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Part 3a: Display Technology Shootout

Comparing CRT, LCD, Plasma and DLP Displays By Dr. Raymond M. Soneira

This is Part IIIa of a three part article series describing an in-depth comparison between CRT, LCD, Plasma and DLP display technologies in order to analyze the relative strengths and weaknesses of each. In Part I we measured, analyzed and compared primary specs like Black-Level, Color Temperature, Peak Brightness, Dynamic Range, and Contrast for each display. Click this link to read Part I .

In Part II we continued with Gray-Scale, Gamma, Primary Chromaticities and Color Gamut to see how they all affect color and gray-scale. Click this link to read Part II.

Here in Part IIIa we continue with Artifacts and how they affect Image Quality.

In Parts I and II we measured the photometry and colorimetry for each of the displays. If all of the pixels in a display performed in exactly this manner, there wouldn't be anything more to discuss because images are just made up of pixels. The whole would just be the exact sum of its parts.

Unfortunately, things aren't quite that simple because a considerable amount of processing is required between the input connectors and the actual display device. All of this processing affects and modifies what each pixel winds up displaying, often adversely affecting image quality and accuracy and frequently introducing artifacts into the image. An artifact is simply any distinct feature that is visible in an image that doesn't belong there. How the different display technologies respond to these issues will be the subject of Part IIIa.

First we'll consider the question of Analog vs Digital processing for intensities, pixels and signals. Then we'll examine Native Resolution and Image Rescaling, followed by discussions on Image Noise, Motion Artifacts and Signal Processing.

Analog vs Digital
Three fundamental issues for any display are the way in which it generates pixels and intensities and how it processes the input signals. Each one of these can exist in either the analog or digital domain. While most people assume that digital is somehow automatically better, that really isn't the case. Each method has its own advantages and disadvantages and either one can produce excellent image and picture quality. It's the details of the implementation that determines the quality of the end result and the kind of artifacts that are produced. We'll explore these issues next.

Analog vs Digital Intensity
The intensity (brightness) of a display pixel can be controlled by either an analog or digital process. Each display technology must do it in a particular way. While any display can accept both analog and digital signals, they must be converted into the proper native mode by the time they get to the actual display device.

Both CRTs and LCDs produce their intensity scale through analog voltage control of the device. The range of brightness that is produced is both perfectly smooth and infinitesimally graduated. Some signal processing is required in order obtain the desired brightness relationship, as discussed in the Gamma and Gray-Scale sections in Part II. If it's all done via analog control then the display retains its pure analog nature.

On the other hand, both Plasma and DLP displays have only digital on and off pixel states so they must produce their intensity-scale by rapidly switching between the two and varying the percentage of time that is spent in each. For example, for 25% brightness, a quarter of the time is spent on and the other three-quarters is spent off. In principle, it's possible to produce a infinitesimally graduated intensity scale just like the analog devices above. In practice the switching frequency is fixed and the states are digitally controlled so only a discrete set of intensity levels that can be produced. At low intensities the on time is so brief that it is visually perceived as image noise. (See Image Noise and Pulse Width Modulation, below). In most consumer devices the goal is to produce 256 intensity levels (8-bits per color or 24-bits total). Signal processing generally reduces the total number of levels that are actually available, so in reality less than 256 levels are provided. This introduces gray-scale artifacts that we'll discuss further under Signal Processing, below.

Analog vs Digital Pixels
The pixel structure of the display can also be either analog or digital. This is determined by either the inherent nature of the display technology or the implementation chosen by the manufacturer.

DLP and Plasma displays have an intrinsic discrete pixel matrix format that is fixed at the time of manufacture and cannot be changed. Each DLP pixel is made up of a micro-mirror and each Plasma pixel is made up 3 gas cells that contain red, green and blue phosphors. This discrete image structure is referred to as digital pixels. There are many competing advantages and disadvantages to this approach. If you have a digital image that has a resolution identical to the display's then it can be reproduced exactly. If it's at a different resolution then it must to be rescaled to match the pixel structure of the display, which introduces errors and artifacts into the image (see Image Rescaling, below). If the image is analog and not digital then it must be digitized, which also introduces errors and artifacts. (See Analog vs Digital Signals, below.)

CRTs as pure analog devices don't have any preordained pixel structure built in so they are free to support a very wide range of image formats. The phosphor dots or stripes in color CRTs do introduce a grain into the image and can produce moiré patterns but don't impart any particular pixel structure. Technically CRTs are pixel-free, but this lack of structure is still referred to as analog pixels because they can be thought of as formed and stretched in whatever way is necessary. For example, if you have a CRT monitor connected to your computer then you can select from a wide variety of resolutions that will be accurately reproduced without pixel or scaling artifacts. LCDs are also analog devices and don't have any intrinsic digital pixel structure (because they are filled with a uniform liquid crystal layer), however, they are generally manufactured with a pixel matrix structure just like the DLP and Plasma technologies. So all of the flat panel technologies are identical in this regard.

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