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2.6 HORIZONTAL AND VERTICAL PAYMENT FREQUENCY The field speed of 60 Hz is the vertical scanning frequency. This is the rate at which the electron beam completes its cycle of vertical motion from top to bottom and back up again. Thus, the vertical deflection circuits for either the camera tube or the picture tube operate at 60 Hz. The time of each vertical scanning cycle for one field is second. The number of horizontal scanning lines in a field is half of the number of 525 lines for a complete frame, because one field contains every other line. It gives 262 horizontal line for each vertical field. Because the time for one field is seconds and because she contains 262 line, the number of lines every second is: 262 x 60 = 15,750. Or assuming 525 lines for a successive field pair which is a frame, we can multiply the frame rate by 30 by 525 which gives the same 15,750 lines traveled in 1 second. This 15,750 Hz frequency is the rate at which the electron beam completes its cycle of horizontal motion from left to right and back again to the left. Thus the horizontal deflection circuit for one of the camera tubes or picture tubes operates at 15,750 Hz. TIME FOR HORIZONTAL LINE The time for each horizontal checkout line (H) is second. In microseconds time H = s = 63.5 s (approach). This time in microseconds indicates that the video signal for image elements in horizontal lines can have high frequencies, namely in the order of megahertz. Note that the frequency f is equal to . If there are more lines, the scan time will be shorter which results in higher video frequencies. Actually in our system of 525 lines, the highest video frequency is limited to close to 4 MHz due to the 6 MHz limitation for commercial television transmitting channels. 2.7 HORIZONTAL AND VERTICAL ALIGNMENT The time spent in scanning is related to the distance in the image (shadow). Because the electron beam in the camera tube scans the image, it covers the different elements and provides the appropriate image information. Thus, when the electron beam strikes the picture tube screen at the receiver, the scan must be timed properly to get the picture information in the correct position. On the other hand, the electron beam in the picture tube may scan the part of the screen where the person's mouth should be when the image information is received at the appropriate time for the person's nose. To keep transmitter and receiver scans in line, special alignment signals must be sent along with image information to the receiver. The tuning pulses are emitted as part of the complete image signal to the receiver but they occur during the blanking time when no information is emitted. The image is cleared during this period while the electron beam is repeating the trace. A horizontal alignment pulse at the end of each line determines the start of the horizontal trace repeat. Note that the alignment is at the start of the trace loop or the end of the trace, and not at the start of the trace. The repeating horizontal trace of the electron scanning beam starts from the right of the figure. The vertical alignment at the end of each terrain determines the start of the vertical trail loop. At this time the electron scanning beam is at the bottom of the image. Without vertical field alignment, the image reproduced on the receiver does not persist vertically. He scrolls up or down on the picture tube screen. If the paylines are not aligned, the image does not persist horizontally. It slides left or right and then dismembers into diagonal segments. In summary, the horizontal line scanning frequency is 15,750 Hz. The frequency of the sync pulses is also 15,750 Hz. The frame repeat rate is 30 per second, but the vertical field scanning frequency is 60 Hz. The frequency of the vertical sync pulses is also 60 Hz. Note that the display frequencies of 15,750 and 60 Hz are appropriate for monochrome television but only approximations for color television. In color transmission, the horizontal line scanning frequency is exactly 15,734.26 Hz and the vertical field scanning frequency is 59.94 Hz. These precise scanning frequencies are used to minimize interference between the color enhancement carrier signal at 3.579545 MHz and the luminance (single color) signal. This technique is described in Chapter 8: “Color Television Sequences and Signals”. However, the horizontal and vertical viewing frequencies can generally be considered 15,750 and 60 Hz because the deflection circuits are automatically tuned to the required scanning frequency for both monochrome (single color) and color broadcasts.