Often I am asked what the advantages and disadvantages are of flatscreen CRTs vs traditional curved CRTs. My answer is that I strongly recommend getting a curved CRT, below I explain some of the reasons why.
Next you should learn some terms:
- Linearity: The proportional accuracy of the horizontal and vertical geometry of the image displayed on the CRT.
Vertical linearity can be tested with the Linearity test of 240p test suite, which displays perfect circles. If you see ovals instead of circles, your CRT is drawing the image with incorrect proportions. You can also test vertical linearity with the Grid pattern - if the height of the grid squares is not consistent in the top/bottom row of the screen vs the center of the screen, your vertical linearity is bad. Vertical linearity can almost always be adjusted somewhere on the CRT.
Horizontal linearity can be verified using the Scroll Test. If you see ripples or waves (otherwise called the "magic carpet distortion") in the ground of the Sonic stage in the scroll test, your CRT has bad horizontal linearity due to the design of the CRT. Horizontal linearity cannot be adjusted or corrected, and if it bothers you then keep reading this page to understand why it's happening. - magic carpet distortion: A geometry distortion which appears across the left and right vertical thirds of the screen. It is especially distracting on 2D scrolling games like Sonic - It will look like the ground is rippling and waving vertically as the graphics pass over the middle of the screen. This effect cannot be improved at all, it is a direct result of the slim/flat CRT design.
Curved CRTs
Traditional (all brands besides Sony)
Traditional CRTs have a curved face in both the horizontal and vertical axis. Most people refer to this as "bubble curve" because it looks like a bubble. Without getting too far into the technical reasons, this design is intentional because CRTs project their image from a central point. Basic geometry rules tell you that corner distortion occurs the further out from center you get on a projected flat surface, because the distance of the projection is not consistent and neither is the angle it strikes the surface. If you curve the projected surface inwards at a given ratio vs. the distance from center, it will mitigate corner distortion and project a more perfect image because the length of the electron beam stays consistent from center to edge, and the angle of the surface is adjusted to match the angle of the electron beam coming from the center.
Because of these reasons, traditional curved CRTs have the best possible geometry and linearity of all consumer CRT types.
Sony Trinitron
Sony changed the traditional design of curved CRTs with their Trinitron tubes. Trinitron CRTs are only curved on the horizontal axis, while remaining flat on the vertical axis. This prevents the "magic carpet" distortion while still providing maximum viewable size and angles by eliminating the vertical curve found in other brands of CRTs. If you have a good grasp of the electron beam explanations given for curved screens you might realize that having no curve on the vertical axis should introduce some linearity issues in the upper and lower edge of the screen, and you would be correct. However it is pretty minor and most people don't notice / aren't bothered by it. Sony also provides adjustments to fix these issues on their higher end consumer TVs and all of their broadcast monitors.
Another major distinction for Trinitron tubes, which I'll only touch on briefly since it is beyond the scope of this article, is that Sony developed their own mask type for Trinitron called "aperture grille" or AG for short. AG masks are unqiue because they have infinite vertical resolution. This is achieved because the mask is only divided horizontally. This gives Trinitrons a different "look" and you may find you prefer it over other mask types, or you may not. Arcades never used AG masks or Trinitron tubes, so if you are going for the most authetnic arcade experience you should get a traditional curved tube instead.
Flatscreen CRTs
One important thing to understand about flatscreen CRTs is that they went through an evolution over the years - early ones in the late 90s were known as "flat-curve" CRTs because they weren't actually completely flat and still had a visible curve to them, albeit far less than a traditional curved CRT. As the years went by and manufacturing techniques were perfected they were able to get them flatter and flatter until the curve was imperceptible.
Flat-curve CRTs
Flat-curve CRTs can have good geometry but the flatter the curve is, the worse the magic carpet effect will be. Similarly, the slimmer the CRT is (i.e. the depth of the chassis when measured from screen to back) the worse the corner convergence/distortion becomes. This is also directly related to how large the tube is diagonally - so larger and slimmer is multiple degrees worse in the corner geometry department.
There are some flat-curve CRTs which still have standard tube depth (90 degree deflection angle) and a visible (albeit flatter than traditional) curve to the tube face (for an example see the Sanyo VMC-8614F or JVC TM-1750CGU. These will have no distortion because the tube curve is sufficient to avoid magic carpet distortion and they have a good deflection angle to preserve corner geometry.
Truly Flat CRTs
Truly flat CRTs have the worst linearity of any CRT you could pick. They will have the worst "magic carpet" distortion because the distance traveled by the electron beam is changing drastically from edge to center, and they will have the worst corner geometry because the beam has to be bent so drastically to strike the flat corner surfaces.
The only advantage I can say for truly flat CRTs is that they were so late in the "CRT era" of televisions that they are almost guaranteed to have component inputs on the back. Depending on where you live it can be harder to find curved CRTs with component inputs, so flat CRTs can be better in that regard if this is your first CRT.
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