Video Signals


Cathode Ray Tube black and white

The video signal

The Scan Line

Standard Video Voltages


Cathode Ray Tube -- Color

blanking

O s c i l l o s c o p e

Source List

EZ OS-5020G oscilloscope
Cathode Ray Tube black and white
A standard TV-set is built with a vacuum tube, which has a phosphor screen that an electron canon shoots at. When the electrons from the cannon hits the screen, light is emitted from the phosphor as long as the canon shoots electrons at it, and it also has a short afterglow. The electron beam from the electron-cannon can be bent using magnets so it shoots at different parts of the screen. If this is controlled so it draws horizontal lines all over the screen repeatedly, while the intensity of the beam is controlled, an image can be drawn on the screen. The scan process is called interlaced scanning, which means that the process in which lines are scanned is all the odds first and then all the even lines. The screen is redrawn 29.97 times per second on a NTSC system, but to reduce flickering the image is interlaced, showing first all odd lines then all even lines, so the image is partially updated 60 times per second.

The video signal
The image seen on the screen has different intensities. As the electron beam sweeps over the screen, the intensity that should be at the position of the beam, is sent as a voltage level in the video signal.. There is no information in this intensity information about where the electron beam is on the screen. To solve this, a synchronization pulse is sent in the beginning of each line to tell the TV that the current line is finished and move down the electron beam to the next line. (Like the <Enter> key on the keyboard, when writing a text with a computer) The TV must also know when a new image is coming, this is done by making a special synchronization pattern. (Like the "new document" function when writing a text with a computer). An image that is updated 29.97 times per second would be quite flickering, so therefore all even lines are drawn first and then all odd, this method shows 60 half images per second, making the picture have less flickering. The information whether the image contains even or odd lines are sent in the vertical synchronization pattern, as different patterns for odd and even images. The video signal has a voltage range 0 to 1V, where 0.3V represents black, and 1.0V is white (gray intensities have voltages between these values). Levels close to zero represent synchronization pulses.

The Scan Line
The image is divided into scan-lines, it is the most important part of the image since it contains the image data. The scan-lines are all 64us long. First a 4us long sync pulse is sent, by setting the signal level to 0V, to tell the TV that a new line is coming. The old TV's was kind of slow, so they needed 8us after the sync-pulse to get the electron beam in position. During this time the signal is kept at black level. The 8us delay is followed by the image data for 52us, drawn on the screen from the left to the right with the intensities obtained from the video signal. Black is represented by 0.3V and as the voltage increases the intensity increases, with the maximum intensity at 1.0v (white). See 4.gif for a picture of one video scan line..