analog stereo recording. However, a decoder then turns around and splits the 2-channel recording into four channels (we'll discuss this process later). To bring Dolby Stereo out of the movie theater and into the home and car, Dolby introduced Dolby Surround and the Pro Logic decoder in 1987 (it was introduced in the car a decade later). You'll still find Pro Logic on older equipment and Dolby Surround tracks are still found on movies, but the world has gone digital and audio has gone with it.
The Emergence Of Digital: Dolby Digital And Dts Surround Sound 5.1
Digital formats (e.g., CDs and DVDs) allow engineers to pack much more data into a smaller space. With Dolby Surround Pro Logic, the decoder had to split the 2-channel signal into four channels due to lack of storage space on analog media. With digital, you have the luxury to send more channels (and more audio information) in separate signals. The discrete channels deliver more active audio information to a listening setup than Pro Logic's matrix sound provides. Two companies, Dolby and Digital Theater Systems (now DTS), introduced discrete 5.1 digital surround for movies within a year of each other: Dolby Digital in 1992 and DTS Surround Sound 5.1 in 1993. Both technologies deliver encoded sound to 5.1 channels-left, center, right, left surround, right surround and the low-effects channel (AKA, the rumbling explosion channel).
DD and DTS rely upon material encoded specifically for their systems. Therefore, the process begins with the master recording. Instead of four original channels like with Dolby Surround, 5.1 encoded material is mixed from masters that have five channels: left, center and right plus two separate surrounds. Sounds simple enough, but there's a problem. Despite the greater capacity of digital formats, there's still not enough room on the media to transmit all of the audio. That's where encoding comes into play.
All digital formats must be encoded. For example, CDs take the analog audio wave and encode it using 16-bit PCM (pulse code modulation). It takes samples of the wave 44,100 times per second at 65,536 levels or gradations. The coded numbers don't match the analog wave exactly, but it's so close that it includes all the audio most human ears can identify (some audiophile holdouts point out the coldness of CDs and keep a tight death grip on their vinyl record collections and eight-tracks). Sixteen-bit PCM packs a lot of data for 2-channel stereo. Imagine adding 3.1 channels along with the massive digital video track. There isn't enough room on a DVD, so perceptual or lossy coding systems were developed.
The basic principle behind perceptual or lossy coding is that a master recording has duplicate audio materials and sounds that the human ear can't make out. Perceptual or lossy coding tosses those sounds out and only includes data that recreates the sound we actually perceive. Therefore, some of the signal is "lost" (it's called "lossy" for a reason). To actually do this, encoders utilize algorithms, coding schemes developed to utilize psychoacoustic principles, which decide what to code and how. Audio signals of each channel are fed through the encoder and split into different frequency bands and sub-bands. The algorithms analyze the bands and decide what is perceptually irrelevant data while making sure that enough bits are retained to recreate the signal. After all the analysis, bit-management routines and data packing, we have encoded material.
Both DD and DTS 5.1 use the same sort of techniques to reduce the amount of bits needed to reproduce sound, but that's where the similarity ends. Their algorithms and coding differ. Dolby argues that its encoder is more powerful than DTS', enabling it to more efficiently provide excellent sound quality at lower bit rates. Those bit rates generally range from 384kbps to 448kbps. Whereas for DTS, bit rates range from 768kpbs up to 1536kpbs. Obviously with DTS there is much less compression, up to four times less when you compare the lower Dolby Digital rates with the highest DTS rates. Can Dolby Digital's claim of more efficient coding offset the disparity in bit rates? You'll have to listen for yourself. Some people think they sound roughly the same; others perceive DTS as sounding better. While many DTS tracks have wowed me with excellent dynamic range and frequency response, I have also been astounded by superbly mixed Dolby Digital soundtracks.
Newer Releases: Dolby Digital Ex & Dts Es
Dolby and DTS haven't been sitting around twiddling their thumbs since introducing DD and DTS 5.1 in the '90s. Expanding on their core offerings, extended versions bring an additional surround channel to the listening experience - the center rear. When sound circles the listening space, the center rear channel makes up for any slack between the two rear surrounds. This results in better directionality. Dolby Digital EX takes the rear channels and, through the decoder, extracts the additional matrix channel. Some DVDs are also encoded with Dolby Digital EX to provide a discrete sixth channel. The DTS version, DTS-ES (Extended Surround) performs similarly. It either uses DTS-ES encoded material to provide a discrete sixth channel, forms a matrix signal from 5.1 material for the sixth channel, or creates a phantom center channel from the existing two rear surround speakers.
