APPENDIX 397How analog video is displayedAnalog video is displayed on an NTSC or PAL compatible device, such asa television set. The tube of a television set is made up of a series of lineswhich display the pictures that we see. The number of lines determines theresolution and quality of the video. (NTSC devices have 525 lines and PALhave 625.) When the video signal is received, it moves across and downthese lines, with each pass referred to as a scan, and one complete pass asa field. Once it has reached the end of the scan, it repeats the process,updating the display each time. This all happens very quickly, with thetube being scanned 60 times a second for NTSC devices and 50 for PALdevices.When television was first developed, the sets at the time had troublescanning so many lines for each field. To compensate for this, it wasdecided to interlace the display, (scan alternate lines rather than every line).For example, the first scan would pass over every even line, the secondevery odd. (As such it took two fields of information to display an actualimage.) This was still quick enough to fool the human eye into believingthat it was seeing a continuous and uninterrupted picture.The world of digital videoInstead of having the video represented by a series of waves and signals,digital video’s data is made up of a series of zeros and ones. Convertinganalog signals into digital data involves digitizing or capturing data. This isdone using a special hardware card called the video capture board. Thereare several varieties of video capture boards in the marketplace. Some aredesigned to perform professional broadcast videos, and others, to processthe more consumer-orientated home video.
