Q. Explain Television signal transmission?
Television signals in television signal transmission are the electric signals generated by converting visual images through raster (TV image area) scanning. The two-dimensional image or picture is converted into a one-dimensional electric signal by sequentially scanning the image and producing an electrical signal that is proportional to the brightness level of the image. A television camera, which optically focuses the image on a photo cathode tube that consists of a photosensitive surface, is used for scanning. An electron beam produces an output current or voltage that is proportional to the brightness of the image, known as a video signal. The scanning of the electron beam is controlled by two voltages, as shown in Figure, applied across the horizontal and vertical deflection plates. In the raster scanning in an NTSC TV system, the image is divided into 525 lines which define a frame, as illustrated in Figure. The resulting signal is transmitted in 1/30 second. The number of lines determines the picture resolution and, along with the rate of transmission, sets the channel bandwidth needed for image transmission.
However, the time interval of 1/30 second to transmit a complete image is not generally fast enough to avoid flickering, which is annoying to the average viewer. Therefore, to overcome the flickering, the scanning of the image is performed in an interlaced pattern, as shown in Figure, consisting of two fields, each of 262.5 lines. Each field is transmitted in 1/60 second. The first field begins at point a and terminates at point b, whereas the second field begins at point c and terminates at point d.