Chapter 5. Digital Video Processing

Processing Definitions

Besides encoding and decoding MPEG, NTSC/PAL, and many other types of video, a typical system usually requires considerable additional video processing.

For example, since many consumer displays, and most computer displays, are progressive (noninterlaced), interlaced video must be converted to progressive (interlaced-to-noninterlaced). Progressive video must be converted to interlaced to drive a conventional analog VCR or interlaced TV, requiring noninterlaced-to-interlaced conversion.

Many computer displays support refresh rates up to at least 75 frames per second. CRT-based televisions have a refresh rate of 50 or 59.94 (60/1.001) fields per second, and refresh rates of up to 120 frames per second are becoming common for flat-panel televisions. For film-based compressed content, the source may only be 24 frames per second. Thus, some form of frame rate conversion, or scan rate conversion, must be done.

Another not-so-subtle problem includes video scaling. SDTV and HDTV support multiple resolutions, yet the display may be a single, fixed resolution.

Alpha mixing and chroma keying are used to mix multiple video signals or video with computer-generated text and graphics. Alpha mixing ensures a smooth crossover between sources, allows subpixel positioning of text, and limits source transition bandwidths to simplify eventual encoding to composite video signals.

Since no source is perfect, even digital sources, user controls for adjustable brightness, contrast, saturation, and hue are always desirable.