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  Sequential or Progressive Scanning

Introduction to Sequential Scanning or Progressive Scanning

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The electron ray begins to scan from line one and follow constantly (trace line and retrace line) making full scanning of 15625 in a second is termed as progressive scanning. In this technique there was a not avoidable problem happened.

FLICKER EFFECT:

Even though the rate of 24 pictures per second in motion pictures and that of scanning 25 frames per second in television pictures is sufficient to cause an fantasy of continuity, they are not sufficiently fast to permit the brightness of one picture or frame to blend smoothly into the next by the time while the screen is blanked among successive frames. This out comes in a specific flicker of light which is extremely annoying to the observer while the screen is made alternately bright and dark.

This difficulty is solved in motion pictures through displaying each picture twice that are why 48 views of the scene are displayed per second even though there are still similar 24 frames per second. The result of the raised blanking rate, flicker is removed.

INTERLACED SCANNING:

In television pictures are effective rate of 50 vertical scenes each second is used to decrease flicker. This is accomplished through raising the downward rate of travel of the scanning electron beam that is why every alternate line gets scanned in place of each successive line. After that, while the beam arrives at the bottom of the picture frame, it rapidly returns to the top to scan those lines which were missed in the preceding scanning. So the total numbers of lines are separated into two groups that are known as 'Fields' V Each field is scanned alternatively. This technique of scanning is termed as interlace scanning and is demonstrated in the diagram It decreases flicker to an acceptable level because the area of the screen is covered at two times the rate. This is similar to reading alternate lines of a page from top to bottom one time and after that going back to read the left over lines down to the bottom.

In the 625 line monochrome system, for very successful interlaced scanning, the 625 lines of every frame or picture are divided into sets of 312.5 lines and each set is scanned alternating to cover the entire picture area. To attain this the horizontal sweep oscillator is ready to work at a frequency of 15625 Hz(312.5/25 = 625 lines), however the vertical sweep circuit is run at a frequency of 50 in place of 25 Hz. Note : because the beam is now deflected from to top bottom in half the time and the horizontal oscillator is yet operating at 15625Hz, only half the total lines, that is 312.5(625/2=312.5) get scanned in each vertical sweep. Because the in a half line first field ends and the second field commences at middle of the line on the top of the target plate or screen, the beam is capable to scan the left over 312.5 alternate lines throughout downward journey of it. In total then, the bean scans 625 lines (312.5X2=625) per frame at similar rate of 15625 lines (312.4X50=15625) per second. Hence, with interlaced scanning the flicker effect is removed with no increasing the speed of scanning that in rum does not require any raise in the channel bandwidth.

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