The Digital Fact Book - Quantel

AAspect ratio1. – of pictures. The ratio of length to height of pictures. All TV screens used to be 4:3,i.e. four units across to three units in height, but now almost all new models, especiallywhere there is digital television,are widescreen,16:9. Pictures presented this way are believedto absorb more of our attention and have obvious advantages in certain productions,such as sport. In the change towards 16:9 some in-between ratios have been used fortransmission, such as 14:9.See also: Anamorphic, Widescreen, HDTV2. – of pixels. The aspect ratio of the area of a picture described by one pixel. The ITU-RBT.601 digital coding standard defines luminance pixels which are not square. In the 525/60format there are 486 active lines each with 720 samples of which 711 may be viewable dueto blanking. Therefore the pixel aspect ratios on 4:3 and 16:9 screens are:486/711 x 4/3 = 0.911 (tall487/711 x 16/9 = 1.218 (wide)For the 625/50 format there are 576 active lines each with 720 samples of which 702are viewable so the pixel aspect ratios are:576/702 x 4/3 = 1.094 (wide)576/702 x 16/9 = 1.458 (wider)The digital HD image standards all define square pixels.Account must be taken of pixel aspect ratios when, for example, executing DVE movessuch as rotating a circle. The circle must always remain circular and not become elliptical.Another area where pixel aspect ratio is important is in the movement of images betweenplatforms, such as computers and television systems. Computers generally use squarepixels so their aspect ratio must be adjusted for SD television-based applications.See also: ARC, PixelAsynchronous (data transfer)Carrying no separate timing information. There is no guarantee of time taken but a transferuses only small resources as these are shared with many others. A transfer is ‘stop-go’– depending on handshakes to check data is being received before sending more.Ethernet is asynchronous. Being indeterminate, asynchronous transfers of video filesare used between storage devices, such as disks, but are not ideal for ‘live’ operations.See: Ethernet, Isochronous, Synchronous26