The circuitry in the TV drives these coils in a specific manner to create the raster scan pattern. The location of the electron beam on the phosphor screen is controlled by horizontal and vertical deflection yokes ( electromagnetic coils) located on the neck of the tube. The illumination provided by the phosphor coating will quickly decay - and we normally want it to quickly decay to reproduce good-quality motion animation - which means the image must be constantly refreshed (redrawn) on the screen. The kind of analog video signals referred to in the question are based on the concept of raster scan, or scanline raster.Įxcept for the early experiments with mechanical scan television, scanline raster-based video signals are primarily designed to be displayed on cathode-ray tube displays, or CRTs for short.ĬRT displays draw their picture on a phosphor coating using a “sweeping”, or “scanning” electron beam. Just to summarize, some points which people may sometimes not fully realize about analog video: So: is it only related to memory size, or is there a problem with the monitors too? Looking on photographs of the monitor, it looks like there's enough space above and below the usual display area to expand it by 20% and get square pixels. I've also checked the timings of the 71.2 Hz Atari SM124, and adding 80 lines to get 640x480 doesn't seem too much. I'm also aware that before the Mac, nobody (or at least not many) felt the need for a 1:1 pixel aspect ratio, so I can understand that there was not much of a market for it.īut could it have been done physically with composite or TTL monochrome monitors with NTSC-like (60 or 30 Hz) frequencies? The standard allows for 480 visible lines on the 525 total, so it's at least plausible. But assuming a flat memory model, and a main memory shared by interleaving access between the CPU and the video controller, one could use a few additional KB to display more lines. I'm well aware that 200 lines can fit nicely in 16 KB (in 640x200x1), and that it's of an overwhelming importance with some display controllers. Reviewing Raffzahn's answer about CGA emulators for Hercules displays, and especially his initial (now corrected) note about 720x350 being PAL's natural resolution, I was wondering if 200/400 line displays of the era (IBM CGA, Atari ST) could theoretically have been made larger by an additional 40/80 lines and still be displayable on monitors of the era.