How does a TV know what to display? We are going to discuss NTSC tv signals. In order to look at how an image is generated by Computer Space, we must first understand how a TV picture is composed. NTSC was develouped in 1941. It was a black and white only standard. In 1953 it was modified to allow color signals. NBC was broadcasting before 1941, using a 441 line standard. When NTSC was released it allowed for 525 lines vertically. This was a compromise other companies at the time wanted up to 800 lines. Which is more than modern SVGA monitors. All video signal must deliver 525 lines vertically. However no TV actually displays all of the lines. Each image is broken into 2 fields. Each field is half the lines of a single screen.The first field is all the odd lines, 1,3,5,…383. And the second is all the even 2,4,6,….384. This is referred to as interlacing.
This is to reduce flickering. If the image was simply drawn from top to bottom it would become noticable that it was being drawn this way. The second reason we don’t see flicker is because of the properties of phosphur. When it gets excited by the electron beam it emits light, but when the beam is shut off it doesn’t immediatly stop glowing. It slowly stops glowing. So that when the next time the beam would hit the same pixel again it isn’t completely dark yet. Helping to create the flicker free illusion.
Why don’t we get to see all 525 lines? That’s because this standard was originally concieved of using mechanical televisions. The beam of electrons scans from side to side, and then returns to the top. It was determined that it would take the equivalent of 70 lines for each field, for the beam to return to the top.
There are 646 pixels accross the screen, on the average TV This is the number of pixels that are needed in order to have the dots on the screen be square. In reality this number can vary. The television itself has small holes for each pixel. This well determine the number of pixels. However since this is a analogue signal, there can be any number of pixels. The TV will display whatever it is capable of. On very small TV’s they may only display half or a quarter of this. At he beginning of each line, there is a window of time that the line is not drawn as well, because the beam would need time to return to the other side of the screen. Reducing the maximum amount of picture that is display, this is slightly different from TV to TV as well. Because of the way crt’s work, every TV displays a slightly different number of lines on the screen. Older TV’s also drifted, so that after a while they would display different lines.
Why was NTSC designed to be 60 fields per second, or 30 frames per second. This is because of the U.S. having 60 Hertz power. TV’s could use the power frequency to make sure they don’t vary from 60 Hz. In the U.S. we actually have two seperate frequencies. The east and west parts of the country are both at 60 Hz, however they aren’t the same phase. A.C. power is alternating between 170 volts positive and 170 volts negative, 60 times per second. This power comes to us as a sine wave. To be in phase each signal has to be at positive 170 at the same time, and negative 170 at the same time. The east side of the country does not have their peak voltage of 170 at the same time as the west side. Older televisions would simply watch for when 170 volts occured, and would use this to synchronize. This is not important for this. But I simply wanted to help explain phase. Newer televisions use an oscillator to make sure they keep the proper frequency, but these were too expensive in the first tv’s. The reason for 525 lines as also because of the power frequency. It was easy to multiply and divide the frequency, and 525 is 3 × 5 × 5 × 7.
During each second their is time to draw 15750 lines. But the video truly only needs to display 11550 lines each second. This gives us 63 microseconds for each line of the screen to be displayed in. The voltage of the signal determines is black, white or a shade inbetween is display. The signal varies between 0 to 1 volts. 300mv was considered black, and one volt was white. With everything in between being shades of gray. So that for each line you would vary the voltage between .3 and 1 volts, to describe the image to the TV. There’s no pauses between pixels, which is why they are not defined for the horizontal. The display can use the voltage at any point for a pixel. If we use the 640 pixel horizontally, we see 10 pixels every 1 microsecond. Next week we’ll look at how Computer Space creates this signal.