| Cinema Cynergy January 2003
Extended Surround: Confusing the Consumer
During a recent conversation with the
design engineers of a major speaker manufacturer, I got into a discussion involving the
nomenclature of surround speakers -- specifically, the additional surround speaker(s)
required for a 6.1 system. Although the generally accepted designation for the additional
speaker is "back surround," many of us at Home Theater & Sound like
to refer to it as the "center surround." Others have preferred to call it the
"rear surround." To set the record straight, the extra surround channel should
be referred to as the "back surround" and the regular surround channels that are
found in both 5.1 and 6.1 systems should be referred to as simply the left and right
surrounds. They are named this way because the left and right surrounds are generally
placed to the sides of the listeners and the back surround is placed somewhere behind the
listener. The answer to exactly where these speakers should be placed in a
particular system is a topic for another column and then some, as it depends on many
variables.
If the so-called experts can’t even agree on the name
of the extra surround channel in a 6.1 system, then what chance does the average consumer
have? And that is exactly my point. There are now so many variations of surround-sound
formats, it’s no wonder that consumers are often confused. Although I have described
many aspects of 6.1-channel surround reproduction in previous columns, I think it is
important to go back and properly describe all of the relevant details of how various 6.1
systems work.
Before we move on and just to be absolutely clear on this,
the names of the channels in a 6.1 (pronounced "six point one") system are: left
and right (mains); center; left and right surrounds; back surround; and the subwoofer or
LFE (low frequency effects) channel. Some systems utilize two back surround speakers, so
these would presumably be called the left back surround and right back surround.
Extended Surround (EX versus ES)
6.1 surround formats, or "Extended Surround"
formats as they are sometimes called, are an extension of the standard 5.1 discrete
digital surround formats. Dolby calls their system Dolby Digital Surround EX, and DTS
calls theirs DTS-ES. Both Dolby Digital Surround EX and DTS-ES are fully compatible with
standard 5.1 audio systems, so a DVD that is labeled as having an EX- or ES-encoded
soundtrack will play back normally on a 5.1 system. The back surround channel simply
splits equally into the left and right surround channels.
The technique in which a back surround channel is added to
a discrete 5.1-channel mix is quite ingenious. Dolby describes Surround EX in the
following manner:
The system uses 5.1-channel Dolby Digital and delivers
an additional surround channel by mixing the signal into the left surround and right
surround channels using a process called matrix encoding. This additional channel, called
the back surround channel, is then matrix decoded upon playback and sent to one or more
speakers located in the center rear of the theater.
Dolby Pro Logic and other matrix-decoding schemes also
derive a center channel and surround channels from out-of-phase information that can be
present on two-channel, stereo recordings.
Matrix versus Discrete
Both DTS-ES and Dolby Digital Surround EX can matrix decode
the back surround channel. DTS-ES can also digitally encode a fully discrete back surround
channel. Thus, two types of DTS Extended Surround exist: DTS-ES Matrix, and DTS-ES
Discrete 6.1. DTS-ES Discrete 6.1-encoded material is also fully compatible with all
standard DTS 5.1 and DTS-ES Matrix systems. Here’s how: The left and right surrounds
always include the matrixed back surround channel. If the decoder detects the optional
discrete back surround channel, it can subtract it out of the mix.
The 6.1 misnomer
The term "5.1" refers to standard DTS and Dolby
Digital soundtracks that have five discrete full-range channels along with a
bandwidth-limited LFE channel. Therefore, it follows that a 6.1 format should have six
discrete full-range channels along with an LFE channel. However, that is only the case
with DTS-ES Discrete 6.1. DTS-ES Matrix and Dolby Digital Surround EX are not true 6.1
formats. In fact, Dolby states in their literature:
Dolby Digital Surround EX carries 5.1 discrete audio
channels with a matrix-encoded back surround channel. It is therefore not a true
6.1-channel system, and the term "6.1" should not be used to describe it.
Having said that, it is commonplace for most people,
including myself, to refer to any system that has a back surround channel as a 6.1 system,
even though it is technically incorrect.
THX Surround EX
When Dolby Digital Surround EX was first introduced for
home use a couple of years ago, THX exclusively licensed it and marketed it as THX
Surround EX. THX requires additional processing of the surround channels, including:
Re-Equalization; comprehensive bass management; time delays for the left and right back
surrounds; and the sensing of mono surround signals, which are then re-routed to the left
and right surrounds. Routing them just to the back surrounds would cause the sound to
collapse to the back of the room. They also require the use of two back surround speakers,
which is only a recommendation from most manufacturers. Although THX Surround EX is a
comprehensive and high-quality method of playing back EX material, it is not an option
most manufacturers include on budget-priced equipment due to the cost of the additional
circuitry and licensing fees.
Now that Dolby Digital licenses Surround EX for home use
with less stringent guidelines than THX, it is becoming available on more and more
inexpensive receivers and processors. However, THX continues to license THX Surround EX as
sort of an upscale or more "high-end" version of Surround EX processing.
