d in succession a rate of 60 frames per second, even though complete pictures are still built up at a frame-per-second rate. It should be noted that interleaving improves resolution as well as apparent persistence of the cathode ray tube (CRT) by tending to reinforce the scanning spots. It has been found convenient to equate flicker frequency to power line frequency. Hence in North American practice, where power line frequency is 60 Hz, the flicker is 60 frames per second. In Europe it is 50 frames per second to correspond to the 50-Hz line frequency used there. Following North American practice, some other important parameters derive from the previous paragraphs: 1. A field period is 60 1 sec. This is the time required to scan a full picture on every horizontal line. 2. The second scan covers the lines not scanned on the first period, offset one-half horizontal line. 30 1 sec is required to scan all lines on a complete picture. 4. The transmit time of exploring and reproducing scanning elements or spots along each scanning line is 15,750 sec (525 lines in 30 1 sec) = 63.5 usec. TELEVISION TRANSMISSION Figure 16.2 Development of a sinusoid wave from the scan of adjacent squares. 5. Consider that about 16% of the 63.5 usec is consumed in flyback and synchro- nization. Accordingly, only about 53.3 usec are left per line of picture to transmit information. What will be the bandwidth necessary to transmit images SO described? Consider the worst case, where each scanning line is made up of alternate black-and-white squares, each the size of the scanning element. There would be 652 such elements. Scan the picture, and a square wave will result, with a positive-going square for white and a negative for black. If we let a pair of adjacent square waves be equivalent to a sinusoid (see Figure 16.2), then the baseband required to transmit the image will have an upper cutoff frequency of about 6.36 MHz, provided that there is no degradation in the intervening transmission system. The lower limit will be dc or zero frequency.