Interlaced and progressive are the raster scanning technique widely used in analog video system. These techniques are distinguished according to the scanning process where the interlaced scan employs the scanning of odd and even marked lines of a picture that are displayed consecutively as two isolated fields which are superimposed to generate one frame or picture on the screen. On the other hand, progressive scanning sweep over the whole image at a time.
Content: Interlaced Scan Vs Progressive Scan
Comparison Chart
| Basis for comparison | Interlaced Scan | Progressive Scan |
|---|---|---|
| Basic | Scanning through dividing a single frame. | Whole frame is scanned at once. |
| Efficiency | Low | High |
| Quality | Degraded | Better |
| Combing effect | Present | Absent |
| Speed of display | One field in 1/60th seconds | One image in 1/60th seconds |
Definition of Interlaced Scan
The interlaced scanning performs scanning by partitioning a frame into two parts. These parts are known as fields which consist of half of the lines exists in a frame, called as 2:1 interlace. The field interval is the time interval between two fields which is specifically half of the frame interval. According to the MPEG standards, the field comprised of the first line and the following alternate lines and this is known as “top field”. While the field including the second set of alternate lines is called “bottom field”. The sampling of these fields depends on the system sometimes top field is sampled first while in some cases bottom field sampled first.
However, there is a major drawback of the interlaced scan known as “combing” which affects the video quality. Combing generally caused due to the slow separation of the two adjacent lines in a frame in accordance with field interval or when fast moving objects are displayed. This effect produces zig-zag artifacts within an interlaced video and images.
An interlaced scan was popular in the early days because it consumes less bandwidth by which it enhances temporal resolution and reduces flickering. Earlier the channels were broadcasted over TV’s where data flung over the airwaves or coax cable. Interlaced videos save on bandwidth by only sending half of a complete frame at a time, and this was helpful for older TV sets for improving refresh rate and creating smoother motion.
Definition of Progressive Scan
The progressive scanning is distinct from the interlaced because it forms a frame in single pass sequentially. However, the working mechanism of progressive scan involves an electronic beam continuously scanning the image area from top to bottom again following the top. The outcome raster signal contains a series of frames separated by frame interval Δt. Each frame has a group of consecutive horizontal lines parted by a regular vertical spacing.
The difference between scanning of the top and bottom line is one frame interval. Although, progressive scan displays also take the interlaced signal as input. How does this work then? It uses concept deinterlacing which is also implemented in the modern set-top boxes and Televisions.
Key Differences Between Interlaced and Progressive Scan
- Interlaced scanning devices divide the whole frame into two part and each field has alternate lines which are combined at the time of displaying it. On the other hand, the progressive scanning devices go through the whole frame at a time in a sequence from top to bottom.
- Progressive scan is more efficient than the interlaced as it can easily capture and display the fast-moving objects clearly.
- Quality of progressive scan is better supported by the fact that the video in 720p (progressive) would be more smoother than the 1080i (interlaced). That is the reason it is more preferred for displaying sporting activity.
- Interlaced scan displays suffer from combing effect which degrades the image and video quality.
- The speed of displaying a video is faster in progressive scan approximately double of the interlaced.
Conclusion
Interlaced and progressive scan are the raster scan techniques where progressive is newer technology and interlaced older but widely used. Although, progressive video making and scanning costlier than interlaced but it produces clearer images and videos.
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