|
Choosing Surround Separates
By Michael Riggs
Playthings once native only to the screening rooms of the rich and famous,
separate audio/video preamplifiers, surround processors, and multichannel
amps now span a wide price range, from affordable to ultra-high-end.
Although premium-priced preamps and amps have proliferated, the greatest
growth has been in the "low end of the high end." Now just
about anyone can get into separates.Tread carefully, though. We're talking
here about the very hub of your home theater system, and whether you
opt for a preamp/amp combo or receiver matters less than whether you
opt for a good one. Mediocre surround separates may not outperform a
really good receiver. They may just make your system more complicated,
adding cables and expense. For the discerning listener, however–especially
one outfitting a large, dedicated home theater room–separates are
definitely the way to go. At their best, they can be both more powerful
and more versatile than a receiver. For best results, you may want to
consult a custom installer certified by the Custom Electronic Design
& Installation Association (CEDIA). Surround amps, preamps, and
processors split the functions of an A/V receiver into two or more boxes.
There are two basic reasons for wanting to add such apparent complexity.
One is simply additional flexibility. Having the power amplifiers separated
from the switching and processing functions makes upgrading easier and
allows for a wider range of system configurations. The other is performance.
Packing switching, processing, and amplification for five or more channels
of audio into a single box almost necessarily involves some compromise.
With separates, you can keep low-level signals away from the hum fields
of the big transformers in power amps while at the same time allowing
for much beefier power amplifiers. You can even use a number of separate
mono, stereo, or multichannel power amplifiers instead of a single multichannel
job.
What is Surround Sound?
But before going too much further, let’s take a look at exactly
what we mean when we talk about surround sound. Essentially, surround
sound is an extension of what we have come to know as stereo that allows
the creation of a more vivid and enveloping sound field. When audio
made the move from single-channel monophonic reproduction to two-channel
stereophonic, record producers could create the illusion of instruments
and voices arrayed across an imaginary stage in front of the listener.
And to some extent, they could create an impression of acoustic ambience.
The sense of realism that can be conveyed by such a system is ultimately
very limited, however, and for best results, the listener is restricted
to a very small seating area. Modern surround sound adds a third front
speaker, midway between the traditional left/right stereo pair, and
at least two surround speakers to the sides of the listening area. (The
surrounds can go in back of the listening area if necessary, but side
placement usually is better.) Often a dedicated low-frequency speaker,
called a subwoofer, is also included to handle deep bass sounds. Surround-sound
recordings contain four or more channels, including three front, at
least one surround, and possibly a limited-range low-frequency effects
(LFE) channel designed to carry only extraordinarily loud movie sound
effects. Play such a recording through a properly configured surround-sound
system, and you will hear more stable and accurate positioning of sounds
across the front together with a much more convincing impression of
acoustic space. A well-made surround recording, well-reproduced, can
yield a you-are-there sensation conventional two-channel stereo cannot
come close to achieving.
Surround-Sound Formats
Although there are quite a few formats for delivering surround sound
these days–some complementary, come competitive–only two or
three basic channel configurations are in common use. So before getting
on to the delivery formats, it’s a good idea to get a handle on
exactly what it is that is being delivered. The current standard channel
configurations are four-channel, five-channel, and 5.1-channel. In four-channel,
there usually are three front (left, center, and right, or LCR) and
one surround, with the single surround channel reproduced through two
speakers to improve spaciousness. (Surround music recordings sometimes
are mixed with two front and two surround channels.) Five-channel recordings
have three front and two surround channels. And 5.1-channel adds a low-frequency
effects channel that is active only below about 120 Hz. The LFE is perhaps
the most misunderstood channel. It is not, for example, "the subwoofer
channel," although it often winds up being reproduced by a sub.
