poynton@vector.Sun.COM (Charles A. Poynton) (08/08/89)
Chad Fogg <cfogg@blake.acs.washington.edu> asks in Rec.video: > What is composite/component digital? Is it the assignment of, say, > 12 bits for chroma and 12 bits for luminence per pixel/ 8 bits of > shade for each red, green, blue element of each pixel? Briefly, YUV means luminance (Y, roughly brightness) in one signal, and two colour difference signals which are scaled B-Y (blue minus luminance) and R-Y (red minus luminance). Virtually no digital video hardware actually deals with quantities of hue or saturation as such; think of U and V as being roughly equivalent to rectangular coordinate versions of intuitive [polar] hue and saturation. 4:2:2 implies YUV, and additionally means that U and V are subsampled horizontally by a factor of two with respect to luminance. 4:2:2:4 adds an alpha/matte/key/transparency channel, typically denoted A for 'alpha' (for historical reasons), to 4:2:2. These notations say nothing about the number of bits in each component. 4:4:4 refers to a system with no horizontal subsampling, and may be either RGB or YUV. 4:4:4:4 is obviously equal-bandwidth RGBA or YUVA. D-1 is a 4:2:2 YUV (component) tape format which records 8 bits of each component. All of the above exist in 525/59.94 and 625/50 versions. Of course we can't call these NTSC or PAL, can we, because they're COMPONENT systems. D-2 refers to a COMPOSITE a digital VTR, which records NTSC digitized to 8 bits. Since the digitization must include sync, burst, and modulated chroma, the luminance range is reduced to very nearly seven bits. Any D-2 recording has been stomped upon by the NTSC encoding footprint, never to be fully restored to components. However, little additional impairment will be added in subsequent generations. D-2 is supposed to be introduced for PAL, but hasn't been yet. No current VTR records an alpha/matte/key/transparency channel, although certain processing devices handle it. Although D-1 and D-2 properly refer to digital videotape formats, the terms are in the process of becoming confused with interfaces. Following this article I'll post a copy of my Technical Note 33 titled "Overview of Digital Video Interfaces" which, although it talks about interfaces for digital video in the studio, contains some information about studio DVTRs such as D-1 and D-2. C. ----- Charles A. Poynton Sun Microsystems Inc. <poynton@sun.com> 2550 Garcia Avenue, MS 8-04 415-336-7846 Mountain View, CA 94043 "Japan has no laws against damage to its flag, but it has strict laws forbidding the burning of foreign flags lest this give offense to the country in question." -- The Economist, July 1, 1989, p. 19. -----