Playing Music in Surround: Part One
In the 1970s, quadraphonic recordings
made their public debut in a variety of formats. These included Columbia’s SQ,
Sansui’s QS, and CD-4 Quadradisc. A lack of interoperability among these different
surround standards was one of the reasons early quad never managed to catch on.
Nowadays, there’s a similarly diverse set of standards
for surround-encoded music on the market: Dolby Digital, DTS (Digital Theater Systems),
SACD (Super Audio CD), and DVD-A (DVD-Audio). But unlike in the earlier surround format
wars, the computing horsepower available in modern equipment means that many pieces of
playback equipment are capable of playing back two or more of these formats, with some
units handling all four.
Analog versus digital audio
There are three main ways audio equipment connects to a
surround system. Presumably you’re familiar with using speaker cable to connect your
speakers. Usually components like a CD player or tape deck are plugged in with an
interconnecting cable terminated with what are called RCA jacks. Both the speaker signal
and the line-level interconnects carry an analog signal that varies continuously up and
down, based on the recording.
In contrast, the content on a CD or DVD is actually encoded
digitally using a series of 0s and 1s that represent the audio signal sampled at a high
rate of speed (at least 44,100 times per second). Many CD players and all DVD players
include a digital output that lets this information transmit directly to a receiver, a
surround processor, or a similar device with a digital input. One advantage of this: The
sound quality doesn’t degrade easily when a digital signal is moved from one
component to another. The other advantage: These digital signals are capable of holding
surround sound instead of just stereo music.
In order to decode most surround formats, a digital cable
runs between the CD or DVD player and the receiver/surround decoder. Eventually these
signals switch back to analog again before they reach the speakers. (Only a handful of
speakers have facilities for receiving a digital input.) Some DVD players include the
capability to decode the surround sound and convert it back into analog from inside their
chassis. In this case, they deliver the signal via a series of six analog outputs. One
thing to watch for is that most receivers only have a single six-channel analog input, so
you could run into problems trying to connect multiple DVD players in this fashion. This
generally comes up only if you’re trying to use, say, a dedicated player for SACD and
another for DVD-A.
At their core, almost all of the digital sound-encoding
schemes on the market, from the original one used on the CD to the latest DVD-A titles,
are fundamentally based on technology called Pulse Code Modulation (PCM). PCM schemes are
defined by two main parameters. The sampling rate is how many times per second the
audio is digitized. Typical sampling rates are 44,100 times per second (or 44.1kHz as
it’s abbreviated), 48kHz, 96kHz, or 192kHz. This determines the maximum frequency the
system is capable of handling, which is one half the sampling rate, so the CD-quality
44.1kHz gives a maximum frequency of 22.05kHz. The bit depth of a PCM recording
determines how many distinct volume levels are capable of being distinguished, and that
determines the dynamic range of the medium. CD quality consists of 16 bits, which
translates into 65,536 levels capable of handling 96dB of dynamics.
Dolby Digital
Although Dolby Digital first appeared in theaters in 1992
with the release of Batman Returns, most consumers initially encountered it with
the introduction of DVD players to the market in late 1997. Most DVD surround-sound
content is available in Dolby Digital. Note that Dolby Digital doesn’t necessarily
imply surround: The format is flexible enough that it can even handle a mono soundtrack.
Using an encoding scheme that Dolby refers to as AC-3,
Dolby Digital normally includes what’s referred to as 5.1-channel surround: five main
speakers (left, center, right, left surround, right surround) and the ".1"
channel, which has content specifically designed for a subwoofer. In most cases, a digital
cable transmits this 5.1-channel signal from the DVD player to a surround receiver. Many
DVD player models include a Dolby Digital decoder inside of them, usually with six
separate analog outputs for use in this mode. Normally, the Dolby Digital decoder inside a
receiver has better capabilities for surround decoding than the ones built into a player.
Common extra features are improved subwoofer support (or "bass management" as
it’s often called) and more programmable delay times for rear speakers.
More recent Dolby Digital titles are actually encoded with
6.1 channels, adding a back surround speaker(s) directly behind the listening position. If
you’re trying to transmit that over analog cables, it takes seven of them to handle
this newer format.
The main advantage of Dolby Digital is its ubiquity: All
DVD players are required to support it, every modern surround receiver supports it, and
you rarely encounter any issues playing the titles back. The only real disadvantage of the
format is that Dolby Digital usually has lower sound quality than the others listed here
because it compresses the audio into a smaller amount of space.
DTS
In 1993, theaters across the country first shook to the
sound of the Tyrannosaurus rex in Jurassic Park, brought to you via DTS surround.
DTS first appeared in the home by a somewhat odd path. The DTS Entertainment Company
decided to release DTS-encoded music in surround on standard compact discs. Your CD player
would read this data, treat it as standard music, and then output it over its digital
output. The surround decoder at the other end would recognize that it was actually a
compressed DTS signal rather than a regular stereo one, and then switch to a surround mode
to play it back.
DTS CDs are still on the market today, normally packaged in
regular CD cases but with warning notes that they will only play in a DTS system. Recently
some of these (like the wonderful Alan Parsons surround version of On Air [HDS
710215-4414-2-9]) have been re-released in the larger case size normally used for
DVD-Audio. Make sure to check the labels to confirm you’re getting what you expect.
Note that the original DTS CDs were usually somewhat forced surround remixes, putting lots
of effects in the rear channels. Try to listen before you buy so that you know you’ll
like the mix.
When the first DVD players shipped, they couldn’t
handle a DVD with DTS sound on it. All current DVD players from any of the major vendors
include support for DTS DVDs, but to be sure you can check for the DTS logo on the player
or in its manual.
DTS normally dedicates more space on the DVD to sound than
Dolby Digital recordings do, and many believe DTS sounds better partly because of this.
Some DVD titles, both movies and music, ship with both types of surround sound on the
disc. A menu function on the DVD normally lets you swap between the two soundtracks. A
good example is the DVD Peter Frampton: Live In Detroit [Image D88161GDVD], which
sounds great both ways. The video is pretty good too.
Next month we’ll look at the two newest emerging
formats for surround sound: DVD-Audio and SACD.
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