Other 6.1 decoding processes
While Dolby Digital Surround EX, THX Surround EX, and
DTS-ES are patented processes owned by their respective companies, additional
manufacturers offer other 6.1 matrix surround-decoding processes. Some of these are
available from companies such as Circle Surround or Crystal/Cirrus Logic who, in turn,
license them to hardware manufacturers. Manufacturers such as Yamaha, Onkyo, and Sony,
just to name a few, even have their own proprietary extended-surround decoding processes.
7.1-channel systems?
Although extended-surround formats only provide a single
discrete or matrixed back surround channel, Dolby and others recommend the use of two back
surround speakers to spread out the sound so that it doesn’t seem to collapse towards
a single point behind the listener. THX actually requires this. In most cases, the mono
back surround signal is simply split and sent to the two back surround speakers. Just as
most systems labeled 6.1 are not true 6.1 systems, I am not currently aware of any 7.1
consumer systems.
Extended surround software
One common misconception is that consumers must use EX- and
ES-designated software to enjoy the benefits of a back surround channel. Any DTS or Dolby
Digital 5.1 soundtrack with out-of-phase information recorded in the left and right
surround channels can be decoded to provide a back surround channel. Although some
soundtracks are now specifically mixed to provide a back surround channel, many discrete
5.1 soundtracks that predate EX and ES processing will provide a matrixed back surround
channel when played on a system that is equipped decode them.
Raising the flag
One of the problems with Dolby Digital and THX Surround EX
receivers and processors is that although they can be set up to recognize Dolby Digital
Surround EX-encoded DVDs automatically, many of the early discs did not contain the flag
for this in the digital data stream. An updated version of the software for the Dolby
Digital encoder was required to include the EX flag, and most production facilities did
not get this new version of the software right away. Presumably most, if not all, major
video-production facilities that produce Dolby Digital-encoded DVDs now have this new
software so that Surround EX processors and receivers can automatically detect more recent
Surround EX discs. DTS-ES software has all reportedly been marked with the correct flag
for either Matrix or Discrete decoding since its inception.
The labeling nightmare
 So how do you know when you purchase a DVD
whether it is EX or ES encoded? Dolby requires the use of the standard Dolby Digital logo
along with the phrase "Dolby Digital Surround EX" or "Surround EX"
somewhere on the packaging to mark these discs. They also have created a new sound-mode
icon with a hollow box indicating the matrix-decoded back surround channel, which can be
distinguished from discrete channels that are indicated as filled boxes. Right now its use
is optional. DTS, on the other hand, has a new logo for DTS-ES-encoded material, but the
labeling does not always clearly distinguish between DTS-ES Matrix and DTS-ES Discrete
6.1. There are only a handful of DTS-ES DVDs available, and most of them are of the
discrete variety anyway.
Taking a look at the back covers of a few recent DVDs that
are EX and/or ES encoded is a confusing exercise. Some releases such as Star Wars:
Episode 1 -- The Phantom Menace include the Dolby Digital logo and the wording
"Dolby 5.1 Surround EX" as prescribed by Dolby, but they are printed separately
at the top and bottom of the cover. Recent releases such as Blade II and The
Lord of the Rings: The Fellowship of the Ring (Platinum Series Extended Edition) have
incredibly tiny Dolby Digital and DTS-ES logos that are almost illegible, but correctly
identify the Dolby Digital Surround EX 5.1 and DTS-ES Discrete 6.1 soundtracks in the
text. Gladiator correctly uses the sound-mode icon with a filled box for the
discrete back surround channel for the DTS-ES Discrete 6.1 soundtrack, which is also
described in the text. However, it uses a surround-mode icon with a hollow box for the
matrixed back surround channel for the Dolby Digital soundtrack, but does not credit it as
being a Dolby Digital Surround EX soundtrack. Also, it incorrectly states that "Dolby
Digital and DTS soundtracks contain up to 6.1 channels of discrete audio" which is
only true for DTS soundtracks. The DTS version of The Haunting, which was the first
DTS-ES Discrete 6.1 DVD, credits the soundtrack correctly in the text, but then uses the
5.1 surround-mode icon, labeled as "6.1." It also incorrectly states,
"Dolby Digital soundtracks contain up to 6.1 channels of discrete audio."
The two DTS-ES Matrix DVD titles that I have on hand are
also cryptically labeled. Terminator 2: Judgment Day -- Ultimate Edition correctly
identifies the Dolby Digital Surround EX and DTS-ES Matrix (described as "DTS 5.1
ES") soundtracks in the text, but only the Dolby Digital logo is present at the
bottom of the back cover. Jurassic Park III uses the surround-mode icon with the
hollow box for the matrixed back surround along with the standard Dolby Digital and DTS
logos, but does not credit either of the soundtracks as being EX or ES encoded.
The more things change, the more they stay the same
So that’s the story behind extended-surround formats,
or at least most of it. Which reminds me of another conversation that I had a few years
ago with a well-respected speaker designer from another very large speaker-manufacturing
company. He lamented the lack of standards in DVD authoring, and how certain DVDs
defaulted to the two-channel Dolby Surround soundtrack instead of the 5.1 Dolby Digital
soundtrack. This meant that some home-theater users might unknowingly miss out on the full
discrete digital experience. If only things were that simple today.
...Roger Kanno
roger@hometheatersound.com |
