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CRT Primer

Updated: April 2006


Analog vs Digital Convergence

A test pattern before convergence is done

As described previously, the images of the three CRT’s need to be precisely aligned on the screen to form a single picture. This is done by first physically aligning the projection tubes, and then adjusting up to 300 points on the image so that all three CRT images line up exactly.

While this sounds like a daunting task (thus one of the appeals of LCD or DLP), in most cases the full alignment can be completed without extra test equipment, and nothing more than a screwdriver. Even a first time CRT owner should be able to get about 95% of the capability of a projector out of the set by completing the alignment themselves without the help of a tech. Those not interested in learning the procedure can hire a tech to align or converge the projector for them. The last 5-10% of the image quality is done by a CRT tech, or those CRT owners that find pleasure in ‘tweaking’ the image from time to time, to compensate for tube wear or convergence drift. Note that these tweaking adjustments take only a few minutes to complete, but of course there are those CRT owners that tweak more than they watch the projector.

Analog convergence is found on older or entry level projectors, and typically consist of 30 to 50 controls under the cover of the projector to align the geometry and convergence of the unit. This procedure is a little harder to learn than digital convergence, which is done via on screen menus that guide you through the procedure, and a wired or wireless remote control lets you go right up to the screen to perform exact convergence settings for an accurate picture.

While analog convergence is an older technology and a bit harder to learn, great deals can be had on projectors with analog convergence, and as long as the set was well built, I have not found that analog convergence drifts more than digital convergence.

Drifting convergence is usually as much due to a shifting earth’s magnetic field, physical jarring of the projector (vibration) as is due to drifting component values of the convergence board.

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