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EE 281 October 21, 2002
Embedded Systems Design Laboratory Handout #7
Laboratory Assignment #4:
“TV Paint”
Due Wednesday, October 30, 2002
1. How To Use Your TV
Have you ever imagined using your TV as a display for a home project? If not, then perhaps you should
have. Televisions, like most mass-produced technological wonders, have standard interfaces. For
example, those little yellow RCA connectors on the back of your TV and/or VCR are designed to accept a
video signal that’s described by the US standard RS-170A (NTSC). If you build something which
outputs video in accordance with the standard, you can plug it into a TV and it’s more or less guaranteed
to work. What’s even more interesting is that a carefully programmed AVR processor can quite easily
generate just such a video signal! (Did you see that coming?) This section will briefly describe the
details of video signals and how they’re displayed.
Televisions are raster-scan devices, which mean that they trace out their images by quickly moving a
single electron beam in a “Z” pattern across the display tube. The beam starts in the upper left corner of
the display, and traces quickly across to the upper right “painting” the first horizontal line of the TV
image. When it reaches the right side, the beam turns off, returns to the left side, and paints the second
horizontal line. This process continues until all the lines in the TV frame have been painted. A complete
TV image is 525 lines tall and is fully painted 30 times per second. However, in the early days of TV
(1930s), it was discovered that the 30Hz refresh rate produced too much flicker to watch comfortably. To
reduce this flicker, each full image is split into two frames of 262.5 lines each, and thus frames are
displayed at a rate of 60Hz. The first frame contains lines 1,3,5,7… of the original image, and the second
frame contains lines 2,4,6,8, etc. Each frame fills the entire screen, but only paints every other line. For
practical purposes, you can discard the final 0.5 line of each frame to make it an even 262 lines/frame. If
you take a moment to do the math, you will find that each line takes 63.5us to paint (15.75kHz).

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