Apple MA261Z/A User Manual - Page 1033

XIII 720 x 486 Versus 720 x 480 Another issue that comes up is the subtle difference between NTSC SD formats that use 486 lines per frame (Digital Betacam, D-1, D-5) and formats that use 480 lines per frame (DV, DVCPRO, DVD). Why is there this subtle difference? The reason is simple: 480 is divisible by 16, and 486 isn't. Divisibility by 16 is important for any MPEG-like compression codec, because each frame is broken into 16 x 16 pixel blocks (known as macroblocks) during compression. All of the video formats that use 480 lines per frame are DV or MPEGbased compressions, all of which use similar approaches to compression. The only time this should be a concern is if you are converting between a 486-line format like Digital Betacam and a 480-line format like DVD. However, the absence of 6 lines is hardly noticeable on an analog television, especially at the top and bottom, which is usually masked anyway. Pixel Aspect Ratio A pixel usually refers to a physical picture element on a video display that emanates light. But a pixel is also a term for a sample of light intensity-a piece of data for storing luma or chroma values. When stored on tape or on hard disk, the intensity of a pixel has no inherent shape, height, or width; it is merely a data value. For example, one pixel may have a value of 255, while another may be 150. The value of each pixel determines the intensity of a corresponding point on a video display. In an ideal world, all pixels would be captured and displayed as squares (equal height and width), but this is not always the case. The ITU-R 601 specification makes it possible to transmit either NTSC or PAL information in a single signal. To achieve this goal, both NTSC and PAL video lines are sampled 720 times. This results in either a 720 x 486 frame (NTSC) or a 720 x 576 frame (PAL). In both NTSC and PAL, the frame displayed has an aspect ratio 4:3, and yet neither 720 x 486 or 720 x 576 creates a 4:3 aspect ratio! The solution to this problem is to display the pixels (the samples of light intensity) taller-than-wide, or wider-than-tall, so that they fit into a 4:3 frame. This results in the concept of "rectangular pixels"- pixels that must be stretched or squeezed to fit in the 4:3 frame. To further confuse matters, most standard definition video devices actually use 704 or 708 pixels for picture information, not all 720. Appendix A Video Formats 1033