|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.