Guide to Home Theatre®, Technical Editor Thomas J. Norton cleared away a lot of unanswered questions

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Circle Surround® 5-2-5™ Applications Notes No. 2

DVD 5.1 Channel Playback using a Circle Surround 5-2-5 Decoder by Henry J. Root

From the Behind the Scenes section in the Fall, 1997, issue of Stereophile Guide to Home

Theatre®, Technical Editor Thomas J. Norton cleared away a lot of unanswered questions

and misinformation that has been fostered in other publications in the following excerpt in

which he admonishes us to Read the Fine Print: DVD and AC-3.

"Early adopters of the new DVD format are about to discover-if they haven’t already - that

not all DVD titles contain discrete 5.1 channel Dolby Digital® soundtracks. Instead, some

discs have good old-fashioned matrixed Dolby Surround mixes, which must be decoded by

Dolby ProLogic (or Circle Surround®, ed.) circuitry for users wanting surround-sound

playback. A few titles will even be monophonic.
"What’s that?" You thought all DVDs sold in North America had soundtracks encoded

using Dolby's AC-3 data-compression algorithm? Well, you're right-they do. We’ve become

so accustomed to thinking Dolby Digital (AC-3) as a discrete 5.1 channel format that we

forget AC-3 may be used to encode any number of channels (with or without matrix

surround information). Or even one.
"Why would anyone use Dolby Digital (AC-3) data compression to put plain-Jane,

two-channel matrixed Dolby Surround, or even mono, on a DVD? Because ordinary PCM

digital - the non-compressed audio format you get with CDs and the conventional digital

audio tracks on laserdiscs - would use approximately 15 percent of the available data space

on a DVD alone-never mind multiple languages. And that’s too much. Two-channel,

uncompressed PCM audio tracks could be used on DVD for shorter programs such as

music videos, but it's highly unlikely that you II see anything but data-compressed audio

tracks - Dolby Digital and perhaps DTS® in the U.S. and MPEG in Europe-on movie DVDs

where data space is critical.
So we now have two AC-3 encoded tracks carrying matrixed Dolby Surround information

(ignoring, the time being, the mono option). To play these tracks back, they must first be

AC-3 decoded, then routed through a ProLogic (matrix, ed.) decoder to derive the matrix

surround. For most users, this will not be a problem; all DVD players can decode two

channels of AC-3 and perform the D/A conversion needed to produce the required left and

right channel (LT/RT, ed.) analog outputs. Hook up these outputs to any ProLogic (or other

matrix, ed.) processor and you’re in business for surround (or stereo or mono) DVD

playback. To play back DVDs having full 5.1 channel Dolby Digital tracks, you use a

separate digital feed from the DVD player to the Dolby Digital processor.
"Using the DVD players analog outputs for everything except 5.1 channel Dolby Digital will,

however, restrict the quality of ProLogic (matrix, ed.) Or mono replay to that permitted by

the DVD players on board, two-channel AC-3 decoder and D/A converters. Some high-end

surround processors (such as Denon AVP-8000 and the Theta Casablanca) avoid this

limitation by accepting the DVD players digital output for all types of DVD soundtracks.

These processors automatically (or via a user-control switch) accommodate whatever format

is camed by the Dolby Digital data-stream, be it discrete 5.1, matrixed Dolby Surround, or

"Though we've just begun our serious evaluation of the DVD format, the first batch of titles

we received from Warner® is an interesting mix of the above possibilities. If you look at the

fine print on the bottom rear of a DVD box, you II see what amounts to a thinly disguised

code in probably the smallest legible typeface Wrner could find."

Dolby Surround 5.1 = discrete surround

Dolby Surround Stereo = matrix surround

Dolby Digital = mono

[All DVDs can be decoded through a Circle Surround decoder. Ed.]
One word of reassurance for those without 5.1 channel Dolby Digital replay equipment:

aside from any two-channel Dolby Surround mixes that may be recorded on DVD and are

handles as discussed above, all current players can also internally process the 5.1 channel

tracks, when present, to reduce them to two-channel (LT/RT) Dolby Surround (matrix, ed.)

mix. This mix is then decoded (it s still in AC-3 form), converted to analog, and routed to the

player s analog outputs. These two analog output channels may then be fed to your matrix

surround processor and converted for replay in the same manner as any matrixed material.
A number of listening tests have been performed comparing the Circle Surround 5-2-5 matrix

decoder (in Video Mode) against the 5.1 channel discrete digital decoders. The worst result

was a draw using a DVD during a shoot-em-up segment of Eraser. All other segments of

movies compared, using both DVDs and laserdiscs, the Circle Surround 5-2-5 decoder was

the clear preference.
There was no contest involving music sources. The Circle Surround 5-2-5 Music Mode did

maintain the same frontal imaging contained in two-channel stereo recording, but the

listeners heard a hard center channel (without the image collapsing into the mono) and the

full bandwidth, stereo surround channels reproducing the non-coherent ambience contained

in the music. Using encoded music, it is possible to hear true stereo from the surround Left

and Right channels.

Both modes produced a sonic environment that enveloped the listeners with sound that even

made converts of hardcore stereophiles. The lack of image wandering caused by the

dedicated dominant center channel of all 4-2-4 systems was a real attention grabber. Even

though some listeners couldn’t verbalize what they heard, they all agreed on the superiority

of the Circle Surround 5-2-5 technology.
Richard Hardesty in review of the Circle Surround technology in Wide Screen Review wrote,

with this level of spatial accuracy and freedom from coloration available from a matrix

device, it brings into question the need for discrete multi-channel systems, especially those

that require substantial data reduction. . .

It effectively demonstrated the system s ability to place sound precisely, anywhere in a

380° degree circle with complete imaging between any two-speaker pairs including the left

and right rears.

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