| Cinema Cynergy April 2001
Almost Everything You Need to Know about
Surround-Sound Formats for Home Theater
It’s no wonder that people become confused by the
myriad of audio formats that are available today. We are bombarded by such names and
acronyms as Dolby Pro Logic, Dolby Digital, DTS, THX, PCM, SACD, DVD-A, MP3, Dolby Digital
EX, DTS-ES, Pro Logic II, and these are just the most common ones.
Although there are even more audio formats than those I
have just mentioned, when it comes to surround-sound formats for home theater, there are
really only three major ones. I will discuss the others later just to avoid confusion, but
the three that we must concern ourselves with, and that I will discuss in detail are:
- Dolby Surround (matrixed surround): Sometimes
referred to as Dolby Pro Logic. An analog surround format used mainly for VHS tapes and
broadcast television shows. Contains four channels of sound folded or matrixed onto
two-channel recordings.
- Dolby Digital (AC-3): A discrete digital surround
format featuring up to five full-range channels and one limited-bandwidth low frequency
effects channel. Mandatory on all DVDs.
- DTS (Digital Theater Systems): A discrete digital
surround format with five full-range channels and one limited bandwidth low frequency
effects channel similar to Dolby Digital, but using a higher bit-rate to transfer the
audio data. Optional on DVDs, which must also provide a mandatory Dolby Digital track of
some sort (although not necessarily 5.1) if DTS is used.
Note that Dolby Digital and DTS are digital formats and as
such are only available on digital media such as DVD, digital satellite broadcasts,
digital cable, CDs, and laserdiscs. They cannot be encoded onto analog audio sources such
as VHS tapes or standard cable and broadcast television. They are often referred to as 5.1
(five-point-one) or 6.1 formats depending on how many channels of audio they contain.
Dolby Surround
This is the surround format that started it all. It is
often referred to as Dolby Pro Logic, which is actually an enhanced version of the
decoding method used to extract the four channels of material that are encoded onto what
is essentially a two-channel stereo recording. This means that it can be used with any
analog stereo media. Consequently, just about every movie on VHS tape is encoded with
Dolby Surround, as are the broadcasts of many television shows.
A Dolby Surround signal contains four channels of
information: left and right front channels, a center channel, and a mono surround channel
that is split in two and sent to both the left and right rear speakers even though the
signal is monophonic. The technique by which these four channels are encoded onto two
channels and later extracted is quite ingenious and is referred to as a matrix system.
Without getting too technical, the left and right front channels are recorded as they
normally would be, but the center channel is added equally to the left and right at a
level that is 3dB lower than it was originally recorded. This is to maintain the original
output level because a 3dB increase in sound is equivalent to doubling the perceived
volume. The mono surround channel is also decreased by 3dB and added equally to both the
left and right channels but each is phase-shifted by 90 degrees in the opposite direction
so that they are 180 degrees out of phase with each other.
This matrixed two-channel signal can then be decoded into
left, right, center, and surround channels by receivers or processors using Dolby Pro
Logic or other methods such as Circle Surround, Logic 7, DTS Neo:6, and now Pro Logic II,
among others. Additionally, when such a recording is played back through a conventional
stereo system, the center channel will be reproduced as a sharp phantom image from between
the two speakers and the surround channel will tend to image diffusely from all over
because it contains out of phase information.
Dolby Digital
This is the discrete digital audio format that is mandatory
on all DVDs (which was referred to as AC-3 on laserdiscs), and is one of the major
advantages that DVD has over analog sources such as VHS tape, in addition to the much
higher picture quality. Discrete digital formats such as Dolby Digital will have superior
separation of channels when compared to matrixed surround formats, which tend to leak
sound into adjacent channels. Additionally, all but one of Dolby Digital’s channels
are full range, while the surround channel of Dolby Surround is rolled off in both the
high and low frequencies to decrease the amount of leakage into the other channels. This
limits its use to mainly adding rear ambience to soundtracks.
Dolby Digital soundtracks can have up to five channels
(left, right, center, right surround, and left surround) of full range audio and one low
frequency effects (LFE) channel, which is often referred to as the .1 or subwoofer
channel. This means that Dolby Digital soundtracks can contain anything from a mono signal
up to the full 5.1 palette of channels, or anything in-between. Although the use of Dolby
Digital may be mandatory on DVDs, because the number of channels is flexible, many DVDs of
older films will contain only two-channel recordings. Ironically, many of these
two-channel Dolby Digital tracks will be of the original matrixed surround soundtracks,
which can be decoded with Dolby Pro Logic or any other appropriate surround processing
after the analog matrixed signal has been extracted from the Dolby Digital bitstream.
Dolby Digital has quickly become the standard method of
delivery for discrete multichannel movie sound to consumers due to its adoption as the
mandatory soundtrack on all DVDs. It is also being used with some digital satellite and
cable television broadcasts, and is included as part of the HDTV standard.
