Dual Scan: An Economical Compromise?

Each column or row of pixels in passive-matrix displays uses three transistors to control red, green, and blue signals. By contrast, each pixel in an active-matrix screen is controlled by three color-signal transistors, which produces more vibrant colors and higher image quality. Six of the color portables in this report use dual-scan color displays. With average prices of $3400, dual-scan systems offer a value point between passive-matrix and active-matrix color models.

Dual-scan technology, a type of passive-matrix display, divides the screen into top and bottom halves. Each half is scanned simultaneously and in theory produces higher contrast ratios and wider viewing angles than traditional passive-matrix displays. The physical screen size and fundamental technology are t he same for both types of screens.

However, we found little difference in color quality between passive-matrix and dual-scan displays. The average dual-scan display received an overall screen-quality rating of 3.1 compared to 2.6 for the average passive-matrix display. (The average active-matrix rating was 7.2.)

We compared the Texas Instruments TravelMate 4000E WinDX2/50 active-matrix and dual-scan color models ($4678 and $4178, respectively). The active-matrix model (7.3 pounds) is only an ounce heavier than the dual-scan unit, and battery life was identical. The active-matrix model received an "excellent" rating for color purity compared to the below-average rating for the dual-scan model. We also compared the Zenith Data Systems Z-Note 433Lnp+ ($3878) dual-scan notebook with the Z-Star 433Vlp Model 200 ($2699) passive-matrix model. We noticed little difference in color quality, contrast, or viewing angles.

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Photograph: Passive-matrix: Economy, but fair quality.

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Photograph: Dual-scan: Slightly better passive-matrix color.

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Photograph: Active-matrix: The best, for a price.