(Depending on the low-frequency capabilities of the various speakers
in a system, deep bass from any or all might wind up going to the subwoofer,
if there is one.) It is simply a place soundtrack mixers can put low-frequency
sound effects that are much louder than anything else in the movie,
which enables them to avoid sacrificing dynamic range in the main channels
to make room for them. The second-most misunderstood channel is the
center. Because in movie and TV soundtracks all, or nearly all, dialogue
goes into the center channel, people sometimes think that it contains
nothing else. In fact, the center is the normally the main channel for
pretty much everything in the soundtrack, including music and, especially,
effects. Most of the time, the least important channels are, ironically,
the surrounds.
Dolby Surround and Pro Logic
The first modern surround-sound delivery format is known as Dolby Surround
(or, in theaters, Dolby Stereo). It uses what is known as a 4-2-4 matrix
to pack four channels of information (left, center, right, and surround)
into two (left and right) during recording and to recover, as nearly
as possible, the original four channels during playback. Dolby Surround
soundtracks are compatible with stereo and even mono playback, though
you lose the surround information in those cases. And they can be carried
by both analog and digital media. Surround soundtracks on videocassette
are in Dolby Surround format, for example. The standard way of extracting
a complete four-channel surround mix is by way of a Dolby Pro Logic
decoder. Any A/V preamp you can buy today will include at least Dolby
Pro Logic decoding circuitry.
Dolby Digital
The advent of digital media made possible a dramatic advance in surround-sound
delivery, known as discrete-channel coding. That is, each channel is
kept completely independent of the others. This is very different from
matrix-encoding schemes such as Dolby Surround, which attempt to embed
additional channels into just two and to recover them on playback–a
process that always involves some loss of directional information. The
only problem is that the type of coding used for CD and most other high-quality
digital media–pulse-code modulation, or PCM–requires a lot
of storage capacity for each channel. Dolby got around this problem
in its Dolby Digital format (originally known as AC-3) by using a technique
known as perceptual coding. Perceptual coders take advantage of the
characteristics of human hearing to reduce the amount of data used to
convey sound information with little or no audible quality loss. Dolby
Digital actually manages to convey 5.1 channels of very high-quality
audio with only about one-twelfth the data that would be necessary using
equivalent PCM coding. Dolby Digital is the standard audio coding format
for DVD and for the North American digital television (DTV) system.
Today, all but low-end A/V preamps and processors can decode Dolby Digital.
In general, Dolby Digital provides wider dynamic range, greater detail,
and better directional effects than Dolby Surround. In 1998, Dolby and
Lucasfilm announced the development of an extension to Dolby Digital,
currently called Dolby Digital Surround EX in theaters and THX Surround
EX in home equipment. (See below for more on THX.) It matrix encodes
a third, center surround channel into the left and right surround channels
of a standard 5.1-channel Dolby Digital mix. First used in Star Wars:
Episode 1, the technique is being used in increasing numbers of movies,
and we are starting to see some home equipment that decodes the center
surround channel, which is reproduced by one or two additional speakers
at the back of the room. Surround EX soundtracks are backward compatible
with standard Dolby Digital 5.1 decoding, however, so you don’t
have to extract the extra channel to enjoy them.
DTS
DTS is a competitor to Dolby Digital that does essentially the same
thing. It is an optional audio format for DVD, which means that it can
be included in addition to, but not instead of, a standard audio format
(Dolby Digital or PCM). Because it is only an optional format and requires
several times as much storage space as Dolby Digital, DTS is not nearly
as common as Dolby Digital. Despite its much higher data rate, DTS offers
about the same sound quality as Dolby Digital. Nowadays, pretty much
any A/V preamp or processor that includes Dolby Digital will also include
DTS. There is also a DTS equivalent of Surround EX, called DTS ES, which
is supported by a few processors.
THX
A number of years ago, Lucasfilm established a standards program for
movie theaters called THX. (The name came from "Tom Holman’s
Experiment," after the program’s originator, and coincidentally
from the title of George Lucas’s first feature film, THX 1138.)
