ZOURCE CODE : FLICKER

(a) Flicker
Although the rate of 24 pictures per second in motion pictures and that of scanning 25 frames per second in television pictures is enough to cause an illusion of continuity, they are not rapid enough to allow the birghtness of one picture or frame to blend smoothly into the next through the time when the screen is blanked between successive frames.
This results in a definite flicker of light that is very annoying to the observer when the screen is made alternately bright and dark.
This problem is solved in motion pictures by showing each picture twice, so that 48 views of the scene are shown per second although there are still the same 24 picture frames per second.
As a result of the increased blanking rate, flicker is eliminated.
Interlaced scanning. In television pictures an effective rate of 50 vertical scans per second is utilized to reduce flicker.
This is accomplished by increasing the downward rate of travel of the scanning electron beam, so that every alternate line gets scanned instead of every successive line.
Then, when the beam reaches the bottom of the picture frame, it quickly returns to the top to scan those lines that were missed in the previous scanning.
Thus the total number of lines are divided into two groups called ‘fields’. Each field is scanned alternately. This method of scanning is known as interlaced scanning and is illustrated in Fig. 2.4. It reduces flicker to an acceptable level since the area of the screen is covered at twice the rate.
This is like reading alternate lines of a page from top to bottom once and then going back to read the remaining lines down to the bottom.
In the 625 lime monochrome system, for successful interlaced scanning, the 625 lines of each frame or picture are divided into sets of 312.5 lines and each set is scanned alternately to cover the entire picture area. To achieve this the horizontal sweep oscillator is made to work at a frequency of 15625 Hz (312.5 × 50 = 15625) to scan the same number of lines per frame (15625/25 = 625 lines), but the vertical sweep circuit is run at a frequency of 50 instead of 25 Hz.
Note that since the beam is now deflected from top to bottom in half the time and the horizontal oscillator is still operating at 15625 Hz, only half the total lines, i.e., 312.5 (625/2 = 312.5) get scanned during each vertical sweep.
Since the first field ends in a half line and the second field commences at middle of the line on the top of the target plate or screen (see Fig. 2.4), the beam is able to scan the remaining 312.5 alternate lines during its downward journey.
In all then, the beam scans 625 lines (312.5 × 2 = 625) per frame at the same rate of 15625 lines (312.5 × 50 = 15625) per second.
Therefore, with interlaced scanning the flicker effect is eliminated without increasing the speed of scanning, which in turn does not need any increase in the channel bandwidth.
It may be noted that the frame repetition rate of 25 (rather than 24 as used in motion pictures) was chosen to make the field frequency equal to the power line frequency of 50 Hz.
This helps in reducing the undesired effects of hum due to pickup from the mains, because then such effects in the picture stay still, instead of drifting up or down on the screen.
In the American TV system, a field frequency of 60 was adopted because the supply frequency is 60 Hz in USA.
This brings the total number of lines scanned per second ((525/2) × 60 = 15750) lines to practically the same as in the 625 line system.
Scanning periods. The waveshapes of both horizontal and vertical sweep currents are shown in Fig. 2.5. As shown there the retrace times involved (both horizontal and vertical) are due to physical limitations of practical scanning systems and are not utilized for transmitting or receiving any video signal.
The nominal duration of the horizontal line as shown in Fig. 2.5 (a) is 64 s (106/15625 = 64 µs), out of which the active line period is 52 µs and the remaining 12 µs is the line blanking period.
The beam returns during this short interval to the extreme left side of the frame to start tracing the next line. Similarly with the field frequency set at 50 Hz, the nominal duration of the vertical trace (see Fig. 2.5(b )) is 20 ms (1/50 = 20 ms).
Out of this period of 20 ms, 18.720 ms are spent in bringing the beam from top to bottom and the remaining 1.280 ms is taken by the beam to return back to the top to commence the next cycle.
Since the horizontal and vertical sweep oscillators operate continuously to achieve the fast sequence of interlaced scanning, 20 horizontal lines get traced during each vertical retrace interval.
Thus 40 scanning lines are lost per frame, as blanked lines during the retrace interval of two fields.
This leaves the active number of lines, N a, for scanning the picture details equal to 625 – 40 = 585, instead of the 625 lines actually scanned per frame.