Enter The Matrix: Dolby Pro Logic, Dts Neo:6 & Srs Circle Surround
Initially, 5.1 surround sound could be heard only if you played 5.1 recordings. If you only had analog stereo recordings - tough luck, your surround speakers would get no action. With sophisticated audio technology being further developed, this wouldn't do for long. Dolby Labs and DTS came up with their answers: Dolby Pro Logic II and IIx, and DTS Neo:6.
Extracting a full surround sound signal from 2-channel material involves feeding the left and right channels into a decoder that analyzes the signal. The original Dolby Pro Logic detects the dominant signal from four cardinal vectors (center, left, right and surround) by measuring power levels from the incoming signal. Once the dominant channel is determined, the other sounds are distributed to the appropriate channel using signal canceling. Take this example: If the dominant signal is the center channel, the decoder takes the left channel and phase inverts it by 180 degrees. The decoder then adds it to the right channel, thus canceling out the center channel component in the right channel. It does the same to the left channel. The end result is that the left and right channels have a copy of each other but none of the center channel, yet the center channel contains components of both right and left. Voltage control amplifiers (VCAs) control the inverted copies in the right and left channels, maintaining a constant level in the soundtrack while allowing for spatial separation.
Taking up from where Dolby Pro Logic left off, Dolby Pro Logic II pulls 5.1 channels from stereo and Dolby Surround as opposed to just four channels from Dolby Surround. Instead of four cardinal vectors, it has six (an additional left surround and right surround), but the biggest difference is it employs a feedback system. This system allows Pro Logic II to create an even greater separation between channels. Dolby provides the example of dialogue placed between the center and right channels. Previously, the dialogue would not be cancelled completely from the surround channels because the level in the left total channel is less than that in the right total. However, with Pro Logic II, a feedback "servo" circuit compares and adjusts the VCA to equalize the two channels and then creates the center channel output by adding the now-balanced VCA signals. The process involves more technical details (if you're dying to know everything, download some technical pdfs from Dolby's website), but the basic end result is that crosstalk between channels is reduced and channels have greater separation.
Now don't forget DTS also has its answer, DTS: Neo:6. As with previous surround sound technologies, the basic concepts and techniques behind Neo:6 and Pro Logic II are the same. Neo:6 will generate a multi-channel mix dependent on what kind of speaker setup you may have, from two channels with a phantom center to five channels to six channels. For 6-channel capability from Dolby, Dolby Pro Logic IIx steps up to the plate. It even goes further by enabling 7.1 sound from stereo or 5.1 mixes. The speaker lineup now goes like this: left, right, center, left surround, right surround, left rear and right rear (don't forget the sub)-how many speakers can a home theater (much less a compact space like a car) hold?
DTS and Dolby Labs aren't the only kids playing on the surround sound field. SRS (Sound Retrieval System) Labs' Circle Surround works with Circle Surround encoded material in the same manner that DTS and DD work with their own encoded material. But like Dolby Pro Logic II and DTS Neo:6 it also creates a surround sound mix through matrix decoding. The extended version, Circle Surround II will decode up to a 6.1 channel mix. CS and CS II use technologies similar to DTS' and Dolby Digital's, the complex decoding systems briefly described in the preceding paragraphs. What makes Circle Surround particularly special is its steering generator, which directs surround information to the appropriate channels. While maintaining channel separation, it smoothes out sound as it pans from left to right or front to back. Rather than dipping as it travels from speaker to speaker, the sound maintains constant output, immersing the listener in the sound field.
Specifically for vehicles, SRS offers Circle Surround Automotive. Including the same technologies as Circle Surround, it includes additional features to tackle challenges posed by a car's interior. For instance, if you don't have subwoofer or two installed in your ride, TruBass makes up for it. It uses psychoacoustic techniques to create the perception of deep bass. By boosting a series of low bass harmonics, speakers that can't go very low appear to the ear to produce very low frequencies. Another challenge cars pose is low speaker mounting locations. Since our ears aren't located on our calves, SRS Focus elevates the soundstage. It does this using the science of HRTFs (head-related transfer functions), special filters that have a frequency response with curves that model our auditory systems. SRS explains that by manipulating the sound contour from speakers, our ears can interpret the sound from coming from a specific angle or location. Circle Surround Automotive has a number of other features that we just don't have room to go into more detail on, including the phantom center speaker.