The idea was to ensure that theaters carrying the THX designation would
deliver the best possible picture and sound. Some time later, Lucasfilm
expanded the program to cover home theater products. The Home THX standard
incorporated both required performance levels and certain processes
intended to correct home playback of soundtracks originally produced
for large movie theaters. (Note that THX is not itself a surround format;
it is just a set of performance standards coupled with some processing
applied to signals after surround decoding–Pro Logic, Dolby Digital,
DTS, or whatever. There is no such thing as a THX soundtrack.) In electronics,
the main processes were re-equalization–a mild treble rolloff to
correct for acoustical differences between auditorium- and domestic-size
rooms–and "decorrelation" of the outputs to the two surround
speakers, to enhance the spaciousness of the mono Dolby Surround surround
channel. The specification also required a specific subwoofer crossover
in the preamp or processor. When discrete 5.1-channel formats arrived,
Lucasfilm modified the decorrelation portion of the standard to accommodate
the new stereo surrounds, but that has been the only basic change. However,
there are now two categories of THX components: THX Ultra, corresponding
to the original standard, and THX Select, which assumes smaller rooms
and therefore does not require so high a power specification. Currently,
only THX-certified preamps and processors incorporate THX Surround EX
decoding. One final but important point about THX is that while manufacturers
certainly have good reasons for adopting it, they can also have good
reasons for not doing so. Usually this has to do with some element of
design philosophy that conflicts with the THX requirements. And many
now incorporate some aspects of the THX approach without seeking the
Lucasfilm seal of approval.
Surround Enhancements (and Simulations)
Many A/V preamps and processors are equipped with digital signal processing
(DSP ) modes intended to mimic the acoustical ambience of a concert
hall, the wide-open feel of a stadium, the intimacy of a jazz club,
and so forth when playing ordinary stereo music recordings. Done well,
such processing can use the additional channels and speakers of a surround
system to reproduce stereo recordings with a degree of realism that
simply is not possible through only two speakers. Unfortunately, it
is often done very badly, which has given the whole idea a bad name
in some circles. It is so wonderful when done well, however, that you
should take serious note of the ambience-enhancement capabilities of
any surround preamp or processor you are considering buying. The best
such modes usually base a good deal of their action on ambience cues
buried in the recording itself, rather than simply making something
up. A good example is Harman’s Logic7 processing, which yields
exceptionally realistic multichannel playback of two-channel sources,
almost regardless of the type of music. But Logic7 actually goes a step
beyond that, in that it improves reproduction of multichannel sources
as well, delivering a smoother, more spacious, more integrated sound
field than you will get from raw Dolby Pro Logic, Dolby Digital, or
DTS decoding.
Other A/V Preamp Issues
One of the chief virtues of an audio/video preamp is upgradability,
though some models are more upgradable than others. Is the unit you're
considering constructed in a modular design to accept circuit boards
for new surround technologies? Are there any extra (unused) switch positions
and jacks to accommodate new options? Is there a set of analog 5.1-channel
inputs to work with an outboard processor or DVD-Audio player? The more
you’re spending, the more reason you have to consider the future
upgrade path in your buying decision.
Amplifier Issues
Muscle amps for surround sound typically have five channels (assuming
a self-powered subwoofer in the system) or six (usually assuming a passive
subwoofer). Permutations are endless, however. Some three- and four-channel
models are available for systems that already have good stereo amps.
As in stereo, you can even use monoblocks, or stereo amps with two channels
bridged to one, giving each surround channel its own separate amplifier
chassis. You might also use two-channel amps to handle two channels
at a time. (For more on two-channel amps, see Choosing Stereo Separates.)
Whatever amp configuration you end up with, do the power and impedance
ratings of the five active channels represent a good match to your speakers
and volume requirements? The power question is the easier of the two,
since people generally tend to overestimate their needs in that department.
