It can totally ruin a movie night if the picture and sound don't match up properly. Along with tips for avoiding this irritating problem, we show you how to get the best sound from your home-theater system - it's all a question of correct listening distances and correct center-speaker positioning.

If the picture is very wide, you can even put the speakers behind the projection screen.

It's irritating when the picture and sound are out of sync: The actors' lips and voices don't match up, spoiling the movie fun. The inconsistency is even more obvious - and irritating - during explosions or other loud action scenes. The spanner in the works might be a poorly adjusted home-theater setup, in need of re-adjustment; or the movie's sound may just have been badly synchronized.

TV manufacturers use all kinds of tricks to get the best sound possible from a TV's built-in speakers, but you'll always get better sound from a decent external sound system. Nevertheless, there are a few things to watch out for when positioning the speakers: Stereo speakers, for example, should always stand at least 1.5 meters from one another. With smaller screens - 32 inches, say - this means the speakers will stand further from the TV; with larger screens, they'll be closer.

Still, the distance between the speakers should never be too large, for two reasons: If you push them far into the corners of the room, the first thing to suffer is the sound - you must always keep a certain minimum distance from the walls. Secondly, an arrangement like this will disturb the viewer's picture-sound perception: For example, a sound that should come from the left of the picture reaches the ear not from slightly to the left, but from the extreme left - this sounds unnatural.

It's important to find a compromise, especially with smaller screens: If you frequently use the setup for just listening to music, it's better to keep a slightly larger distance between the speakers. If the speakers primarily pump out movie and TV sound, you should put them closer to the screen. Incidentally, the ideal speaker arrangement for listening to music - and the setup commonly used in recording studios - is to have five speakers (plus the center speaker) arranged in a circle around the listener.

Traditionally, the center speaker sits below the TV screen. No wonder: After all, many TV stands provide an extra compartment just for this purpose, housing the center in a tidy place that's free of cable clutter.

Although practical and attractive, this solution also causes some problems, for example, due to the center's proximity to the floor. This produces slight reflections, and the ear finds it difficult to block out these blurry, imprecise sounds. A rug in front of the center speaker can help, as can tilting the speaker slightly upwards.

Furthermore, when positioned below the TV, the center speaker is actually relatively far from the picture, separating the sound from its supposed source. Dialog almost always takes place in the upper third of the screen (people don't speak from their stomachs or feet), so it's more sensible to position the center speaker above the screen.

Where should the center speaker go - above or below? Both options have their ups and downs.

Positioning the center speaker above the screen stirs up a new set of problems: How do you mount the speaker? Doesn't it look strange there? Indeed, a bulky speaker directly above a flat-panel TV looks anything but attractive and, actually, completely out of place. Mounting the speaker higher - near the ceiling - would be even worse for the acoustics than putting it below the TV.

So, this isn't really an option for wall-mounted TVs; you're better off putting the TV on a rack or pedestal and fitting an additional stand or bracket for the center speaker - now things are getting complicated!

Nevertheless, there are some relatively simple solutions. For example: A fixed-frame projection screen hanging in front of a bookcase means you can position the center speaker just above the screen on one of the bookcase's shelves. If you have a dedicated home theater, it's even easier to put the center speaker above the screen: Mount the screen a short distance in front of the wall so that the center and the other speakers can sit neatly behind it.

Invisible speakers improve the relation between sound and picture yet further - what you can't see is harder to localize. In order to do this, however, you need an acoustically transparent screen that doesn't dampen the sound excessively. The speakers should be at least 20 centimeters behind the screen - ideally further - and you should cover the whole wall behind the screen with a black, dampening material to eliminate reflected light.

With the center mounted above the picture, it almost forms a line with the tweeters of the front speakers.

If you can't decide between above and below, consider one further option: two center speakers! With one above and one below (but never at the sides), the sound actually comes from the center of the picture. The catch? This takes a lot of effort to get right.

First of all, you need two high-quality center speakers, which will cost a lot of money. Secondly, you would ideally use a second power amplifier, pushing the cost up even further. Thirdly, you have to position both center speakers precisely - the distance from each speaker to the listeners must be exactly the same; otherwise, the effect is lost. The brain can only form a "phantom source" in the middle of the picture if the two sounds arrive at exactly the same time.

If you want to use two center speakers, make sure they are exactly the same distance from the viewers.

A completely different approach: no center speaker at all! The so-called "quadraphonic" system manages with just four equal speakers and no center speaker, producing instead a "phantom center". The advantage: If the center channel is split equally between the right and left speakers, voices appear to come from the exact middle of the picture. Noises crossing the picture - a passing car, for example - also move realistically.

There's a simple reason, however, behind the quadraphonic system's failure to establish a broader user-base, remaining instead an enthusiast's pursuit: It only works perfectly for one listener/viewer, sitting precisely in the middle of the speakers. Some insist that it will also work for two people, but this is the absolute limit if you want accurate sound.

There are other disadvantages, too: The systems lack force because their mid-range frequencies are weak -there's one less speaker - and it takes extremely high-quality (and expensive) systems to produce phantom sources competently.

The sound and picture have to match up in time, as well as in location. Unfortunately, in the era of digital signal-processing, this isn't always the case. The picture and sound get out of sync because one type of signal can take longer to process than the other. The picture generally takes longer than the sound, because pictures contain significantly larger amounts of data. Experts disagree on how long the delay can be before it becomes irritating for the viewer.

The best solution comes from modern AV receivers with a "lip-sync" function, which equalizes the delays of the audio and video signals reliably. If your receiver has no lip-sync function, you can try using an external delay box. Currently, these are only available for S/P-DIF signals, so they cannot handle the HD audio formats of Blu-ray discs (Dolby TrueHD and DTS-HD).

Furthermore, finding the precise setting can be tricky: You first have to look for a suitable test scene (a car door slamming, for example) and then make manual corrections. Be aware that the delay can differ between different formats, such as between DVD and Blu-ray, making a tricky process even harder.

Delay boxes, such as the Master Delay from Primare, can synchronize sound and picture - but only for S/P-DIF signals.

Matching picture and sound is no luxury; rather, it's an essential requirement for home-theater satisfaction. With a correctly positioned and accurately adjusted setup, a good movie night doesn't consist of 50 percent picture and 50 percent sound, but rather 100 percent movie fun.