Lissajous Curve Is An Ideal CRT Learning Project

Lissajous curve with shorter exposure

It was satisfying to see our CRT test rig showing Lissajous curves. [Emily] and I both contributed components for this cobbled-together contraption, drawing from our respective bins of parts. While the curves have their own beauty, there were also good technical reasons why it makes such a great learning project for working with salvaged cathode ray tubes. Mainly for things we don’t have to do:

Focus: We weren’t able to focus our beam in our first work session. We couldn’t count on sharp focus so we appreciate that Lissajous curves still look good when blurry. Thankfully, we did manage better focus for better pictures, but it was not required.

Modulation: To create a raster image, we must have control over beam brightness as we scan the screen. Even if doing arcade vector graphics, we need to be able to turn the beam off when moving from one shape to another. In contrast Lissajous curves are happy with an always-on dot of constant brightness.

Deflection: To create a raster image, we’d need a high level of control over the tube’s deflection coils. We’d need to create a constant horizontal sweep across the screen, as well as scanning vertically. HSYNC, VSYNC, all that good stuff. In contrast driving deflection coils for Lissajous curves require far gentler and smoother coil changes.

Geometry: Unlike modern flat panel displays, CRT can have geometry distortions: pincushion, trapezoidal, tilt, they’re all annoying to adjust and correct in order to deliver a good raster image. Fortunately, a Lissajous curve suffering from geometry distortions still look pretty good and allow us to ignore the issue for the time being.

There is a long way to go before we know enough to drive these tubes at their maximum potential. For one thing, it is running at a tiny fraction of its maximum brightness level. The tube’s previous life in a rear projection television was a hard one, visible in the above picture as a burned-in trapezoid on its phosphor layer. Driven hard enough to require liquid cooling, it would be so bright to be painful to look at and that’s when the beam is scanning across the entire screen. A Lissajous curve covers only a small fraction of that screen area. Concentrating a full-power beam in such a small area would raise concerns of phosphor damage. As pretty as Lissajous curves are, I don’t want them permanently burned into the phosphor. But we don’t have to worry about it until we get beam power figured out.

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