DTS
 Functionally, DTS is pretty
much the same as Dolby Digital (although I am sure that the folks at DTS and Dolby Labs
would vehemently disagree with me), except that it utilizes a higher bit-rate to
reportedly achieve higher fidelity. DTS and Dolby Digital are incompatible with each
other, but most current receivers and processors can handle both formats. DTS is an
optional format on DVD so the majority of discs are still encoded with Dolby Digital only,
although a few are now available with 5.1 soundtracks in both formats. Previously, DTS
versions of DVDs were mostly available as separate releases from the standard Dolby
Digital versions, but dual-format discs (those containing both 5.1 DTS and 5.1 Dolby
Digital) are now becoming more common. Some examples of these discs include: Gladiator,
Terminator 2: Ultimate Edition, and American Beauty, just to name a few.
Presumably, because they have positioned themselves as a
high-quality, multichannel alternative to Dolby Digital, DTS is only used on DVDs with
soundtracks that contain 5.1 discrete channels of sound. I would agree that DVDs recorded
with DTS generally sound better than those recorded with Dolby Digital, but the
differences are not night and day. Instead, they are relatively subtle improvements to
soundstage depth and width, increased detail and imaging, etc.
Johnny come lately
Recently, both Dolby Labs and DTS have introduced
improvements to their digital surround formats that allow the addition of a "rear
center channel" to provide 6.1 channels of sound. This added rear information can
certainly enhance the surround-sound experience, but it requires an additional rear
speaker and also another channel of amplification if you are using a processor and power
amp combination.
Fortunately, these new and improved formats, which are
called Dolby Digital EX and DTS-ES, are backward compatible with existing hardware, which
can still be used to play back the new software. One thing that should be mentioned is
that Dolby Digital EX uses analog decoding to extract the rear center channel so that even
software that is not specifically encoded with EX can yield a rear center signal if there
is a mono signal recorded equally on the left and right rear channels. This type of
processing can be thought of as adding analog matrix decoding such as Dolby Pro Logic to
the rear channels after they have been extracted from the Dolby Digital bitstream. DTS-ES
is a discrete 6.1 format (i.e. a format with a discrete, digitally-encoded rear center
channel), but like Dolby Digital EX, it too can extract a phantom rear center signal from
5.1 soundtracks. These two variations of DTS-ES are referred to as the
"Discrete" and "Matrix" versions.
Finally, there is also a new type of Dolby Surround
decoding for matrix surround recordings that is called Pro Logic II. At first glance, this
might appear to be something of a throwback, but think of all the existing VHS tapes,
broadcast television programs, laserdiscs, DVDs of older films with only two-channel
matrixed sound, and music CDs that are out there. Yes, you heard me right. Music CDs could
stand to benefit greatly from this new decoding scheme that is reported to do a much
better job of spreading the sound across the three front channels, and not making
everything so center-channel oriented, as with Dolby Pro Logic. Additionally, Pro Logic II
uses some of the left and right front stereo cues to differentiate the mono surround
signal and make it somewhat directional. Pro Logic II was developed by Jim Fosgate who had
previously produced similar matrix-surround decoding methods such as 6-Axis and Logic 7,
which were the precursors to Pro Logic II.
And the rest
 As previously mentioned there
are other audio formats out there that you should be aware of, but which are not
particularly applicable to home theater. PCM is the digital format used by compact discs.
Because it is so ubiquitous, it really isn’t an issue and is rarely mentioned as
every DVD player, receiver, processor, etc. is PCM compatible. SACD and DVD-A are the new
high-resolution multichannel formats primarily for audio only use. MP3 is the highly
compressed two-channel audio format that is used mainly to transfer songs over the
Internet. THX is not really an audio format or a decoding scheme per se, although it does
specify some processing of the rear channels and bass management for surround-sound
playback. It is actually a set of licensing specifications developed by Lucasfilm,
covering everything from motion picture sound reproduction in both theaters and in homes
all the way to the mastering and duplication of DVDs, and is a topic complex enough to
warrant its own future column.
The bottom line
If you are thinking of getting into a surround-sound system
today, you will need a minimum of Dolby Digital and Dolby Pro Logic decoding. You might as
well get DTS while you are at it because it is usually included with most processors and
receivers anyway. There are so few DTS discs out there, however, that it is not a big deal
even if you don’t have it. It would also be prudent at this point to get Dolby
Digital EX and DTS-ES even if you don’t plan on adding the extra rear speaker right
away just to keep your future upgrade options open. Pro Logic II also appears to be a very
promising new method of decoding matrix surround recordings that you should definitely
consider if investing in an A/V processor or receiver.
...Roger Kanno
roger@hometheatersound.com
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