CRT Fun

There are many projects you commonly see on DIY electronics sites but I find that those involving Cathode Ray Tubes are less prolific.

CRTs have long been a fascination of mine: one can do all sorts of neat stuff with them including even video photography! (Yes, the TV can watch you if you add a few components).

The ability to display high speed repetitive phenomena has long been of important utility to many fields including electronics, physics, medicine, mechanical design, music, computer science, radio, etc. Almost all naturally occurring phenomena involve some periodicity (hence the difficulty in establishing true randomness). Things like the movement of the sunrise, the respiration of a mammal, the vibration of a piano string, the life cycle of an insect species; all are observable in terms of their repetitious effects. This observation usually reveals a great deal of information that is otherwise essentially hidden from view. Slow-occurring phenomena like ocean tides, moon position, plant growth, etc are generally quite easy to graph in terms of time and amplitude/position by human efforts alone. Fast things like piano strings vibrating on the other hand are impossible to graph out temporally by hand and this is where the relatively high speed of electronic and mechanical systems comes into play.

Early cathode ray tubes were developed to display information that was too fast to be measured by their predessors: mechanical oscillographs. Such a mechanical unit might have a moving paper tape under an electrically or physically actuated pen. These systems, essentially fast chart recorders, work well for slow signal changes but become completely useless before the frequency range of human hearing is even accomplished. Many things we'd like to measure are far far beyond the range of human or any animal hearing system. The first commercially popular CRT, the Western Electric 224A, was capable of deflection rates at about 100KHz. Deflection means bending the electron beam emitted by the tube's gun so that it strikes the phosphorescent coating of the face in arbitrary places. To be able to move the dot from one edge of the screen to the other a hundred thousand times each second meant that this was an enormous technical advantage over anything previously invented. The ultimate development of CRT technology in terms of deflection speed was probably in the Tektronix  tubes being developed in the late 70s. These were linear well into the hundreds of megahertz (hundreds of millions of deflections per second).

The most extensive use of CRT technology is likely to have been in televsion in the many decades of its use until LCD and Plasma techonologies superseded it. Instead of displaying waveforms by deflecting the beam about the screen, the beam is set up to deflect in a particular pattern known as a raster. The scan of a TV image starts from the upper left corner to the upper right, then makes another scan below that and so on until about 480 lines have been drawn and at which point the entire process repeats. (For those in the know, the NTSC signal we use actually contains 525 lines but some of them are used up for timing and data transfer and don't actually carry picture information). The intensity of the beam is modulated to make up a picture and this all happens so quickly it appears to be continuous.

After this brief introduction I digress to show off some of the things I've been doing with my collection of cathode ray tubes in the further sections under the "CRT Fun" heading on my site. Enjoy!


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