Interlaced and progressive video scanning in CRT tv (scan lines effect on TV)
Interlaced scanning vs progressive scanning
Scanning is used in TV transmission to send the video signals from transmitter to the receivers. The video that is played on the TV screen is basically made up of a lot of frames. Each frame can be considered as a picture. Hence we can say that a set of large number of picture frames are sent in continuation to form the video.
Due to the persistence of eye vision, when 25 or more that is 30 frames are shown in front of our eyes with in one second than a effect is created due to which still pictures appear to gain motions or we can say that the still images starts to convert into videos.
When these frames are sent for transmission in tv system the signal is not sent in the formof picture frames. Each picture frame is converted into set of horizontal lines. One picture frames consists of up to 500 horizontal lines. Theses horizontal lines are made by the cathode ray tube signal beam which scans the sensor of camera. These scanned horizontal lines are than sent from transmitter to the receiver tv. These horizontal lines must be reproduced on the tv screen in synchronization with the camera transmitted signals.
When only 25 frames per seconds are shown on the screen than the problem of flicker occurs. To prevent the problem of flicker double of 25 frames that is more than 50 frames are needed to be displayed on the screen. If we double the frames to 50 than problem of large bandwidth arises. To prevent this problem the number of horizontal scanning lines are divided into even and odd horizontal lines.
Firstly the odd number of horizontal scan lines produce the frame, than even horizontal lines are used to produce the second frame. In this way without increasing the bandwidth the frames displayed are doubled and hence problem of flicker is also prevented. This way of scanning is known as interlaced scanning.
In progressive scanning like reading of book, the electron beam do the scanning on tv screen from top to bottom by going from left to right for each line. This process continues to produce the video signals on the screen.
While moving back of scanning beam from right to left the movement should be quick to prevent any loss of picture information. This time required for right to left movement is known as fly back time or retrace time.
Scanning and synchronization process plays a very important role in tv transmission and hence must be done carefully to produce the videos in an optimal way.