A normal television screen uses interlacing to acheive a good frame rate with the limited control hardware of the 1960s. To save on costs, the steering magnets there flick the beam up and down 60 times per second -- so it could paint an entire image each time, if the horizontal steering magnets were up to it. But since a TV screen is 544 by 372 pixels, that would require them to aim the beam back and forth 337*60=20220 times. 1960s-era control circuitry couldn't do that at a reasonable price.

The solution used was to use worse phosphor for the screen, which kept glowing for longer after it was hit -- so moving images would be blurrier. In fact, the phosphor keeps glowing for a bit over a thirtieth of a second. Then, the first time the beam goes down the screen, it paints row one, then is off while the magnets reset, pulling its path back through row two, paints row three, is pulled back through row four, and so on through all the odd rows. When it reaches the bottom, it resets up to the top of the screen and starts painting the even rows. So while there's a new image every 60th of a second, you only have a totally new image every thirtieth of a second. This is called interlacing.

Computer monitors and HDTVs use progressive scan. All that means is that what I've described above doesn't happen. Instead, the entire picture is drawn at once. The benefits are:

  • Better phosphor leads to less blurring.
  • Less jitter in the beam means a cleaner image.
  • You don't have to worry about overlapping the odd rows and the even rows, so there are no interlacing artifacts.
  • Almost all progressive-scan HDTVs support digital input, so you don't torture your images through the digital-analog-digital process which happens to DVDs played on normal sets.
  • Most (but not all) progressive-scan DVD players will have 3:2 pull-down circuitry, which assists in translating the 24 fps rate of film into the 30 fps rate of video.