Types of motor

Video compression is composed of several  basic Processes. The first four are essentially preprocessing steps. Though they are Not considered compression, they can have a dramatic effect on data reduction.

¡Filtering

Images which lack sharp edges are easier to compress than Images which have sharp edges. Thus a preprocessing step in MPEG is to filter The image to reduce the sharp edges (i.E. High frequencies) by filtering.

The number of pixels used in the average is called the number Of taps. MPEG recommends using a seven tap filter.

¡Color space conversion

¡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.

The Advantage to this approach is that the Y value can be digitized at full Resolution while the U and V can be digitized at lower resolutions without Causing the eye to notice any reduction in information or detail.

Digitizing

There are Various different sampling schemes used for different compression methods and Video formats.

In studio Quality video, the U and V components are sampled twice for every four Y Samples (4:2:2 YUV).  Scaling

The largest Reduction in data occurs by scaling (subsampling) the image when it is not Displayed in a full 640 X 480 pixel window or when a reduction in spatial Resolution is tolerable

The Processes that directly result in compression are

In MPEG and JPEG, the low frequency DCT Coefficients are quantized using approximately four bits meaning they have 16 Different levels. The higher frequency coefficients are quantized with fewer Bits, usually, 2,1, or zero bits. This means that many of the high frequency Coefficients are converted to zero which causes them to be eliminated.