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One of the best things about digital information is that it is easy to move around and can be used in many different ways. That's the theory, but the reality isn't always so straightforward. This is never truer than when dealing with digital video. With so many formats and playback devices in common use, the possibilities for watching video are seemingly endless, but so too is the potential for confusion.

Digital video is a minefield because it has so many facets. Your new mobile phone may have MPEG4 Video emblazoned across its packaging but the file you load on to it might not play if it is not the right variation of MPEG4, encoded at the correct resolution, frame rate and bit rate. It will also need to use the correct audio format, sample rate and bit rate. And it needs to be wrapped in the correct container file type. If that sounds hopelessly complicated, don't despair. With a few key concepts under your belt, you'll be able to watch video on your own terms rather than be at the mercy of the technology.

Over the next four pages we explain the ins and outs of video formats and how to go about converting files. Armed with this knowledge, you should be able to find the right software and settings to enjoy video on any combination of devices, now and in the future.

What's in a video file?

We're tempted to compare the various parts of a video file to an onion; in fact, it's more like a walnut. The shell is the file type, otherwise known as the container. The type of container

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used is denoted by its file extension. Windows doesn't display file extensions by default, but when dealing with video it's handy to be able to see them. Open My Computer and click Tools, Folder Options, select the View tab, untick Hide extensions for known file types and click OK.

Common container types are AVI, MPG, WMV and MOV, although there are plenty of others. These names don't necessarily tell you much about the data within the file and some containers can hold only certain types of video data. For example, a WMV container supports only WMV video and WMA audio. Other containers, such as AVI, can hold a theoretically limitless variety of video and audio formats.

The main practical purpose of a container, from the user's point of view, is to provide compatibility with a specific hardware playback device, and to specify which software fires up when you double-click the file on your PC. Typically WMV files will open in Windows Media Player and MOV files will load in QuickTime, but most playback software can open a wide variety of container types. To change the default player for a particular container, right-click the file and select Open With, Choose Program... You'll be given a list of compatible software, so select the one you want, tick the box marked Always use the selected program to open this kind of file, and click OK.

Inside the walnut shell, things get more interesting. Like a walnut kernel, a video file has two halves - the video and the audio. They're clearly linked, but they can be thought of as entirely separate components. The only thing they always have in common is their duration.

Video content

The video component is made up of lots of discrete images, or frames, played in quick succession. These have two key variables: the resolution of each frame and the number of frames per second. UK TV broadcasts and DVDs use 25 frames per second (fps), and each frame comprises 720x576 pixels. This is known as the PAL standard. However, as if to prove that nothing is ever simple with digital video, neither specification is quite what it seems.