Broadcasting Telecasting (Oct-Dec 1953)

plate is composed of thousands of separate light-sensitive cells. When the light comes through the lens of the camera, it forms an image of the scene on the plate. The individual cells of the plate store up energy in proportion to the light which falls upon them. Thus, the cells in the bright parts of the image are filled with considerable energy while the darker parts are filled with less energy. In order to release this stored energy to the transmitter, an electron beam is caused to scan the image from left to right and top to bottom. The beam can be thought of as an electron gun which successively punctures the individual cells thus releasing the stored energy to the transmitter. The transmitter provides a vehicle for transporting the camera signals to the receiver. This is done by generating a radio frequency signal in a part of the spectrum which has previously been determined to have the desired properties with regard to propagation, available bandwidth, etc. The camera signals including the image signals and certain other electrical pulses needed to maintain synchronism between transmitter and receiver are then superimposed on this radio frequency carrier. At the receiver a scanning beam similar to that at the camera is directed against the face of a viewing screen. The receiver scanning beam travels from left to right across each of the lines of the picture in exact synchronism with the beam at the camera. Therefore, the amount of electrical energy released at the camera at any instant will affect the beam at the receiver at that identical point in the picture. Thus, the picture is recomposed. Ill THE COMMISSION'S STANDARDS 4. When the monochrome television standards were adopted, the Commission allocated 6 Mc (6 million cycles per second) for each television channel, nearly all of which is utilized for transmitting the video portion of the composite video-sound signals. The Commission's rules for monochrome require that thirty complete pictures each comprising 525 lines be transmitted each second. In a 4 Mc video bandwidth 4 million pairs of elemental light and dark areas can be transmitted each second, or 8 million separate distinguishable elements. The number of elements is a limiting factor in determining the sharpness or resolution of the television picture and these may be arranged vertically or horizontally in an infinite variety of combinations. If it is decided, and the Commission's rules so provide, that it is necessary to scan 30 pictures each second in order to maintain continuity of motion and prevent flicker, 266,666 elements (i.e. 8 million divided by 30) are available for each "still" picture. Since the Commission's standards provide for 525 lines of vertical resolution, the horizontal resolution is fixed at 505 elements (i.e. 266,666 divided by 525). It should be noted that since the picture is 4 units wide for each 3 units of height that the horizontal elements are thus about 40% wider than the vertical elements. In practice, the values of the number of lines and elements are reduced about one-third due to the time required for sending the synchronizing signals, limitation of equipment and other factors. IV OPERATION UNDER PROPOSED SIGNAL SPECIFICATIONS A. THE CAMERA Since most colors can be duplicated by the mixture of proper amounts of three properly selected primary colors, it follows that a color television system can be based on the transmission and reception of images in the three primary colors. 5. The first step in the transmission and reception of images in the three primary colors is accomplished in the television camera. The camera generates three different signals from the information in the picture. These may be the signals corresponding to the red, green and blue components in the picture but other combinations of three such independent sets of information could be used. 6. One method that has been used is the equivalent of three monochrome cameras. These cameras are operated from a single set of controls so that the view televised by each camera is identical. In front of each camera lens there is placed a red, a blue and a green filter, respectively. Thus, while the view in front of each camera is identical, the scene reaching the light sensitive plate of each camera contains only the components passed by the red, blue or green filters. Hence, this camera produces an image in each of the primary colors, and changes the optical images into their equivalent electrical energy. B. THE TRANSMITTING SYSTEM 7. The three components obtained from the camera are electrically processed in such a manner as to obtain a brightness signal and two color-minus-brightness signals, namely redminus-brightness and blue-minus-brightness. The brightness signal is channeled into one circuit with the other two signals being dealt with in a separate circuit. (1) The Brightness Component — 8. The brightness circuit of the color transmitter is similar to the conventional monochrome transmitter. Both have the same function of transmitting the relative brightness of the picture in monochrome. Thus, the two systems may be considered compatible, since a receiver performing satisfactorily