Digital Video

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Compression

Manual Compression

A person recording video for digitization can drastically affect the later compression steps. Video in which backgrounds are stable (or change slowly), for a period of time will yield a high compression rate. Scenes in which only a person's face from the shoulders upward is captured against a solid background will result in excellent compression. This type of video is often referred to as a 'talking head'.

Filtering

Filtering itself does not achieve any compression but it is a necessary step due to the artifacts of compression. Filtering is a preprocessing step performed upon video frame images before compression. Essentially it smoothes the sharp edges in an image where a sudden shift in color or luminance has occurred. The smoothing is performed by averaging adjacent groups of pixel values. Without the filtering preprocess step decompressed video exhibits aliasing (jagged edges), and moiré patterns as in the example pattern at the right.

What You Can't See Won't Hurt You

The human vision system is more sensitive to changes in luminance than in color. The second preprocessing step is a conversion from the RGB color scheme of a computer to the YUV color scheme that television uses. The Y value is termed the luminance and the U & V values are the chrominance. (In S-video two cables are used one for Y and one for UV. In component video each value is carried across a separate cable.) This color-space conversion allows for separate luminance and chrominance sampling in the digitization step.