Home > Articles > Home & Office Computing

How Surround Sound Works

In the first part of his series on surround sound, Michael Miller explored the history of the technology. Good background info, but how exactly does sound from a surround sound system "surround" you? He explains in this article that it's actually less complicated than you might think.
Like this article? We recommend

Like this article? We recommend

By definition, surround sound is sound that surrounds you—in particular, music or movie soundtracks that come at you from all sides, thanks to multiple audio channels fed through speakers positioned in all corners of the room. In a home theater system, the surround sound is initially encoded onto a programming source—a DVD, a cable or satellite broadcast, and so on. When the programming is played, the surround sound tracks are extracted from the source by a piece of electronics called a surround decoder. The individual surround channels are then amplified and fed to the appropriate speakers. The result is sound that envelops you as you watch your movie or listen to your CD.

How do all those channels of audio get from their source to your ears? It's all a matter of technology. Two primary technologies are used for surround sound today: the older matrix surround, typified by Dolby Pro Logic, and the newer discrete surround, as used in the Dolby Digital system. We'll look at both, starting with the oldest first.

Matrix Surround

The original home surround technology tried to pull off the impossible—turning two things into four. That's because, at the time (we're talking the early 1990s here—a virtual lifetime ago), the videotapes and laserdiscs we were watching barely had space to carry two stereophonic channels, let alone the four channels necessary for a complete surround experience. Engineers could fit right and left channels onto the tape or the disc, but no more than that. So where to put the extra channels?

To stuff four channels of information into the space normally used by two, the engineers at Dolby Laboratories used a matrixing technology. This enabled them to combine four streams of information into two tracks, and then retrieve the original four channels on playback—sometimes referred to as a 4-2-4 processing system. (They took four channels, crammed them into two channels, and then separated them into two channels again: 4-2-4.)

Does that sound a little tricky? It was. Imagine mixing streams of red, green, blue, and purple sand together in a bucket, carrying the bucket across a room, and then trying to extract the individual colors at the other end. Messy at best, and perhaps even impossible to separate into the original colors.

The Dolby engineers figured out how to do it, however. By analyzing the audio information contained within the left and right channels, they could identify the information that was the same in both channels. They realized that they could mix information from a third channel into both the left and right channels, compare the two channels, and then extract that information (the third channel) that was identical in the two channels. They could even get a fourth channel into the mix, by recording it out of phase with the third channel and then also feeding it to the left and right channels.

Using this approach, the Dolby engineers devised the following data streams:

  • Stream A (left channel)

  • Stream B (right channel)

  • Stream C (the data that's identical in streams A and B)

  • Stream D (the difference between the data in streams A and B)

Obviously, streams A and B carried the normal left and right channel information. The engineers used stream C to carry center channel information, and stream D to carry information for the surround channel. (This technology only allowed for a single surround channel, even though this single channel was often fed to two rear speakers.)

Here's how the whole thing works. Left channel information is fed into data stream A, and right channel information into stream B. Center channel information is placed into both the A and B streams—thus resulting in the "identical" stream C. The surround channel information is also recorded on streams A and B, but placed out of phase to the second stream. This creates stream D, the "difference" stream.

Thus, the videotape, videodisc, or television program—anything that can carry a stereo signal—is encoded with these four data streams. If you listen to the program through a normal stereo audio system, it sounds like normal stereo—the center channel information comes out (in equal portions) through both the left and right channels, as does the surround channel information. No one knows the difference; the positioning of the new streams is effectively "hidden."

But when you run the program through a surround decoder, everything changes. The decoder passes the data in streams A and B directly to the left and right amplifiers. It identifies the identical data between these two streams and extracts it into the decoded stream C, which is sent to the center channel amplifier. The decoder then identifies the differential data in streams A and B (the out-of-phase information), shifts them relative to each other so they're back in phase, and sends the stream to the surround channel amplifier.

The only bad thing about this process—called Dolby Pro Logic—is that it's not terribly precise. That's because when everything gets all mixed together, it doesn't always separate out completely, resulting in some degree of leakage from one channel to another. But the results are surprisingly effective—particularly when the surround channel contains primarily ambient effects. However, if you want to clearly position a sound source in the surround channel, the effect is less than impressive; no matter how effective the decoder, there will always be some duplication of the sound in the front channels, which spoils the effect. How, then, to better separate the surround channel sound?

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020