Aliasing

Aliasing is shimmer in the video image, and is fundamentally an artefact of interlaced video. Sharp lines and fine details in images are affected, and shimmer whenever there is movement. The typical image components that show aliasing are; venetian blinds, automobiles with lots of chrome, automobile grilles and finely detailed images of greenery. Aliasing does not show up until these elements are in motion, so it is typical to see a finely detailed image periodically shimmer whenever the camera pans, particularly in a diagonal direction. A simulation of the cause and appearance of interlacing is shown below. What is simulated below is a close-up, magnified and slowed-down representation of an upwards camera pan across a diagonal line.

Some examples of severe aliasing on DVDs can be found on the following Region 4 DVDs;

Thelma & Louise - pick almost any scene, and watch the extreme shimmer as the camera movies up or down
Backdraft - once again, pick almost any scene with camera movement in it, and severe aliasing crops up. In particular, Chapter 3 is severely affected.
Die Hard 3 - All versions of this DVD suffer to a greater or lesser extent with aliasing, but the very first DVD version that was released to the Australian marketplace was particularly bad in this regard.
The Thing

Original Film Image

Interlaced Representation

What Exactly Is Interlacing?

Film is a series of complete still images (frames), displayed one after the other, 24 per second. This is quite straightforward.

Video, on the other hand, is not recorded or displayed as a complete image one after the other, but rather as two half-images one after the other.

Video is made up of individual picture lines. In the case of PAL video, there are 576 lines that make up the image frame. These lines are not recorded or displayed in sequential (progressive) order. Instead, first the odd lines are shown (1,3,5,7,9 ... 575), and then the even lines are shown (2,4,6,8,10 ... 576). This is known as interlacing, and these half-frames are known as fields.

The reason this system was adopted in the first place was because the technology was not advanced enough to allow any other form of broadcasting, recording or reproduction of a video signal at the time television was introduced. It was a compromise between resolution and bandwidth that still exists to this day.

Motion is a problem for any interlaced video system. The problem is that motion can occur between the odd and the even video fields, so that motion occurs in a jagged fashion across an image, much as illustrated in the example above.

How Is This Relevant To DVD?

In the case of a DVD that has been sourced from video material, this is not particularly relevant, as this artefact is inherent in the source material and cannot be readily eliminated.

However, in the case of a DVD that has been properly sourced from film, this is relevant, as this artefact can be removed relatively easily. When a film which runs at 24 full frames per second is transferred to DVD, the video is essentially stored at either 24 or 25 frames per second. There are no aliasing artefacts inherent in the source material on the DVD. Aliasing is created by the conversion by the DVD player from this film format to the appropriate format for display on a TV or other display device.

Progressive scan DVD players are now available which will output a non-interlaced signal from a DVD that is appropriately encoded. Coupled with a display that is capable of displaying a progressive scan signal, these DVD players will eliminate this annoying artefact.