Ed's AV Handbook.com
Home Theater & High Fidelity Stereo Audio

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AV Terminology

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Batting practice for the audio/video pro and a primer for the novice

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Surround Sound
          A friend commented that he appreciated the surround sound descriptions and speaker layouts outlined below.  But he still did not clearly understand surround sound.  In other words, why multiple channels?
          Let's begin with the early attempts to use stereo audio in commercial movie theaters.  
It was successful if a viewer sat in a seat at the stereo 'sweet spot' location or beyond.   However, if they sat closer, plus well left or right of the 'sweet spot',
and viewed an actor screen center; they then heard the actor's voice emanating from the edge of the screen. Audiences found this disconcerting.
          Solution -- the sound of screen images now radiate from their screen location.  Screen left images are directed to the left speaker, screen right images to the right speaker, and center images to a center placed speaker.  
          Theater audio improvements continued with s
urround channels via rear placed speakers that allow for 'cool' effects.  Star ships fly front to back, left to right, diagonally,
and circularly around the the room.  
A dedicated low frequency effects channel was also added that rocks theater audiences with exploding concussive sound.  
he latest Dolby Atmos/DTS-X commercial theater formats now offer up to 64 'object oriented' channels that include above height channels.   Surround sound has come a long way since its early use in 1940.     

Home Theater Surround Sound Formats
          The balance of this page outlines the home theater surround sound formats of Analog Dolby Surround, Dolby Digital, and DTS.  Chapter Six covers speaker and listener placement in more detail.
Analog Dolby Surround
          Analog Dolby Surround is the original home audio surround sound format.  Some may recall vinyl LP quadraphonic audio.  'Quad' concealed additional rear audio channels within a stereo recording.  Analog Dolby Surround is a similar format.  The HiFi Beta VCR, the HiFi VHS VCR, and the Laser Disc were the original home audio sources of Dolby Surround. Dolby surround sound was available in two versions: Dolby Surround and Dolby Pro Logic.

          Dolby Surround routes stereo audio to a circuit that takes the difference between the left and right stereo channels.   This matrix difference is a monaural and frequency limited (almost no bass) channel routed to left and right rear placed speakers.

        Dolby Pro Logic is also matrix derived from a stereo recording.  But Pro Logic adds a center channel, a low pass filtered sub woofer channel, and modifies the left and right channels to set the left/right 'audio edges' of the image.  The rear channel is produced as Dolby Surround.  
          The following outlines the matrix logic that created all five channels from two channels of audio via a digital sound processing integrated chip:
                - Rear = (left channel - right channel)
                - Center = [(left channel + right channel) - (rear channel)]
                - Left edge = (left channel - center channel)
                - Right edge = (right channel - center channel)
                - Sub woofer = low pass filtered (left channel + right channel)

Digital Dolby & DTS Surround Sound      
          Digital surround (for the most part) eliminates 'difference' matrix processing.  
channel is recorded as a discrete channel routed to a dedicated speaker.  The DVD
and Blu-ray
disc employ the Dolby and DTS formats.  Broadcast satellite, cable, and off-air TV use Dolby formats.  Dolby Digital and DTS are produced in the following versions.

Dolby Digital 5.1

          Dolby 5.1 consists of five discrete full range audio channels -- front left/center/right, rear left/right -- plus the 'point 1' low-frequency effects sub woofer channel.

Dolby Digital EX
6.1 & 7.1 ... more rear channels!
          Dolby 6.1 adds one rear center channel recorded discretely or matrix derived from a 5.1 recording.  Dolby 7.1 simply duplicates the 6.1 rear center channel and routes it to another rear placed seventh speaker.  6.1 and 7.1 add more “rear fill” in a larger room.

Dolby Pro Logic II

Pro Logic II creates 5.1 surround sound from stereo movie soundtracks.  

Dolby Pro Logic IIx

Pro Logic IIx adds a center-rear surround channel to 5.1 surround sound derived from stereo movie sound tracks.

(Dedicated To Sound -- formerly Digital Theater Systems.)
uses the same layouts as Dolby.  The difference is Dolby protects small home theater center speakers by limiting center channel dynamics with a complementary limiting of the front left and right channels.  DTS does not.  Therefore DTS has the potential to generate more dynamic playback.

DTS ES 6.1 & 7.1
DTS EX 6.1 & 7.1 offer the same additional rear center channel arrangements as Dolby.

Multi-channel and the Blu-Ray Disc
Lossless DTS HD Master Audio & Dolby HD
Lossy DTS HD & Dolby Digital Plus

Dolby HD and DTS HD Master Audio are lossless digital processing formats that offer almost bit for bit recreations of the original studio master recording.  The concept of lossless recording is addressed in Chapter Three.  The important point: lossless processing is a significant step forward in fidelity.
Dolby Digital Plus
and DTS HD are lossy digital formats.  Lossy formats are a step down from the lossless process.  But they offer improved performance over standard Dolby Digital and DTS.
 Lossy is covered in Chapter Three.  

2014 Dolby Atmos adds two to four overhead "height  channels" to the home theater  5.1,  6.1, and 7.1 layouts.  Atmos is referred to as an "object oriented" effect.  Digital 'Tags' are assigned to objects in the sound mix and directed to a specific speaker.

Dolby Atmos also offers an optional version with 32 channels.  It consists of 24 speakers on the floor paired with 10 overhead speakers.

- DTS  joined the object oriented with above channel surround sound party.

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Ed's AV Handbook.com
Batting Practice for the AV Pro and a Primer for the Novice.
Copyright 2007 Txu1-598-288   Revised 2018