If you use speakers of moderate sensitivity (in the 90 dB/SPL/watt range)
and have a normal-size room, you probably could be very content with
50 watts per channel. Very few people need more than 100 watts per channel
in a surround system. Bear in mind, however, that speaker sensitivity
can have a big effect here. A speaker with, say, 87 dB sensitivity will
require twice as much power to reach any given volume level as a speaker
with 3 dB greater sensitivity. Also note that, all else being equal,
it takes a 3 dB change in maximum amplifier power (a halving or doubling
in wattage) to make a significant audible difference in any case. If,
for example, you’re weighing two amplifiers, one rated at 100 watts
per channel and the other at 120, base your decision on something else,
as that’s not a meaningful difference. The other potential matter
of concern is loudspeaker impedance. Almost any practical audio amplifier
will become uncomfortable when the impedance of the speakers connected
to it is too low, because too much current will be drawn through its
output transistors. Generally speaking, you can expect just about any
amplifier from a reputable maker to work properly with speakers having
rated impedances of 6 ohms or greater. But some will have difficulty
with speakers rated 4 ohms or lower. If you are in any doubt on this
question, make sure you get amplifiers with high current capability,
to ensure that they can handle low-impedance speakers.
Connectivity: Audio/Video Preamps
Count your source components and amplifier channels, then take a look
at the preamp's back panel. Are there enough inputs and outputs to suit
your system's needs? Here's a list of what you might reasonably expect
to encounter for video gear:
- composite-video
inputs
- S-video inputs
- component-video
inputs
- front-panel audio/video
inputs
- monitor outputs
- VCR inputs/outputs
When you see the
phrase "video input" or "audio/video input" with
no other qualification, you probably will find a composite-video connection.
(The same is true of outputs.) These use 75-ohm video cables terminated
with ordinary RCA (phono) plugs. S-video inputs are increasingly popular
and will yield better picture quality from digital video sources such
as DVD and satellite TV. But to take advantage of this feature, your
TV must have an S-video input. If your TV is of recent vintage, it may
well have one or more S-video jacks. These are special multipin sockets
and thus are easily recognizable. Component-video connections allow
slightly higher quality than even S-video, but they are less common.
Unless you've got a digital TV, a high-end analog projector, or a component-out-equipped
DVD player, you needn't worry about this. Even if you do, you may find
it more convenient to make component-video connections directly to your
TV. Front-panel inputs, typically stereo audio with composite and possibly
S-video, enable you to connect a camcorder without having to fumble
around the back panel. Monitor outputs, which allow the preamp to route
video to the TV, may be of any or all types. Normally the output types
available will match the input types found on the preamp. VCR inputs
should always be accompanied by VCR outputs, so that the preamp can
record other sources to the VCR as well as play back from it. On the
audio side, you will find at least six line-level analog audio outputs
(left, center, and right front; left and right surround; and subwoofer),
for feeding your power amps, plus any or all of the following:
- digital inputs/outputs
(optical or coaxial)
- analog stereo
inputs
- loop-through
inputs and outputs for recording media
- a set of multichannel
line-level analog audio inputs
Any preamp or processor
that performs Dolby Digital decoding will have at least one digital
audio input. Most will have more than that, which is good if you have
more than one Dolby Digital source or even if you simply have some components
with stereo PCM digital audio outputs, such as CD players and recorders.
In general, the more digital inputs the better, and it is nice to have
one or two digital outputs as well, to feed digital recorders. In some
A/V preamps, digital inputs are permanently associated with certain
inputs; in others, you may be able to assign some or all of them. Also,
you may find that some of the digital inputs are PCM only, with only
one or two configured to support Dolby Digital and DTS. More often,
however, the input will detect what type of signal it is receiving and
respond appropriately. The other important thing to know about digital
audio connections is that they come in two main flavors: Toslink optical
and coaxial. The former require special fiber-optic cables; the latter
use 75-ohm shielded cables with RCA plugs on each end. It doesn’t
really matter which kind you use. Just make sure that you can mate all
the digital outputs on your source gear to digital inputs on the preamp–optical
to optical and coaxial to coaxial. Note also that coaxial digital audio
connections (like composite- and component-video connections) should
be made with 75-ohm cables, not ordinary audio patch cords. Analog stereo
audio inputs are the regular old two-channel, line-level RCA jacks we’ve
all been using for years. Inputs intended for recording devices normally
will have a set of outputs associated with it as well, for feeding the
signals from other sources to the recorder. These are commonly referred
to as tape-monitor loops, because they originated for the purpose of
connecting audio tape recorders. However, preamplifiers with multiple
tape or VCR inputs sometimes will have outputs for only one of them.
