| Cinema Cynergy November 2001
Almost Everything You Need to Know about
Surround-Sound Formats for Home Theater -- Part Two
Just when you thought it was safe to purchase a receiver or
processor with 5.1 channels (left front, center front, right front, left surround, right
surround, and low-frequency effects) with either Dolby Digital or DTS processing, along
comes the latest generation of products with 6.1 channels of sound. These products add a
center-surround channel (or sometimes even two) and go by a confusing myriad of names such
as Dolby Digital Surround EX, DTS-ES, 6.1, or THX Surround EX.
Fortunately, the manner in which 6.1-channel sound is
implemented makes it fully compatible with all existing 5.1-channel products such as those
that utilize the standard Dolby Digital and DTS formats. Even though 6.1-channel surround
sound is fully compatible with 5.1 systems, there is some confusion as to the nomenclature
of these systems and exactly how they work. They are generally referred to as
"Extended Surround" systems and are dubbed "EX" by Dolby and THX, and
"ES" by DTS.
THX Surround EX (Dolby Digital Surround EX)
Dolby and THX have collaborated to create a 6.1 format
known as Dolby Digital Surround EX in professional theater systems and THX Surround EX in
home-theater products. THX holds the licensing rights to the home-theater version of this
technology, and thus the "THX Surround EX" designation is available only on
THX-certified processors and receivers. Note that there are currently no home-theater
products that are "Dolby Digital Surround EX" certified as this designation is
licensed exclusively by Dolby and is reserved for professional theater systems. Both the
Dolby professional and THX consumer versions of this system are sometimes referred to as
"Surround EX."
 Surround EX is not a true
discrete 6.1 format. Rather it employs matrixing the center-surround channel (often called
the back-surround channel) into the left- and right-surround channels of the Dolby Digital
5.1 bitstream. This technique is very similar to analog Dolby Surround, which utilizes
matrix encoding to store four channels on a two-channel stereo signal. Because Surround EX
is not a true 6.1 format, Dolby authorizes the use of the following logo (right) for
Surround EX-encoded material. Note the hollow square that represents the matrix-derived
center-surround channel placed behind the listener.
6.1 movie soundtracks are mastered on high-resolution
discrete 6.1 systems and are "down-mixed" (compressed and combined into a single
digital bitstream) by an encoder to the 5.1 Dolby Digital bitstream, which includes
matrixing the three surround channels into the two available surround channels. According
to Brent Butterworth of Dolby Labs, the latest version of the Dolby Digital encoding
software allows the digital bitstream to be flagged as Surround EX, which can be read by
any current THX Surround EX processor or receiver to automatically turn on EX processing.
However, there are currently no DVDs that have been authored with this new software to
contain the Surround EX flag.
The Haunting was the first DVD to be released in
DTS-ES Discrete 6.1.
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The consumer version of Surround EX licensed by THX
includes several features that are designed to offer the listener enhanced flexibility to
ensure the best possible sound reproduction of the surround channels. Included in the
specification are provisions for bass management, re-equalization and delays for all
surround channels, and automatic re-direction of mono surround-channel information to both
the left- and right-surround channels so that all of the sound will not be sent to the
center-surround channel. THX also recommends the use of two center-surround speakers.
DTS-ES
DTS has developed a method of extracting 6.1 channels of
sound from a 5.1 soundtrack that is very similar to its counterpart from Dolby and THX.
Like Surround EX, DTS-ES Matrix, as it is called, utilizes matrix decoding to extract
three channels of sound from the two discrete surround channels. However, DTS has also
developed a true discrete 6.1 format known as DTS-ES Discrete 6.1. This method of encoding
actually stores a discrete center-surround channel in addition to the left and right
surrounds making it the only true 6.1 surround format. Fortunately DTS-ES Discrete 6.1
software is fully compatible with standard DTS 5.1 hardware so it can be played back on
any processor or receiver bearing the DTS logo.
Other 6.1 decoding schemes
There are other generic matrix-derived surround-channel
decoding schemes used to extract a center-surround channel from compatible source
material, which are not licensed by Dolby/THX or DTS. These go by such names as
"Cinema 6.1" or simply "6.1" and some companies such as Circle
Surround have their own licensed technologies. It should be noted that these products
still utilize Dolby Digital and DTS to decode digital 5.1 soundtracks even though the
extraction of the matrixed center-surround channel is accomplished by some other decoding
method. This is very common in mid-priced 6.1 receivers, most of which are not THX
Surround EX-certified and contain no DTS-ES decoding.
The DTS- and Dolby Digital-capable Outlaw 950 receiver, which uses a compatible method of
matrix processing to extract a center-surround channel.
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Confused yet?
To sum up, theatrically released films can be mixed in
Dolby Digital Surround EX, but DVDs are decoded in home-theater products using THX
Surround EX processing. DTS utilizes a similar method called DTS-ES Matrix, which also
uses a discrete 5.1 soundtrack with a matrixed center-surround channel. However, DTS also
has a true 6.1 format with six discrete full-range channels and a low-frequency-effects
channel known as DTS-ES Discrete 6.1, which is compatible with all standard DTS 5.1
hardware. There are also many generic 6.1 decoding methods that utilize compatible matrix
decoding of the surround channels that are not certified by Dolby/THX or DTS.
My two cents
6.1 can certainly enhance the home-theater experience, but
for many of us, it is difficult enough to fit five speakers and a subwoofer into a room,
let alone six or even seven speakers. The placement of the center-surround speaker(s) can
be especially problematic and there is the requirement for additional channels of
amplification if you are not using a 6.1 receiver with built-in amplifiers. However, if
you have the finances, space, and inclination to set up a 6.1 surround-sound system then
by all means do so, but you shouldn’t necessarily feel as though you are missing out
if you only have a mere 5.1 system.
Good old Dolby Digital and DTS 5.1 can sound very good.
Heck, even Dolby Pro Logic when properly implemented can sound very good and Pro Logic II
sounds even better than regular Pro Logic. Just what is Dolby Pro Logic II you ask? That
is a whole other can of worms and could probably be the topic for another column.
Let’s just say that if you are currently shopping for a surround-sound system you
should definitely be looking for products that have the latest technologies such as 6.1
and Dolby Pro Logic II capability. However, if you already have a good-sounding 5.1
system, I wouldn’t consider upgrading to a 6.1 system to be a necessity.
...Roger Kanno
roger@hometheatersound.com
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