Sequential and Interlaced Scanning

Sequential and Interlaced scanning: The division of picture into many horizontal lines called scanning. Scanning can be compared with that of reading a page of a book. We start at the top, read all the words in the first line from left to right, and then return rapidly to the left to read the next line, and so on, until we reach the bottom line of the page. Similarly, a camera tube scans the horizontal lines one by one.

The television picture is scanned in a sequential series of horizontal lines, one under the other as shown in figure. This scanning makes it possible for one video signal to include all the elements for the entire picture. At one instant of time, the video signal can show only one variation. In order to have one video signal for all the variations of light and shade, all the picture details are scanned in a sequential order of time.

The scanning makes reproduction of a television picture different from that of a photographic print. In a photograph, the entire picture is reproduced at one time. In television, the picture is reassembled line after line and frame after frame. This time factor explains why a television picture can appear with the line structure form apart in diagonal segments and the frames rolling up or down the screen.

Horizontal Linear Scanning

The TV picture is scanned in the same way as you would read a text page to cover all the words in one line and all the lines on the page starting at the top left in figure all the picture elements are scanned in successive order, from left to right and from top to bottom, one line at a time. This method is called horizontal linear scanning. It is used in the camera tube at the transmitter to divide the image into picture elements and in the picture tube at the receiver to reassemble the reproduced image.

The sequence for scanning all the picture elements is as follows:

1. The electron beam sweeps across one horizontal line, covering all the picture elements in that line.

2. At the end of each line, the beam returns very quickly to the left side to begin scanning the next horizontal line. The return time is called retrace, or flyback. No picture information is scanned during retrace because both the camera tube and the picture tube are blanked out for this period. Thus the retraces must be very rapid, since they are wasted time in terms of picture information.

3. When the beam has returned to the left side, its vertical position is lowered so that the beam will scan the next line down and not repeat the same line. This is accomplished by the vertical scanning motion of the beam, which is provided in addition to horizontal scanning.

As a result of the vertical scanning, all the horizontal lines slope downward slightly from top to bottom. When the beam is at the bottom, vertical retrace returns the beam to the top to start the scanning sequence again.

To obtain the maximum amount of the picture detail and to avoid flicker, interlaced scanning is used. In interlaced scanning, each picture is scanned twice by the camera. The camera tube first scans the odd lines 1, 3, 5 and so on and skips the even lines 2, 4, 6 etc., unit it completes one field from top to bottom. After the first field is over, the camera tube scans even lines 2, 4, 6 and so on and skips odd lines completely its second field. These two fields are interlaced together from the complete picture as shown in figure (a) and (b). Half i.e., 312 ½ lines are scanned during odd line scanning and 312 ½ lines are scanned during even line scanning.

Since two fields are scanned for each frame, the repetition rate of the fields becomes 50 per second. Since 50 Hz is the AC mains line frequency, vertical scanning frequency of 50 Hz simplifies the design of the TV receiver and TV transmitter power supply filters. Also, if interlaced scanning is not used, then there is a large gap between the first line and the 625^th line with the result that bottom of the picture looks brighter as compared to the top. Interlaced scanning is best suited for TV Transmission.

Interlaced Scanning

Advantages of Interlaced Scanning:

1. Avoids flicker

2. It is better than sequential scanning.

3. Conserves bandwidth.

Differences between Progressive and Interlaced Scanning:

No:	Progressive Scanning	Interlaced Scanning
01	In this every successive line is being scanned.	In this the electron beam first scans odd lines from top to bottom and then it scans the lines those are skipped in the previous scanning.
02	The effective no: of pictures scanned per second are 25 frames/sec.	The effective no: of fields scanned per second are 50 frames/sec.
03	Flicker problem will occur.	Flicker problem is avoided.
04	Total no: of lines scanned at a time from top to bottom are 625 lines.	Total no: of lines scanned at a time from top to bottom are 312 ½ lines.