 16.2.1 Additional Definitions 16.2.1.1 Picture Element (pixel or pel). By definition, this is the smallest area of a television picture capable of being delineated by an electrical signal passed through the system or part thereof" (Ref. 1), a picture element has four important properties: 1. Pv, the vertical height of the picture element 2. Ph, the horizontal length of the picture element 3. Pa, the aspect ratio of the picture element 4. Np, the total number of picture elements in an entire picture The value of Np is often used to compare TV systems. In digital TV, a picture consists of a series of digital values that represent the points along the scanning path of an image. The digital values represent discrete points and we call these pixels (pels). The resolution of a digital image is determined by its pixel counts, horizontal and vertical. A typical computer picture image might have 640 X 480 pixels. 16.2.1.2 Aspect Ratio. This is the ratio of frame width to frame height. This ratio is defined by the active picture. For standard NTSC television, PAL television, and computer displays, the aspect ratio is 4:3 (1.33:1). Widescreen movies and HDTV have a 16:9 aspect ratio. 16.3 THE COMPOSITE SIGNAL 16.3 THE COMPOSITE SIGNAL The word composite is confusing in the TV industry. On one hand, composite may mean the combination of the full video signal plus the audio subcarrier; the meaning here is narrower. Composite in this case deals with the transmission of video information as well as the necessary synchronizing information. Consider Figure 16.3. An image that is made up of two black squares is scanned. The total time for the scan line is 63.5 usec, of which 53.3 usec is available for the transmission of actual video information and 10.2 usec is required for synchronization and flyback. 4 During the retrace time or flyback it is essential that no video information be trans- mitted. To accomplish this, a blanking pulse carries the signal voltage into the reference 63.5 us 53.3 us Scanning element Pattern Reference