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Pressing Matters - Compression

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The Christmas shopping should be well and truly over now - and if it isn't you're in deep trouble (particularly if you're reading this article long after the publishing date!). So for the next two weeks we're taking a break from the latest and greatest in digital camcorder products and delving deeply into one of the main technologies which has made them possible - video compression.

Pressing matters - Compression

Video requires huge amounts of bandwidth. In Europe, regular TV - or ‘standard definition' (SD) - operates at a resolution of 720 x 576 with 25 frames per second. Using 24-bit colour, that equates to about 30MB/sec of data. So, long before high definition arrived, video data rates were already well beyond what most consumer hardware could cope with. Encoding-decoding algorithms, or codecs, were an absolute necessity for the digital video revolution we've been enjoying for a decade or more.

Broadly speaking, codecs are either ‘intraframe' or ‘interframe'. Intraframe means that each frame is treated as a separate still image and compressed only with reference to the pixels contained in that frame. Interframe compression, on the other hand, records the differences between successive frames and an initial keyframe. This allows smaller overall file sizes than with the intraframe method, as areas of the frame which remain static can be greatly compressed.

However, there is a much larger processing overhead for encoding and decoding interframe compression. This is because the system must load the keyframe and all the successive frames which refer to it in order to decode any individual frame. For this reason, interframe compression has traditionally been unpopular as an editing format, although that has changed in the last few years.

A common example of an intraframe codec is DV, which we will be explaining in more detail later in this article. The most common interframe codec is MPEG, which we will be looking at in all its flavours next week. However, above codecs are the file formats most of us will be much more familiar with - QuickTime and AVI. These aren't codecs themselves, but containers for codecs, with more than one option available.

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