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Ed's AV Handbook
Batting Bractice for the AV Professional
and primer for the novice

Chapter 1  Page 3

AV Terminology
Surround Sound

Multi-spk River & Canyou illustration

A friend commented that he appreciated the surround sound descriptions and speaker layouts outlined below.  But he still did not clearly understand surround sound.  And he asked, why are there so many format acronyms?  My attempt at explanation began with the use of stereo audio in a movie theater. 

Stereo audio can be effective if a viewer sits in a theater seat near or beyond the stereo sweet spot location.  If they sat closer plus well left or right of the sweet spot as they viewed an actor screen center, they heard their voice emanating from the edge of the screen.  This can be disorienting.

Dolby Labs popularized a solution, a surround sound format that radiates the sound of screen images from their screen location.  They direct the sound of screen-left images to a left-placed speaker, screen-right images to a right speaker, and centered-images to a center speaker.  Plus, as the name implies, they installed rear channel speakers that allow for surrounding enveloping effects.  Star-ships fly front to back, left to right, diagonally, and circularly around the theater.  The addition of a dedicated low-frequency effects channel rocks theater audiences with exploding concussive sound. 

Home Theater Surround Sound Formats

The balance of this page outlines the home-theater surround sound formats of analog Dolby, Dolby Digital, and DTS.  Chapter Six addresses speaker and listener placement in more detail.

Analog Dolby Surround

Analog Dolby Surround is the original home surround sound format.  Some may recall SQ and QS quadraphonic LP records.  They concealed rear channels within a stereo recording.  Analog Dolby Surround uses a similar method. 

The original Dolby Surround home sources include the HiFi Beta videotape, the HiFi VHS videotape, and the LaserDisc.  Analog Dolby surround sound was available in two versions, Dolby Surround and Dolby Pro Logic.

Dolby Surround routes stereo audio through a circuit that takes the difference between the left and right stereo channels.  Dolby routes the difference, a monaural and low-frequency limited (almost no bass) channel, to a left and a right rear placed speaker. The circuit does not affect the reproduction of the front left/right stereo audio.

Dolby Pro Logic adds the movie theater center channel, the sub-woofer channel, and modifies the left and right front channels to set the left/right audio edges of the image.  The rear-channel is as Dolby Surround.  The following details the logic used to create Pro Logic Surround Sound via a Dolby Labs integrated chip.

= (left channel - right channel)

= [(left channel + right channel) - (rear channel)]

Left Edge
= (left channel - center channel)

Right Edge
= (right channel - center channel)

= low pass filtered (left channel + right channel)

Dolby Surround Spk Layout  Dolby Pro Logic Spk Layout

Digital Dolby and DTS Surround Sound.

Digital Surround Sound eliminates difference matrix processing.  Each audio channel is recorded as a discrete channel and routed to a dedicated speaker. 
The DVD, the BluRay disc, and Internet streaming employ the Dolby and DTS digital formats.  Broadcast satellite, cable, off-air ATSC, and soon ATSC 3.0 NextGen TV use Dolby formats.  Dolby Digital and DTS are available in the following versions.

Dolby Digital

Dolby Digital 5.1
Dolby Digital 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. 

Let me emphasize that all of the five main channels are full range - 20Hz to 20Khz - audio.  Large rear speakers can reproduce exciting low-frequency bass effects that small rear speakers cannot.

Dolby Digital EX 6.1 & 7.1
Dolby 6.1 and 7.1 provide more rear fill in a larger room.

Dolby 6.1 provides a 6th discrete, or matrix-derived from a 5.1 mix, center rear channel.

Dolby 7.1 duplicates the 6.1 rear center channel and routes it to another rear placed seventh speaker.

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 channel to Pro Logic II derived from stereo movie soundtracks.


DTS employs the same layouts as Dolby.  The difference -- Dolby protects small home theater center speakers by limiting center channel dynamics with the complementary limiting of the front left and right channels.  DTS does not.  Therefore, DTS has the potential to generate more dynamic playback.  Also, many point out that Dolby compresses 5.1 digital audio to 640kbit/sec, while DTS supports a higher bit rate of 1.5Mb/sec.  Higher bit rate may extend the DTS advantage.  

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

Dolby 5.1 LayoutDolby 6.1 LayoutDolby 7.1 Layout

Cartoon Surround Layout

Multi-channel and the Blu-ray Disc 

Blu-ray offers the Dolby HD & DTS HD Master Audio lossless surround sound formats, 
Blu-ray also offers the Dolby Digital Plus and the DTS HD lossy surround formats.
Each offers the 5.1, 6.1, & 7.1 layouts.
And each requires version HDMI 1.3 or higher.
Chapter 3 discusses the subject of lossless and lossy digital audio formats.

Immersive surround from above

Dolby Atmos Spk Layout

2014 Dolby Atmos adds two or four overhead 'height channels' to the home theater 5.1, 6.1, and 7.1 layouts. 
The Atmos process works via an 'object-oriented' effect.  Digital 'tags' are assigned to film objects in the sound mix directed to specific speakers.

2015 DTS:X introduced the object-oriented tags and above channels.

Blu-ray players include two decode options. Decode via the player or the AV receiver.  However, Dolby Atmos/ DTS-X are decoded only if the Blu-ray player is set to bitstream and decoded by the receiver.  Set the player to Bitstream and connect via HDMI. 

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Ed's AV Handbook   
Copyright 2007 Txu1-598-288 Revised 2024

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