We're so used to the line-by-line, frame-by-frame raster scan on our TV — whether on a now-obsolete CRT or up-to-date flat screen — that we may forget that displays were not always driven this way. The traditional analog oscilloscope was not a raster-scan device; the amplified signal of interest controlled the vertical-axis deflection, while a time base or second signal of interest drove the horizontal axis. Although not raster scan, it was a straightforward X-Y drive setup. So screen drive seems pretty straightforward, and you can go with raster scan or X-Y deflection.
That's why I was puzzled after watching an old WWII movie, with a destroyer in pursuit of a submarine. The destroyer had radar (yes, it was in use towards the end of the war, on land, ships, and even some aircraft), and the display was a round CRT with the classic “sweep” of a bright line emanating from the screen center, rotating around 360⁰ in sync with the antenna (Figure 1). Any reflected signal blip was seen as a bright spot due to the CRT's phosphor persistence as the sweep line went around — all very intuitive.
A little later, though, I started to think about this screen display, and I wondered: What kind of analog-drive signal waveforms were used to generate that circular sweep (and they had to be analog, of course)? Were they related to the classic Lissajous figures used “back in the day” for frequency calibration? Were they sine/cosine signals, and if so, how were they used?
Also, did the CRT of the day use magnetic or electrostatic deflection, and how were the deflection coils or plates arranged? How was the sweep of the screen synchronized to the outside rotating antenna? (Some sort of synchro/resolver, I assume.) Finally, when we see this kind of traditional image displayed on a modern system, such as the TV weather folks like to show us, I assume it is actually a rasterized image of a circular-antenna or phased-array antenna sweep — but is it?
I call these questions “analog mysteries,” but they are really questions with knowable and discoverable answers, once we do the historical research. Unlike a real mystery, which may not have an answer we can find or understand (such as “what is gravity?”), this one requires delving into something that was real and worked but used a technique that now is obsolete.
It's sort of like the story of those huge columns of the classic Greek and Roman temples. Researchers are fairly confident they know how the column sections were fabricated and how their precise placement was plotted, but no one knows how they moved, lifted, and carefully positioned column sections, each weighing at least 10 tons and more, on top of each other. It was a combination of human and animal muscle power plus some sort of “machinery,” but that's about all that is known.
When I have time, I'll dig into the design of early radars and hopefully find some clear answers. Those folks were pretty clever at accomplishing some amazing things with what they had available, although by our standards it was all so very crude.
Are there any similar “How did they do that?” questions you have ever wondered about? One of my other ones is understanding how trackers of space vehicles, ranging from orbiting satellites to deep-space probes, worked propagation-related return delays and relativistic effects into their track-analysis calculations.
Have you found answers to your own analog mysteries?