Technical & Editorial
- Written by Steve Smallcombe
- Published on 30 November 2007
We often get questions about which TV display technology is "better". In particular, the question arises in regard to DLP vs. LCD. This article hopefully will provide some answers.
DLP (Digital Light Processing - Texas Instruments) technology offers several potential advantages over LCD (Liquid Crystal Display - marketed by many companies with trademarked names and variations, such as JVC's D-ILA and Hitachi's LCOS, both of which are reflective LCD rather than conventional transmissive LCD). In particular, DLP projectors have a much higher (higher is better) pixel "fill factor" than transmissive LCD, that is more of the area of an individual pixel is used for the picture as opposed to the grid surrounding the pixel (D-ILA and LCOS do have an excellent fill factor though). With all digital or fixed panel projectors (non CRT), the grid between the individual pixels is static, wasted space, and this can be visible to viewers. It is most commonly referred to as the "Screen Door Effect", or SDE, and many viewers find SDE to be objectionable when it is noticeable. Because of its higher fill factor (pixels closer together, so less space in between pixels), DLP-based projectors generally produce a much smoother looking image with less SDE compared to LCD-based projectors of similar resolution.
Below is a diagram showing pixels arranged with a low fill factor and a high fill factor. The pixels are the same size, but are closer together in a high fill factor panel with the same resolution. With the high fill factor, you cannot see the regions between the pixels as easily, so it has less of a Screen Door Effect.
The second advantage often identified with DLP-based projectors is a high contrast ratio and efficient use of light. Contrast ratio is figure of merit that compares the ratio between the brightest white and the darkest black that a projector can produce. The light output of the projector in lumens and the contrast ratio are generally considered two of the most important performance indicators for HT usage. Generally, a better contrast ratio leads to better black levels and, therefore, a more realistic presentation of darker scenes in movies and TV programs.
While DLP technology has several significant advantages over competing technologies such as LCD, it also has several potential drawbacks, especially in a single chip design like most of them are right now, based on the way color and shades of gray are produced. As mentioned above, when the DLP mirrors flip they can turn light on or off (1 or 0). Video images consist of many different shades of gray or luminosity and, more often than not, also have color. To make the many shades of gray, or regions of varying light intensity, the mirrors have to flip on and off rapidly so that the eye (or the brain) averages or integrates the image and perceives the desired light level. This is called dithering. Other forms of averaging or dithering may also be applied to regions of the screen to further extend the number of levels of gray that can be perceived by the viewer.
With "single-chip" DLP-based projectors, the perception of color is created in a similar way to how the levels of gray are made. To make colored images, light from the bulb passes through a "Color Wheel," a small rapidly rotating wheel with red, green, and blue windows or colored filters, before it hits the DLP chip. (Color wheels on some single-chip DLP-based projectors also have a clear or white section.) To make red, the light from the chip is turned on (the mirrors flip on in the red part of the image) when the red window in the wheel is aligned with the light path, etc. Sequentially turning on the red and blue for a given pixel makes purple, red and green make yellow, etc. Thus by rapidly flipping the mirrors in synchronization with the color wheel, the projector can make all the colors and shades of gray needed for video images. While it is hard to imagine many tiny mirrors flipping so quickly to make a moving image, the system obviously works, and DLP-based projectors are among the most popular and well thought of models available today. For a very informative explanation of how DLP works visit http://www.dlp.com and take the demo.
The potential drawback of this single-chip DLP technology is that in any given instant, the picture on the screen is not the total image, but is instead rapidly alternating between images consisting of the individual red, green, and blue colors. Thus the eye and the brain play the last critical role in making single chip DLP projectors work, by combining or averaging or integrating the picture, so that the viewer perceives the desired image and not the rapidly flashing momentary components of the image.
With a static picture from a single-chip DLP-based projector, it is easy to understand how this averaging works just fine. Where things potentially start to fail is when there is motion in the image, or when one blinks or rapidly moves one's eyes quickly between various parts of the image. In these cases, the perceptual integration of the image may break down and one might see "rainbows" or false flashes of color, in the image. Some individuals have also reported getting headaches after watching single-chip DLP projectors for any length of time. It appears that not all individuals handle this color averaging process equally well. Newer single-chip DLP-based projectors use a higher speed (5x) six segment color wheel, thus greatly reducing the likelihood that an individual will perceive these artifacts. Also, Double Data Rate (DDR) DMD chips reduce artifacts even further by updating the information going to each mirror at a faster rate.
Below is an animated graphic showing what the rainbow effect looks like. You would see it in high contrast areas of the picture when you move your eyes up and down or side to side rapidly.
In contrast, three-chip projectors use a separate chip for the red, green, and blue colors, and thus, simultaneously present the RGB images so that no temporal averaging or integration by the user is necessary. Today, virtually all LCD based projectors intended for HT usage are three-chip projectors. There are some three-chip DLP projectors out there, but these are very expensive and are typically for commercial use only. Hopefully this will be changing.
Thus in today's HT projector market one can select DLP-based projectors with their better contrast ratios, better black levels, and smoother image, but with the risk that some individuals may see artifacts that are not present with LCD projectors.