• Active and passive systems
• Subwoofers
• Decoders and digital audio
• Stereo and multi-channel systems
• Getting surrounded by sound
• Surround formats and compatibility
• 2-way and 3-way systems
• Centre speakers and satellites
• What about wireless systems?
• The little things

Getting surrounded by sound

Both 7.1 and 6.1 systems will play any surround sound format currently used in DVD audio. On the other hand, with a 5.1 system you are limited to only the most common formats - Dolby Digital and DTS. In any case, you will need an amplifier that supports the number of channels (i.e. speakers) you plan to buy. You will also need a decoder that supports the right encoding formats. If you are buying a powered system with a decoder, you won't need to worry about matching everything up. If you're buying or own an amplifier, however, make sure that it supports all the relevant formats for the number of speakers you're getting. These are summarised here:

• Dolby Digital 5.1
• DTS 5.1
• Dolby Pro Logic II 5.1
• THX Surround EX 6.1
• Dolby Digital EX (DDEX) 6.1
• DTS ES 6.1
• DTS Neo:6 6.1
• DTS ES Discrete 6.1
• THX Ultra2 7.1
• Dolby Prologic IIx 7.1

Surround formats and compatibility

Basically, the two big players in the surround sound arena are Dolby and DTS, both of which come in a variety of flavours. Practically every DVD will have a Dolby Digital soundtrack, and almost as many will have DTS. Aside from these two, the remaining formats are less used and you will probably have to seek out titles that support them - particularly THX. Another thing to bear in mind is that not all Dolby Digital soundtracks are recorded in 5.1, some are just plain stereo. Most AV amplifiers will let you choose between using all your speakers or just the front stereo pair for playback of stereo audio, but not all powered systems will be this flexible.

2-way and 3-way systems

It is common for budget systems to utilise a single cone or driver in all the speakers. Speakers with two or three cones are called 2-way or 3-way speakers, and these are designed to separate the middle and high end frequencies into separate drivers. While this can be a sign of quality, it is worth bearing in mind that 2 and 3-way speakers require additional technology, such as a crossover, which means there is need for more precision in the manufacturing process. Because surround systems use a subwoofer, there is a convincing argument that there is no real need to use multiple drivers in the satellites. One possible exception is the centre speaker which may be required to cover a broader spectrum than the others. There are great high-end systems that are 1-way and terrible budget packages that are 2 and 3-way. Consequently, having more speaker drivers isn't necessarily a feature to judge a system by.

Centre speakers and satellites

The centre speaker of a surround sound system is special. This is mainly because it is the place where most movie dialogue is sent. Because of this, some 5.1 systems use a different type of speaker for the centre channel. It may, for instance, be 3-way whereas the satellites are only 2-way. Systems based on 1-way satellites may alternatively use a 2-way speaker for the centre channel. In most cases, the centre speaker is a low-profile version of the satellites, making it easier to place without obstructing the screen. Occasionally, however, all 5 or 6 satellites, including the centre speaker, are identical. This is the case in both high end and budget systems, so this isn't automatically a sign of quality.

What about wireless systems?

If running 6 or 7 speaker cables around your room doesn't sound like your idea of fun, there is always the option of installing a wireless system. Unfortunately, there are some intrinsic issues with wireless technology that makes multi channel audio problematic. Firstly, there is the issue of quality. To circumvent the hiss and hum that is produced by radio frequency (RF) transmissions, some vendors are using digital modulation to provide crystal clear audio over wireless. Unfortunately, this does come at considerable expense. The other issue is that wireless connections can introduce timing errors during playback. Although noticeable loss of synchronisation with the image is unlikely, the time difference between wired and wireless speakers in the same system can result in phase cancellation and other audible interference artefacts. The irony, however, is that no matter how good the end result is, the wireless speakers will still need to be plugged into a power point. This means you can enjoy fewer speaker wires at the expense of a couple of extra power cables.

The little things

Aside from the big issues of whether to choose a powered, 6.1 or wireless system, there are a handful of little features that can make all the difference when you're setting everything up. Firstly, check that the speaker cables are long enough for trailing around the walls of your room. There's nothing more frustrating that getting a system home only to find you can't install it properly until you buy some more cable. Secondly, check the connector type on both the speaker and your amplifier. Most amplifiers and speakers use bare wires that are held in place by clamps or screw connectors, but others may use RCA connectors or banana plugs, for instance. Powered systems will often use RCA connectors and this isn't such a bad thing either, as it is easy to lengthen their reach with extension cables. Last but not least, if you're not overly experienced with installing speakers, see if the cables are colour coded. This way you can be sure you have all the speakers correctly positioned and connected to the right sockets.