SCANNING PRINCIPLES OF TELEVISION
Scanning is a technique for converting the charge images created within a television camera tube into a variable electrical signal. The TV image is scanned in a sequence of horizontal lines that are stacked one on top of the other. It's the same as reading a text page and covering all of the words in one line and all of the lines on the page. All of the picture elements are scanned one line at a time, from left to right and top to bottom.
The scene is scanned in both the horizontal and vertical directions at the same time to generate enough full images or frames per second to maintain continuous motion. Most television systems have a frame repetition rate of 25 frames per second. The following is the order in which all of the picture elements should be scanned. They are,
1. An electron beam passes over a horizontal line, transforming all of the image components in that line. Trace is the name given to this period. Horizontal scanning is used to accomplish this.
2. The beam immediately returns to the left side after each line to begin scanning the following horizontal line. Retrace or fly back are the terms used to describe the return lines. Because both the camera tube and the image tube are blanked out during retrace, no picture information is scanned. As a result, the retraces must be very quick, as they lose time in terms of image information.
3. When the beam returns to the left side, the vertical position of the beam is lowered so that it scans the next line down instead of repeating the same line. The vertical scanning motion of the beam achieves this.
Types of scanning:
a. Horizontal Scanning
b. Vertical Scanning
c. Flicker
d. Interlaced Scanning
(A) HORIZONTAL SCANNING:
The electron beam is deflected across the screen with a continuous, uniform motion for the trace from left to right due to the linear increase of current delivered to the horizontal deflection coils. The sawtooth wave direction swiftly reduces to its starting value at the top of the ascent. The retrace or fly back is the result of these quick reversals. At the left border of the raster, the horizontal trace begins. The finish is on the right edge, where the flyback causes a retrace to the left.
Figure1: Waveform of Horizontal Deflection Coils
Figure 1 shows the waveform of the horizontal deflection coil. The horizontal displacement to the right corresponds to ‘up' on the sawtooth wave. The usable scanning time is indicated by the dark line, while the retrace time is indicated by the dashed lines. To contain a higher number of image elements and hence more information, the number of scanning lines for a single full picture should be large. The horizontal scanning frequency is 15,625 Hz in the 625 line system.
(B) VERTICAL
SCANNING:
While the electron beam is being deflected horizontally, the sawtooth current provided to the vertical deflection coils moves the electron beam at a consistent pace from top to bottom of the raster. As a result, when traveling from top to bottom, the beam creates one below the other. In Figure 2, we can see the vertical scanning waveforms.
Figure 2 Vertical Deflection Waveform
The vertical scanning beam is deflected to the bottom of the raster by the trace component of the sawtooth wave, as illustrated in Fig. After that, the beam is quickly vertically retraced back to the top. At the bottom of the raster, the vertical sweep current achieves its maximum amplitude, delivering the beam to the raster's bottom.
The horizontal scanning is ongoing during vertical retrace, and numerous lines are scanned during this time. The information is transformed into an electrical signal using the scanning beam. The vertical scanning frequency is 25Hz, that is every second 25 frames are scanned.
The scanning beams at the camera tube and picture tube are blanked and no picture information is picked up or reproduced during the horizontal and vertical retrace intervals.
(C) FLICKER:
The television picture's scanning rate of 25 frames per second is insufficient to allow the brightness of one picture or frame to merge seamlessly into the next. The screen is blanked in between each frame as a result of this effect. As the screen alternates between bright and dark, the effect is a distinct flicker of light. All the lines in the frame are scanned in a progressive sequence from top to bottom when progressive scanning is used. The Flicker effect is created because there are only 25 blank-outs each second. 50 blank-outs per the second result from scanning 50 full frames each second. Human eyes can no longer perceive this rapid fluctuation, therefore the flicker effect is no longer visible. The video frequencies associated with the image elements are multiplied in a line as a result of this effect. Interlaced Scanning is the approach used to tackle this sort of problem.
(D) INTERLACED
SCANNING:
Flicker is reduced in these television pictures by using 50 vertical scans per second effective rate. This is accomplished by increasing the scanning electron beam's downward velocity of motion, causing each alternative line to be scanned instead of the next line. When the beam reaches the bottom of the picture frame, it quickly returns to the top to scan the lines missed during the previous scan. As a result, the total number of lines is divided into two groups known as "Fields." Each field can also be scanned separately. This scanning method is referred to as 'Interlaced Scanning.' Interlaced scanning is described in Figure 3 for the 625 line system.
Figure 3 Interlaced Scanning
Each Frame's total lines are split into two fields. They are:
a. Odd field and
b. Even field
Odd lines in the frames are contained in the first and subsequent odd fields. The even scanning lines appear in the second and all even fields. The frame repetition rate is 50 per second, with two fields each frame and 25 full frames scanned per second. In reality, increasing the vertical scanning frequency from 25 to 50 Hz scans every other line in the picture. There are 312.5 lines per field.
The beam originates at A and sweeps across the frame at a constant velocity, covering all of the image elements in a single horizontal line. The beam retraces quickly to the left side of the frame after this trace, ready to scan the following horizontal line. Because the vertical deflection signal creates a vertical scanning motion, which is slower than horizontal scanning, the horizontal line slopes downward in the scanning direction.
The beam is now at the left side, ready to scan line 3, omitting the second line, once line ‘1' has been scanned. The vertical scanning frequency is increased from 25 to 50 Hz to achieve this line skipping. The electron beam now scans all of the odd lines before arriving at a location towards the bottom of the frame, such as point B in the diagram. Because of the flyback on their vertical saw tooth defection signal, the vertical retrace starts at time B. The beam then returns to the frame's top, where the second or even fields begin.
The beam now moves several horizontal lines from B to C. 20 horizontal lines are drawn during this time. Because the scanning beam is turned off at this time, these 20 lines are known as inactive lines. As a result, the raster's second field begins in the center. The beam scans line 2 in the second field after scanning half of a line from point C.
The beam then scans the even lines that were left out of the initial field's scanning. In this field, the vertical scanning action is identical to that of the preceding field. As a consequence, the second field's even lines, down to point D, are scanned. Because the second field begins at a half-line point, points D and B are a half-line distance from each other.
In the second field, the vertical retrace starts at point D. The vertical flyback causes the beam to return to the top from here. The beam completes the second vertical retrace at A because there are so many vertical retrace lines. Because the number of vertical retrace lines in both fields is the same, the beam will always end the second vertical retrace where the first trace began. At point A, all odd fields begin. Point C is the starting point for all even fields. This technique is repeated 50 times each second, which effectively eliminates flicker.