De-interlacing video basics

Today, many television monitors are not CRT-based, and in many cases, they are not even scanned systems. If you take a photograph of a CRT with a fast shutter speed, you only see a portion of the entire picture, since the image is being continually repainted on the screen. If you take a photograph of an LCD or plasma display, you will see the entire image since every pixel is ‘on’ all of the time. In order to display an interlaced image on one of these displays, it must first be converted to a non-interlaced, or progressive scan image. This process is called de-interlacing.

In addition to the new displays, there are new video formats, not all of which are interlaced. The common HDTV standards in North America are usually referred to as 1080i (1080 lines, interlaced) and 720p (720 lines, progressive). Plasma and LCD display devices can also have their own native resolutions, which may not match either of these standards. As a result, de-interlacing is often done in conjunction with the resizing of an image, as a part of the coordinated image processing system inside a TV or DVD player.

There are several methods used to de-interlace a video image, many of which are grossly inadequate. The two most common of these are weaving and blending. In weaving, field 1 and field 2 are shown together during the first 60th of a second. For the next 60th of a second, field 2 and field 3 are shown, and so on, such that half of the lines are updated every successive 60th of a second. Sounds clever, until you realize that you are displaying field 1 and field 2 at the same time, when they were actually captured 1/60th of a second apart. Doing this creates visual artifacts. As an example, imagine how a hockey stick might look moving from left to right across the screen:

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The hockey stick is drawn on the screen as a series of consecutive lines. In the interlaced format, the camera only captures and transmits every other line every 1/60th of a second:

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When two fields that were sampled at different times are shown at the same time, the result is blurring along the direction of motion. Moreover, sharp edges in the original image become jagged – an effect known as “mouse teeth”. Mouse teeth is a common artifact that results from using the weaving method of de-interlacing.

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Another way to easily de-interlace a video image is called blending. In blending, consecutive fields are averaged and then displayed as one frame. Blending has the advantage of not generating any mouse teeth, but it results in a loss of both vertical and temporal resolution – making your high definition TV not so high definition.

The best way to perform de-interlacing is also the most difficult to implement. Rather than simply regenerating field 1 and displaying it along with field 2, each portion of the image in field 1 is analyzed, and a new field 1’ is generated and displayed with field 2. Field 1’ is similar to field 1, but motion-adapted. If the de-interlacer determines that there is a hockey stick moving from left to right across the screen, it generates the new field 1’ with the stick located where it ought to be at the time of field 2. This method produces a video frame with no mouse teeth, at the expense of only a few million calculations per second. The complex algorithms involved in motion-compensated de-interlacing are implemented within high-end image processors such as Gennum’s GF9350 with VXPTM technology.

Gennum implements two distinct algorithms in their chipset to properly de-interlace an image. FineEdgeTM processing is a motion-adaptive de-interlacing algorithm that eliminates mouse teeth. Another algorithm, called TruMotionHDTM, assists FineEdgeTM by processing the time-based (temporal) characteristics of the video.

The VXP technology recognizes that the most common type of processing within the video display is not simply de-interlacing, but also rescaling of the image. This two step processing is most effectively done when moving from higher to lower resolutions; therefore, in a system using the Gennum Image Processor with VXP technology, the input image is usually first up-converted and de-interlaced to a 1080p (progressive) image, then down-converted to the native resolution of the display.

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Next page: Telecine and 3:2 pulldown