Progressive scanning

December 26th, 2009 - Posted by Admin in HD streaming

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Progressive or noninterlaced scanning is a method for displaying, storing or transmitting moving images in which all the lines of each frame are drawn in sequence. The opposite process is interlacing used in traditional television systems where only the odd lines, then the even lines of each frame (each image is called a field) are drawn alternately.

The process taking a stream of interlaced frames and converting it to a stream of progressive frame is called de-interlacing.

With the new improvements of home and office desktop computers, it was discovered that using a traditional television for displaying computer images did not returned good results, especially with text. This was due to the effect of interlaced scan. In order to achieve a more pleasing and precise way of displaying images on a computer, progressive scan was developed.

Progressive scan works in the same manner as your computer monitor. It writes one full frame of video from left to right across the screen every 1/60 of a second. Since the entire image is drawn at one time – as opposed to an interlaced image where the even lines are drawn first, followed by the odd lines – a progressively scanned video image looks more stable than an interlaced one.

Progressive scan differs from interlaced scan in that the image is displayed on a screen by scanning each line (or row of pixels) in a sequential order rather than an alternate order, as is done with interlaced scan. In other words, in progressive scan, the image lines (or pixel rows) are scanned in numerical order (1,2,3) down the screen from top to bottom, instead of in an alternate order (lines or rows 1,3,5, followed by lines or rows 2,4,6).

By progressively scanning the image onto a screen every 60th of a second rather than “interlacing” alternate lines every 30th of a second, a smoother, more detailed, image can be produced on the screen that is perfectly suited for viewing fine details, such as text.

Here is a sample with the difference between interlaced and progressive scan:

Progressive scan

In order to access progressive scan, both the source component, such as a DVD player, HD cable, or satellite box, and the Television display need to be progressive scan capable. The progressive scan signal is transferred from the source component to the television via either a progressive scan component video output or other type of connection, (DVI – Digital Video Interface or HDMI – High Definition Multi-media Interface) that allows the transfer of standard and high-definition progressive scan images to a similarly equipped television.

Standard AV and S-Video connections do not transfer progressive scan video images. If you hookup a progressive scan output to a non-progressive scan TV input, you will not get an image.

On DVD players, the progressive scan signal is sent via component video connections, which are labeled Y, Pb, Pr, or via HDMI or DVI connections.

Component video connections labeled Y,Cb,Cr transfer only interlaced signals.

Feel free to contact me for additional explanations and samples.

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  1. […] (as mentioned above). That means, that it doesn’t play as fluidly as it could be. 3) Bob (Progressive Scan). Displaying every field (so you don’t lose any information), one after the other (= without […]

  2. […] frames is composed by mixing two different fields). You can read more about interlacing techniques here. The number of scanning lines is 525 for each frame (a frame may contain up to 16 million colors). […]