100 Hertz provides a solution to the problem of area flicker, which is noticeable in 50 Hz televisions. As we saw earlier in this chapter, in 50 Hz TVs, the image is scanned 50 times per second - a frequency which is still detectable by the human eye in the form of a flickering picture. Every 1/25 of a second (every 40 milliseconds), one complete image is created. First a half image of all the odd lines in 1/50 second (20 milliseconds), then a half image of all the even lines in 1/50 second (20 milliseconds). (To simplify, we name the odd line image frame A and the even line image frame B). Even though this appears to be fast, one frame in 20 milliseconds is slow enough for the phosphors to fade to black before the next frame is sent to the screen. Due to the fading of the phosphors, we perceive area flicker. It becomes especially visible in bright areas of the image.
In 100 Hertz sets, the frames are scanned at a rate of 100 times per second, which is twice as often. To be able to double the number of image frames, 100 Hz televisions use a digital memory. The digital memory converts all incoming video signals from analog to digital. It stores every new frame for a split second, so that it can be written a second time (from memory) to the screen. After that, the digital signal is converted back to analog and sent to the screen twice as fast.
The A frame is sent to the screen twice within 20 milliseconds and then the B frame twice in 20 milliseconds. The doubling of the frames is refreshing the phosphors twice as often (every 10 milliseconds), which is so fast that the fading is not noticeable to the human eye.
