Interlacing

Normal sequential scanning, i.e. scanning complete pictures (625 or 525 lines) at one time followed by another complete picture scan, introduces unacceptable flicker when the picture is reproduced by a c.r.t. This is because once scanned, the brightness of a line fades away gradually as it awaits to be refreshed. Flicker is greatly reduced by a technique known as interlacing. Interlace scanning involves scanning the ‘odd’ lines 1, 3, 5, etc., first followed by the ‘even’ lines 2, 4, 6, etc. Only one-half of the picture, known as a field, is scanned each time. A complete picture therefore consists of two fields, odd and even, resulting in a field frequency of 2 X 25 = 50 Hz for the UK system and or 2 X 30 = 60 Hz for the US system.

At the end of each field, the electron beam is deflected rapidly back to the beginning of the next scan. The odd flyback ends at the end of the last line of the odd field (point A in Figure 1.3) and the beam is then taken to the start of the first line of the even field (point B) travelling the distance of the screen height. To ensure the same flyback vertical distance and hence same field flyback time for both fields, the even flyback (dotted line) is ended halfway along the last line of the even field (point C) to take the beam to the start of the following odd field (point D) halfway along line 1. This is the reason the total number of lines is chosen to be an odd number.

Since the line scan continues to move the electron beam across the screen during the field flyback, the path traced by the beam during the flyback is as shown in Figure 1.4. In the absence of picture information, scanning produces what is known as a raster.