Progressive and Interlaced Scanning
November 24, 2008
“Persistence of Vision”….. This little magic of nature has worked big wonders for the Television industry. The TV takes advantage of this attribute of the eye, to display motion pictures and make them appear as if in a continuous flow.
It is indeed a thrilling sight when Kevin Pietersen swings his bat and strikes the ball, sending it flying in the air, for a six. The whole motion picture appears like a smooth flow. In reality, the TV camera shoots this in a frame-by-frame format. When these frames are shown in quick succession, they appear like a continuous flow to the eye. You see a similar effect while looking at a rotating fan. You cannot distinctly see each blade. The rotation happening at a high speed makes it seem like a disc. That’s “persistence of vision”. Anything moving at high speed, seems continuous to the eye.
Coming to TV cameras and their frame-by-frame shooting; each of these frames just looks like a still picture. When it comes to displaying these frames, the TV scans each frame, line by line, from top to bottom, and paints it on the screen, that way. In other words, each frame is divided into several horizontal lines. These lines are drawn one by one on the screen by the electron gun in the TV. All this line-by-line drawing and frame-to-frame display, happens so fast that it appears like real movement of objects to the eye.
Frames can be scanned in 2 ways, namely – Interlaced scanning and Progressive scanning.
What is Interlaced Scanning?
Interlaced scanning is commonly used in the Cathode Ray Tube Televisions. In this type of scanning, each frame is divided into 2 Fields - the Odd Field and the Even Field. The odd lines in the frame are scanned first, and this constitutes the Odd Field. The even lines are then scanned to form the Even Field. The electron gun, fires the odd and even fields of each frame in rapid succession, to display a moving picture on the TV screen. Again the ‘Persistence of vision’ comes to play and we experience life-like moving images. In general, the odd field is displayed in a 50th of a second. The even field is then displayed for the next 50th of the second. Therefore, in every second, 50 odd + even fields are displayed. Thus, we can say that 25 frames are displayed per second. This format of displaying 25 frames or 50 fields in a second is called PAL or SECAM. There is another format too, called NTSC, which employs display of 30 frames or 60 fields per second.
The advantage of using interlaced scanning is that only half the bandwidth of the actual frame transmission is required. This is because, only the field information has to be transmitted at a time, and each field is half the frame size. But there are drawbacks too. Inspite of the quick refresh rate, sometimes one does experience flicker or judder (called artifacts), while watching interlaced pictures. This happens because of the alternate display of odd and even fields. A process called anti-aliasing is employed to blur out these artifacts. However, this happens at the cost of the clarity of the picture. Line doubling is another technique used to reduce the flicker. In this method, each line of the field is doubled when displayed.
What is Progressive Scanning?
Progressive scanning, includes the scan and display of each and every line of the frame, sequentially, from top to bottom. This scanning is made use of in LCD and plasma TVs. Since the complete frame is drawn on the screen, there is no judder or artifacts, which are visible in interlaced scanning. The picture appears clear and detailed. The clarity is much better than the anti-aliased or line doubled pictures. If an interlaced picture is stilled, you might possibly see horizontal lines or half of the frame (either odd or even field). But, a still progressive picture, appears complete. However, you require twice the bandwidth of interlaced scanning, for transmission of progressive scan images. This is because, the complete frame details need to be transmitted at once. For compatibility purposes, many of the plasma and LCD TVs do have in-built circuitry to convert interlaced video to progressive video, the process being called de-interlacing (line doubling is one of the de-interlacing methods used).
You might have probably come across terms like 1080i, 480i, 576i or 1080p and 720p. These refer to the vertical display resolution or number of scan lines. The letter ‘i’ denotes “interlaced scanning” and the letter ‘p’ notates “progressive scanning”. The 1080 indicates televisions with the highest possible resolution of this day. Great clarity and minute detail display are achieved with this.
Thus, progressive scanning and interlaced scanning have their own advantages and disadvantages. While one gives greater clarity, the other employs lesser bandwidth. However, progressive scanning definitely seems to be taking the centre stage, as LCD and plasma TVs are taking over the traditional CRTs in many households.
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