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, howeverespecially one outfitting a large, dedicated home theater roomseparates 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, lets 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 FormatsAlthough there are quite a few formats for delivering surround sound these dayssome complementary, come competitiveonly two or three basic channel configurations are in common use. So before getting on to the delivery formats, its 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 LogicThe 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 DigitalThe 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 playbacka 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 mediapulse-code modulation, or PCMrequires 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 dont have to extract the extra channel to enjoy them.
DTSDTS 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.
THXA number of years ago, Lucasfilm established a standards program for movie theaters called THX. (The name came from "Tom Holmans Experiment," after the programs originator, and coincidentally from the title of George Lucass 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 decodingPro Logic, Dolby Digital, DTS, or whatever. There is no such thing as a THX soundtrack.) In electronics, the main processes were re-equalizationa mild treble rolloff to correct for acoustical differences between auditorium- and domestic-size roomsand "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 Harmans 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 IssuesOne 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 youre spending, the more reason you have to consider the future upgrade path in your buying decision.
Amplifier IssuesMuscle 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, youre weighing two amplifiers, one rated at 100 watts per channel and the other at 120, base your decision on something else, as thats 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 PreampsCount 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:
- 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 doesnt 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 preampoptical 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 weve 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 preamps 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.