The days when flat-panel thin-film transistor (TFT) monitors were an expensive luxury are well and truly over. Prices have come down dramatically in the past couple of years, and flat-panel monitors now outnumber their bulky brothers in high street stores.

The space-saving design of flat-panel monitors makes them ideal for home users who don't want a hulking great CRT monitor taking over their bedroom or home office. But size isn't the only difference between TFT and CRT monitors.

You don't need to understand all the technical details of how a TFT monitor works, but there is one important bit of jargon that you do need to know about.

Most TFT monitors use what is known as a digital video interface (DVI) to connect to your PC. Understanding what DVI is and how it differs from the video graphics array (VGA) interface used by old-fashioned CRT monitors is important if you're going to get the best out of your shiny new TFT monitor.

The digital difference
The main difference between TFT and CRT monitors is that TFT monitors are digital devices, whereas CRT monitors (and televisions) are analogue. It's just like the difference between old vinyl records (analogue) and modern audio CDs (digital).

The socket at the back of an ordinary CRT monitor that you use to connect the unit to a PC is called a VGA connector.

At the back of your PC you'll find a corresponding VGA connector that is built onto the graphics card installed inside your PC. You can then use a standard VGA cable to plug the monitor straight into your PC.

That's straightforward enough; all you need to remember is that the VGA interface can only handle an analogue signal.

There's a problem here, though. The graphics card inside your PC is a digital device, and the images it generates are digital.

To display an image on an analogue CRT monitor the graphics card first has to convert its digital signal to analogue so it that can then be transmitted along the VGA cable to the CRT monitor.

There's a risk of losing image quality during this conversion process, although this isn't really a problem for most modern CRT monitors, and they generally do produce excellent image quality.

In fact, until relatively recently, many professional designers and photographers still preferred to use CRT monitors as they had a slight edge for high-quality photography or graphics work.

That is no longer the case, though, and the latest TFT monitors provide superb image quality. But, to get the best image quality that the monitor is capable of, you will need to use a DVI interface rather than VGA.

As the name implies, the DVI sends a pure digital signal straight from the PC to the monitor with no conversion process to affect the quality of the image displayed on the screen.

You will, however, need to have a DVI connector on both the TFT monitor and on the PC's graphics card, along with a DVI cable to connect the two.

It's easy to see if there's a DVI connector on your monitor or at the back of your PC, as it looks completely different to a VGA connector.

The VGA socket on your PC's graphics card has three rows of round pin holes and is trapezoidal in shape. A DVI socket is rectangular in shape, usually white in colour and has three rows of square pin holes, with a further four square pin holes to the right.

You won't find a DVI socket on a CRT monitor, as there's no point putting a digital link on an analogue monitor.

On the cards
Most modern graphics cards will include both DVI and VGA connection, but older PCs and graphics cards may only have a VGA interface. So, to cater for older PCs, many TFT monitors also have both VGA and DVI interfaces.

That means there are two options for connecting to the monitor, and if you've already got a VGA cable lying around from an old monitor then the VGA connection will seem like the obvious choice. Unfortunately, it's also the worst choice.

As we've already mentioned, the VGA interface can only handle an analogue signal, so a PC's graphics card has to convert its digital signal into an analogue signal before sending it along the VGA cable to the monitor.

But the TFT monitor is a digital device, so the analogue signal then has to be converted back into digital format. This means you have converted the signal from digital to analogue and then back to digital once more, with each conversion affecting the image quality slightly.

To get the best possible picture quality you'll need to use the DVI interface on both the monitor and your PC. This allows the PC to send a pure digital signal straight from the graphics card to the TFT monitor, with no conversion and no risk of degrading the image.

Any TFT monitor with a DVI should include the necessary DVI cable, but these are standard cables that will work with any model of monitor so you should be able to pick them up for about £15 from any decent computer store.

There's no need to panic if your PC's graphics card doesn't have a DVI interface. The image quality you get through the VGA interface will be perfectly useable - it just won't have the pristine sharpness and clarity of the original digital signal created by a PC's graphics card.

The decision is yours
There is one other possible fly in the ointment, though. There are now quite a few TFT monitors on sale that only have DVI interfaces. Needless to say, you can only use these monitors if your graphics card also has a DVI interface.

Fortunately, DVI graphics cards are quite commonplace now, and you can buy one for less than £100, so it won't cost you a fortune if you decide to go the DVI route.

In fact, some of the latest graphics cards only have DVI interfaces on them. However, these cards generally include a small VGA adaptor, so that you can still use the graphics card with any monitor that has a VGA interface.

Unfortunately these adaptors won't work the other way around, allowing you to connect a VGA graphics card to a monitor that only has a DVI interface.

If you currently have a CRT monitor and you're happy with it, then you don't need to worry too much about DVI. But if you like the idea of a space-saving TFT monitor, it really makes sense to use its DVI option rather than the old-fashioned VGA interface.