Phantom Speakers: Srs Trusurround XT
The way matrix decoding creates additional channels out of a 2-channel source is rather impressive, but what about creating the impression of multiple speakers where in reality only a 2-channel setup exists? SRS has an answer: TruSurround technology. It takes up to 6.1 channels of discrete information from any multi-channel source and mixes it for two channels so that those two channels sound like five or six speakers. TruSurround XT of course includes TruBass to make up for the lack of a subwoofer. It also utilizes WOW technology to handle non-surround material.
WOW creates a sound field, rather than a mere soundstage, by using those good old HRTFs. This means that anywhere you sit, you get the same listening experience. There's no sweet spot, no best seat in the house. After extracting ambient information, SRS's 3D technology runs it through a frequency response correction curve that restores the appropriate location of the sounds and places them relative to dominant sounds. This creates a three-dimensional sound field. WOW technology can also provide this sound field in something as simple as headphones. Dolby Headphone gives a surround sound experience from headphones as well.
HIGH DEFINITION: MLP Lossless, DTS 96/24 & DTS-HD, Dolby Digital Plus And Dolby Truehd
Although surround sound mates perfectly with watching movies, it also has a growing place with musical recordings. To bring high-definition audio to DVD-Audio Dolby utilizes MLP Lossless. This lossless (opposite of lossy) format reproduces sound bit for bit, without discarding additional information like lossy formats do. In the recording studio, producers record up to six channels of 96kHz/24-bit audio (or two channels of 192kHz/24-bit audio). DTS named their version more to the point: DTS 96kHz/24 bit or just DTS 96/24. It's backwards compatible with older decoders that decode at 48kHz.
With the introduction of HD-DVD and Blu-ray technologies, DTS and Dolby have stepped up with even higher definition sound for movies. HD-DVD and Blu-ray have much more storage space than DVDs, allowing not only full HD video but audio reproduction that is exactly as the moviemaker intended. On Blu-ray discs, DTS-HD Master Audio delivers sound at up to 24Mbps on 7.1 channels. Dolby TrueHD and Digital Plus will come standard on HD-DVD and optional on Blu-ray discs. TrueHD delivers up to 18Mbps of 96kHz/24-bit audio for eight full-range channels. Digital Plus will support data rates as high as 6Mbps with a minimum of 3Mbps on HD-DVD and 1.7Mbps on Blu-ray.
Surround sound specialists like Dolby, DTS and SRS don't have a monopoly on the blank-point-one market. Audio manufacturers have been working on their own to bring 5.1 and more into the car and home. For example, Yamaha and Polk Audio both have home systems that create a surround sound experience from a single speaker "bar" placed in front of the listening area. Harman Kardon's Logic 7 works like Neo:6 or Pro Logic IIx to convert stereo into multi-channel sound. Bose has also taken on the car audio arena with their Centerpoint technology. Even if manufacturers don't pursue their own surround sound technology, more of them sign up with SRS, DTS and Dolby every year. Like stereo decades ago, surround is fast becoming what people want. The number of products and vehicles with surround capability continues to grow and the trend for more channels builds steam as well - poor old mono.
Dolby
Aston Martin: DB9, Vantage
Cadillac: STS, XLR, SRX, Escalade, ESV, EXT
Jaguar: XK
Lexus: GS, ES, IS
Maybach: Maybach
Volvo: S40, S60, S80, V50, V70, XC70, XC90, C70
Land Rover: LR2, Freelander
Acura: TL, RL, RDX, MDX
Infiniti: M45
Chevy: Suburban, Tahoe
Mercedes: S-Class
Acura: TL, RL
Cadillac: STS
Lexus: GS, IS, LS
Mercedes: S-Class
Infiniti: M series
Land Rover: Range Rover
Toyota: FJ Cruiser, Avalon, Sienna, Camry, Matrix
Honda
Mitsubishi
Mazda
Daihatsu
Subaru
Acura: RL
Audio: A6, Q7, A8, A8L
Cadillac: STS, SRX, Escalade/ESV/EXT, DTS
GMC: Yukon Denali, Denali XL, Sierra Denali
Infiniti: M
Maybach: Maybach
Mazda: CX-7
Porsche: 911, 911 Carerra GT, Boxster, Cayenne, Cayman/Cayman S
Acura: TL, NSX, 3.5RL, TSX Sports Sedan, RSX Sports Car, MDX
Mercedes: all models except SL
BMW: 1, 3, 5, 6 and 7 series
Land Rover: Range Rover, Range RoverSport, Discovery/LR3
Note: Not all the listed systems are discrete 5.1 but rather matrix surround sound