Raster Displays

  • TV boomed in the 50s and early 60s
    (they got cheap)
  • B/W TVs are basically oscilloscopes
    (with a hardwired scan pattern)
  • Entire screen painted 30 times/sec
  • Screen is traversed 60 times/sec
  • Even/Odd lines on alternate scans
    (called fields)
  • Interlace - a hack to give
    • Smooth motion
      on dynamic scenes
    • High Resolution
      on static scenes
    • Optimize bandwidth
Lecture 1   Slide 12   6.837 Fall '01

During the late 50s and early 60s, broadcast television, really began to take off. It had been around for a while, but it didn't become a commodity item until about this time. Televisions are basically just oscilloscopes. The main difference is that instead of having complete control over the vertical and horizontal deflection, a television sweeps its trace across the entire face in a regular fixed pattern (the actual details are slightly more complicated, but that's the jist of it). This scanning pattern proceeds from the top-left of the screen to the bottom-right as shown in the diagram. The final result is that the entire screen is painted once every 1/30th of a second (33 mS).

Televisions were mass produced and inexpensive. For a computer to paint the entire screen it needs only to synchronize its painting with the constant scanning pattern of the raster. The solution to this problem was to add a special memory that operated synchronous to the raster scanning of the TV, called a frame buffer. While televisions were cheap, memory wasn't. So there was a long period where the patterns were scanned out of a cheap high-density read-only memories, called character generators. The trick was to use a single 8 bit code to specify an 8 by 12 character pattern from the ROM, and with a few addressing tricks one could build a nice display (80 by 25 character) with only 2 kilobytes of memory. Thus the era of the CRT-terminal was born.