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The Specs Explained

Among the many characteristics that define an audio system, only a relatively small number are really important to making a good buying decision. Here are the key specifications for a home theater audio system. We've divided them into three groups: important, somewhat important, and minor.

Important: Surround Decoding

The surround-sound formats supported by the system will affect the realism and intensity of the experience.

Dolby Pro Logic: This base-level surround decoding will work with essentially any source. Generates front left and right, center, and surround outputs (with the mono surround output normally reproduced through two speakers). Works well only with Dolby Surround-encoded soundtracks. This or Dolby Pro Logic II is an essential feature.

Dolby Pro Logic II: This advanced version of Dolby Pro Logic has now largely supplanted the original system. It provides superior surround decoding with stereo surround output. Also works very well for generating five-channel surround from two-channel stereo music recordings, such as CDs. Highly desirable. If you could have only one surround decoding option, this would be the one to choose.

Dolby Pro Logic IIx: This enhanced version of Dolby Pro Logic II can deliver up to 7.1-channel output. Nice if you can put one or two back-surround speakers in the room in addition to the standard left and right surrounds, but no better than the vanilla version otherwise.

Dolby Digital: Dolby Digital decoding is the best choice for multichannel Dolby Digital soundtracks delivered in that format, as they are on DVD, HDTV, and some satellite transmissions. But it's useless for things like VHS releases or analog TV programs that can't carry Dolby Digital but can deliver Dolby Surround. For those you need Dolby Pro Logic or Pro Logic II, which are, in effect, universal sound decoders. And, if you have to choose just one, choose Dolby Pro Logic II.

Dolby Digital EX: This is an enhanced version of Dolby Digital that allows a back-surround channel in soundtracks in addition to the usual left and right surrounds. Nice to have if you can accommodate the extra speaker or two at the back, but no better than the standard version otherwise. Works well with Dolby Digital EX and some regular Dolby Digital soundtracks.

Dolby Digital Plus: Dolby Digital Plus is the next generation of Dolby Digital.

Dolby TrueHD: This lossless (read: uncompressed) encoding/decoding format is the standard for HD-DVD and is an option on Blu-ray Disc.

DTS: an alternative surround encoding/decoding format that is functionally equivalent to Dolby Digital. It offers no advantage, however, and is not as widely used. Not really needed except for certain special DTS-encoded multichannel CDs.

Important: Frequency Response

Frequency response measures both the range of frequencies that can be reproduced, from lows to highs, and the evenness of their reproduction. For example, you might see something like this: 40 Hz to 20 kHz, +/-3 dB. That means low frequencies from 40 Hz (cycles per second) to high frequencies up to 20,000 Hz (the approximate upper limit of human hearing) are reproduced with no more than 3 decibels of deviation from perfect accuracy ("flat" response).

The part of all this that is worth paying attention to is the low-frequency limit. For most music and many movies outside the action and sci-fi genres, a lower bass limit of 50 Hz will do. Pushing the bass down to 40 Hz will ensure that you never feel seriously deprived. And if you get it down to 30 Hz or below, you can feel some pretty bone-rattling effects.

Important: Audio Inputs

The number and type of audio inputs available in the control center or audio/video receiver determines how many sources you can connect to it and what you can do with their outputs.

Line-level stereo: Your ordinary, everyday analog audio connection, normally consisting of two RCA jacks. Any audio or audio/video source will have compatible outputs. These are fine for stereo or Dolby Pro Logic reproduction.

Coaxial digital: One of two types of digital audio connections. This type uses an RCA jack; the cable should be 75-ohm coax (any cable labeled for video or digital audio). A digital audio connection--either coaxial or optical--is necessary for Dolby Digital (or DTS) decoding. The type doesn't matter; you just have to match input to output.

Optical (Toslink) digital: The other type of digital audio connection, which uses fiber-optic cable for signal transmission.

Somewhat Important: Speaker Configuration

Surround encoding and playback is defined according to a number system.

For example, Dolby Surround is a 4.0 system, with front left and right, center, and mono surround channels. Dolby Digital is a 5.1 system, with front left and right, center, left and right surround, and low-frequency effects channels. (The LFE is the ".1 " channel.)

Dolby Digital EX is a 6.1 system. The speaker systems used to reproduce these formats are usually 5.1, 6.1, or 7.1, with the latter two adding one or two back-surround speakers, respectively, in addition to the standard left and right surrounds. In this context, the ".1 " refers to a subwoofer (which will usually reproduce deep bass from the main channels in addition to the LFE). In some cases one surround channel may be routed through two surround speakers, as in a Dolby Pro Logic decoded Dolby Surround soundtrack in a 5.1 system or Dolby Digital EX in a 7.1 system.

5.1: This is pretty much the base format now, capable of excellent performance.

6.1: This system adds a third surround speaker directly behind the seating area, which is nice when the room can accommodate the extra speaker, but generally not practical when the seats are against the back wall.

7.1: Two back-surround speakers are positioned behind the seating area instead of just one. A 6.1-channel soundtrack usually sounds better with 7.1 playback than with 6.1, so it's not gilding the lily.

Somewhat Important: Video Inputs

These are only somewhat important because you usually don't have to route video signals through the audio control center. But video inputs can be a convenience.

Composite-video: This provides the lowest quality but broadest compatibility. Any device that has video outputs will include composite-video among them. The connection is made with a single 75-ohm coaxial cable between RCA jacks.

S-video: For better quality, most video sources except standard VCRs now have S-video outputs. This is connected with a special cable and multipin sockets.

Component-video: This high-quality spec is the minimum standard for connecting HDTV tuners and progressive-scan DVD players. It requires three 75-ohm coaxial cables of the same type used for composite video.

HDMI: This digital RGB connection, the highest-quality option, is commonly used for HDTV tuners and occasionally for DVD players. It requires a special cable and multipin sockets. You're not likely to find this one on anything other than high-end integrated home-theater audio systems.

Minor: Amplifier Power

Surprised? A speaker's ability to reproduce sound loudly depends on both amplifier power and the sensitivity of the speakers, which is seldom specified for this type of system. Highly sensitive speakers don't need nearly as much power as less-sensitive ones to sound just as loud. For example, a speaker with 91-dB sensitivity will sound just as loud with a 50-watt amplifier as a speaker with 88-dB sensitivity will with a 100-watt amp. And because amplifier power is cheap these days, it's rare for any systems, other than very low-end ones, to be underpowered. The shortcomings tend to be elsewhere.