And They’re Off!

In standard televisions, these beams race across the picture screen, lighting up and combining phosphor colours as they go. They travel in a straight line from the left side of the screen to the right side of the screen, and when they reach the end they race back to the left side. Only this time they start at a slightly lower position, right underneath the original line.

This is known as a “Raster Scan”, and it continues in this pattern all the way down to the bottom of the screen. Then it goes back to the top left corner and starts again. When the beam is activating phosphors in a line it is on, and it switches off when it starts a new line or begins another pattern. The left to right movement is known as “horizontal retrace”, while the movement from the bottom to the top of the screen is known as “vertical retrace.”

Because these lines are tiny and spaced so close together, your brain blurs them into a single image.

The average television uses an interlacing technique where every other line is painted first, and then the remaining unpainted lines are completed on a second sweep. In this way the entire screen is covered in two sweeps and is completed 30 times a second.

Another pattern, known as a “Progressive Scan”, paints each line one after the other with no spaces, and completes the screen 60 times a second. At these speeds, your brain doesn’t see a series of still images spaced fractions of a second apart.

It sees a uniform, moving image.

As a side note, that high pitched whine that some people hear coming from their television is actually the broadcast signal, moving the electron beam across the screen at over 15,000 times a second!