So make sure the preamp’s tape-monitor-loop complement meets your
needs. A/V preamps normally have at least one line-level subwoofer output
for feeding a powered subwoofer. And many now have a six-channel analog
audio input designed to accept the analog output of a multichannel source.
This concept originated as a way to provide Dolby Digital upgrade capability
to Dolby Pro Logic processors. It has survived as a means of accommodating
future surround formats, including such new multichannel music media
as DVD-Audio and Super Audio CD (SACD). For that reason alone, it is
a very good feature to have. Some preamps have built-in FM or AM/FM
tuners, in which case they are known as tuner/preamps. These have antenna
inputs for their tuner sections. Some A/V preamps also have phono inputs
for connecting a turntable, but not all. If you want to play phonograph
records, make sure the preamp you buy has a phono input. And a handful
of A/V preamps have special AC-3 RF inputs for accepting Dolby Digital
signals from laserdisc players. But this is of no importance unless
you have laserdiscs with Dolby Digital soundtracks and a laserdisc player
with an AC-3 output. For all other Dolby Digital sources you would use
the regular digital audio inputs. In rare cases 5.1 ins/outs may use
a multi-pin DBA-25 connector in lieu of six standard RCA-type pin plugs.
Connectivity: Power Amplifiers
The connectivity issues for power amplifiers are minimal–you got
your ins, and you got your outs. Line inputs allow the amp to receive
input from the line outputs of the audio/video preamp. Normally these
are standard RCA phono jacks, though some multichannel amps also provide
a DB-25 connector for single-cable hookup to similarly equipped A/V
preamps. Speaker outputs, also called high-level outs, may be doubled
on the front left and right channels, to allow a stereo feed to a pair
of speakers in a remote room. The binding-post outputs commonly used
on separate power amps provide a more secure connection than the wire
clips more common on receivers.
The Human Factor
While surround amps tend to be stoic powerhouses, needing no user attention
during operation, surround preamps bristle with the same ease-of-use
issues that receivers have. In fact, a preamp may even be a little more
complex than a receiver in that it may offer more surround adjustment
options. Make sure any model you consider offers simple adjustment of
basic functions from the front panel and the remote. Surround setup,
input assignments, and other details should be accessible via on-screen
or front-panel menus, but ease of use may vary wildly. Some preamps
are designed to be set up by the dealer or the custom installer, whereas
other preamps can be operated by regular folks. Know which kind you're
buying! If you're a do-it-yourselfer, is the on-screen graphical user
interface (GUI) easy to navigate? Or are you forced to delve into many-layered
menus to operate basic functions? The preamp's remote is critical to
your system's ease of operation and your sanity. Does it cover the functions
you use most often? Do you like its overall look and feel? Are the keys
differentiated by size, shape, or layout? If you're a true darkened-room
home theater enthusiast, how about backlit keys? A universal remote
will unclutter your coffee table, but many preamp remotes operate only
the preamp. The manufacturer may assume the custom installer will provide
a custom remote-control solution, such as a touchscreen. If you are
shopping for one of the rare audio/video preamps with universal remote,
the most common kind is the preprogrammed remote, which contains a library
of codes to control not only the preamp but also your TV, DVD player,
VCR, and so forth. Smart manufacturers include codes for the products
of others. Even better sometimes is a learning remote, which accepts
codes directly from another remote–a more cumbersome but sometimes
more reliable method of programming.
|
