Date: Sun, 26 Mar 95 22:33:20 EST From: stevekit@eznet.net Howdy Robert. I got a new info file on the different TV systems. I noticed a little file I wrote some time ago on your WWW page, and thought I could do better :) You're welcome to add or replace the file with the following: *** Info on the three color TV systems *** General: All three color TV systems have several things in common. They are all interlaced, where two fields make up one full frame. Interlaced is used so the picture doesn't flicker on the screen. Fields can be drawn twice as fast as frames. A "line" is a horizontal line that sweeps from the left to the right on your television screen. After every line is complete, there is a horizontal sync pulse, telling the TV set to start drawing the next line from left to right. Interlaced means lines 1,3,5,7 and so on are transmitted and drawn on your television first. At the half-way point in the frame, or when one field is complete, a vertical sync pulse is transmitted, telling the TV set to start drawing the next field. The fields are drawn from top to the bottom of your TV screen, or in the vertical direction. Vertical sync pulses are transmitted after every field. Next, the lines 2,4,6,8 and so on are transmitted, which make up the second field. Now, a full frame has been transmitted. The above information is how the picture is drawn on the TV screen. Next, the picture information has to be transmitted. Two signals are transmitted at the same time, the black/white signal and the color signal. The black/white signal is the main signal. It is transmitted using standard AM modulation. The black/white signal has a frequency response of about 0-5 MHz, depending on the TV standard. When TV started, black/white was the only signal transmitted. It was basically an AM transmitter where they fed video to the input instead of audio. Of course, the specifications had to be much higher. Then, color was added. It had to be compatible with black/white transmissions, so they added a subcarrier in the higher end of the black/white frequency spectrum. This would make minimal interference to the black/white signal. Now, this is when the three TV systems take of in different directions. NTSC: The frame rate is chosen to be very close to the frequency of the power system. In the US, we use 60Hz power, so the NTSC frame rate is 59.94 Hz. The reason you want the same rate as the power frequency is to minimize interference. The line rate is 525 lines. The line frequency, frame frequency and color carrier frequency are all related to each other, and are actually locked to each other. All the needed timing signals can be divided from one master clock signal. The color carrier for NTSC is 3.58 MHz, for short. There are many more digits. The color information, consisting of Red, Green and Blue, are all present at the color TV camera. The black/white signal is made by mixing the three colors, 0.30Red, 0.59Green and 0.11Blue. The two color signals needed to make a full color picture is R-Y (red minus black/white), and B-Y (blue minus black/white). If you transmit the Y (black/white) signal and the two combination colors, R-Y and B-Y, you can extract the green color using simple adding and subtracting. Green = Y-R-B For NTSC, the two color signals are modulated on the color carrier using quadrature AM modulation. It's possible to keep the two R-Y and B-Y signals on one carrier. To extract the two signals at the receive end, you must also transmit a color reference signal, called the color burst. This short burst of unmodulated color carrier is placed right after every horizontal sync pulse (about 15000 times per second). The color burst is used to lock the color demodulator in the TV set so the correct R-Y and B-Y signal can be demodulated. Now, the black/white signal and the two color signals can be combined to make Red, Green, and Blue, all the color information needed to make a full color picture on your TV set. PAL: PAL is fairly close to NTSC. Since the power frequency is 50Hz, the frame rate is also 50 Hz. The line rate is 625 lines. However, the Europeans didn't like the color change that can occur if there is a phase change in the transmission of the signal. So on NTSC TV sets there is a HUE or TINT control to correct for any phase change of the color burst/color signal. One way of making an "automatic" hue control is to transmit the R-Y signal alternately with a phase shift of 90 degrees. In every other line the R-Y signal is transmitted inverted. Since our eyes are less sensitive to color compared to black/white, the resolution needed for color is less. As you know, the black/white resolution is about 5 MHz, however the color information transmitted on the color carrier is about 1.4 MHz wide. The frequency of the carrier is 4.43 MHz. So, in PAL, they assume the vertical resolution can be cut in half without "affecting" color resolution. By combining two horizontal lines, using a delay line in the TV set, two lines can be combined and any phase error can be cancelled. Severe phase changes in the transmission of a PAL signal will show up as weak colors, but correct colors. In NTSC it will show up as full color saturation, but the wrong colors! It's also a fact that our eyes are much more sesitive to color hue changes than to color saturation changes. So, you will not see green faces in PAL, but you might see weaker colors. SECAM: SECAM is a totally different system when color transmission is concerned. As for scanning, it's the same as PAL. The French didn't like the PAL nor the NTSC, so they came up with another system. It's based on the information that the color resolution is less than the black/white resolution. So, only half the vertical resolution is needed for color. Instead of transmitting both R-Y and B-Y at the same time, as in NTSC and PAL, SECAM only transmit one color signal at a time. One line transmits the R-Y signal, and the next line transmits the B-Y signal. To get all the colors needed to make a full color picture, you need at least Y, R-Y and B-Y. By delaying one line by exactly "one line", you will have the current Y signal, the current B-Y signal, and also the delayed R-Y signal. Voila!, you now have all the colors. SECAM does not use AM modulation for the color, but FM modulation. Since FM modulation is "not" effected by phase shifts, there is not a problem during transmission that will effect the color hue nor saturation. (old problems with NTSC and PAL). Two frequencies are used for the modulation, 4.2 MHz and 4.4 MHz. One frequency is used for R-Y and the other for B-Y. SECAM has some problems, however. You cannot mix together two SECAM video signals, which is possible for two locked NTSC or PAL signals. Most SECAM TV studios use PAL equipment, and the signal is converted to SECAM before it goes on the air. Also, the color noise is higher in SECAM. Recording the signal to video tape is hard, and you get poor results unless you use professional equipment. Combinations: There are combinations used of the above systems. There might be a typical 625/50 scanning system used for PAL, but the color is actually NTSC. The carrier will be the typical 4.43 MHz PAL, but the modulation on the carrier is NTSC. Vise versa is also used. ********************************************************************* * __/\__/\ Steve Kittelsen __________ * * / /\ \ Rochester, New York \ / * * I I I I " stevekit@eznet.net " \ / * * \ __\/__ / * OPEN YOUR MIND * \/ * *********************************************************************