TVEffects



	What are the World Television Standards?
	To convey motion, television in the US broadcasts 30 still frames per second. At this speed, the eye detects what it interprets as motion. In order to deliver the frames within the bandwidth constraints and frequency ranges that are available to television, each frame is split into two alternating fields: the first field contains the odd numbered lines and the second contains the even numbered lines. Each field contains a total of 241.5 lines to equal a combined 483 lines. The lines are created by an electron beam one line at a time from left to right, top to bottom. The electron beam displays all of the odd lines first, then the even. A vertical synchronization pulse returns the beam to the top of the screen between odd and even fields. One frame can be seen when all horizontal lines have been displayed. Motion is perceived when this process is repeated 30 times per second. The splitting of frames into fields and recombining them at the television is also know as interlacing. So why split the signal in this manner? Interlacing was the solution that was devised in the 1940's to prevent a noticeable flickering or strobe like effect from occurring to the broadcast picture and has remained in existence for over 50 years. However, there are currently three world-wide television standards in existence. · NTSC · PAL · SECAM		NTSC The NTSC standard has been in existence since 1941 but is in a transitional period from analog to digital broadcasts. NTSC is an acronym for the National Television Standards Committee. NTSC represents the television standard for North America as well as many other countries around the world. NTSC specifies a fixed vertical resolution of 525 horizontal scan lines at 60 Hz, however this number ends up being reduced to a maximum of 483 visible lines when the vertical blanking interval (VBI) and other non picture related lines are subtracted. PAL PAL is an acronym for Phase Alternation Line The PAL specification calls for 625 horizontal lines at 50 Hz. While this produces a sharper image than NTSC, it also has a tendency to produce more screen flicker with certain images due to a reduced bandwidth (compared with the 60 Hz NTSC standard). PAL originated in Germany and has become the standard for most of Western Europe. The number of available lines for PAL is 575 when considering the VBI. SECAM SECAM is a French acronym for Séquentiel Coleur Avec Mémoire. SECAM is the broadcast standard adopted in France, Eastern Eurpoe and portions of the Middle East. It is similar to the PAL standard at 625 lines at 50 Hz.