on the monochrome system will also receive the brightness or monochrome signals transmitted in the color system. Since the eye is most sensitive to green, less sensitive to red, and least sensitive to blue, the brightness is obtained by mixing signals in that order of proportion. The specific values of the mixture are 59% green, 30% red and 11% blue.1 Such a mixture will produce a picture on monochrome receivers, in shades of gray. In the color system, this mixture accomplishes the primary objective of transmitting with correct intensity the brightness signal which is one of the two components of the color picture image. (2) The Chroma Component — 9. The color minus brightness signals are derived by subtracting the electrical value of the brightness signal from the electrical value of the color signals. The result is called the "color minus brightness" signal or "color difference" signal.2 Thus, the chroma circuits of the transmitter must process two signals, red minus brightness (Er' — Et'), and blue minus brightness (Ee' — Et'). Only two signals are necessary since the similar relation for the green signal (Eg' — Et') can be recovered at the receiver from a mathematical relationship between the other two.3 The two signals transmitted are the red minus brightness and blue minus brightness. This still presents somewhat of a problem since the two signals must be transmitted in the same circuits without interaction. The method used is to modulate4 the two 'Mathematically, this is written: Et' = .59 Eg' + .30 Er' + .11 Ee' Where Ey' = brightness signal Eg' = electrical signal corresponding to the green components of the picture. Er' = electrical signal corresponding to the red components of the picture. Eb' = electrical signal corresponding to the blue components of the picture. 2 The brightness signal is not separated from the chroma signal until after the color image has been transformed by the camera from an optical quantity to its electrical equivalent. The reason for this is that the electrical quantity "color-minus-brightness" has no physical equivalent since the eye responds only to chroma accompanied by brightness. Chroma minus brightness would be invisible. The subtractions and additions necessary to compose the brightness and chroma signals are accomplished in a matrix unit which is a computing machine for units of electricity. Mathematically, the color minus brightness signal of the blue signal is written: Ee' — Et' 3 Et' = .59 Eg' + .30 Er' + .11 Eb' .59 Eg' = Ey' — .30 Er' — .11 Ee' Eg' — Ey' = 1.7 Ey' — .51 Er' — .19 Eb' Eg' — Ey' = .7 Ey' — .51 Er' — .19 Eb' = .51 Et' — .51 Er/ + .19 Ey' — .19 Eb' = — .51 (Er' — Ey') — .19 (Ee' — Et') 1 The term modulation is used a number of times in the text of the decision. For those unfamiliar with this fundamental process the following may be helpful. A radio transmitter generates a "carrier frequency" on the frequency assigned to the station. (This carrier might be considered as a replacement for the wire in a telephone system.) The intelligence to be transmitted, whether it is sound, picture or facsimile, is imposed upon this carrier by the process of modulation. For radio and television the intelligence or modulation, always a lower frequency than the carrier, is imposed upon the higher frequency for more efficient transportation to the receiver. When the desired intelligence modulates a carrier wave there results a composite signal which has the propagation characteristics of the carrier wave but also contains the original intelligence in one form or another. At the receiver the demodulation process results in recovery of the original intelligence and elimination of the carrier wave which has served its purpose. This is accomplished in a demodulator (detector) and there are a number of ways of performing the demodulation. One way is to generate in the receiver another frequency which is exactly equivalent to the transmitter carrier frequency. When these two identical carriers (frequencies) are placed in the same receiver circuit (demodulator) they have the effect of cancelling each other leaving the original intelligence. This is the system used in the recovery of the color signals described above. (The system of modulation described above is known as amplitude modulation because the modulation varies the amplitude of the carrier.) Other systems based on the same broad principles are called frequency modulation and phase modulation. In the latter cases, the modulation varies the frequency or phase characteristics of the carrier rather than the amplitude; however, the objective of the modulation is the same. signals on a selected subcarrier in a manner designed to prevent interaction.5 The frequency of the subcarrier is carefully selected to reduce the visibility of interaction between the chroma information and the brightness signal on the received picture.6 10. The relative location of the subcarrier within the channel is also an important consideration. If the subcarrier is placed too near the picture carrier there may be interference between the two. On the other hand, if the subcarrier is placed too near the edge of the channel it will restrict the width of the sidebands and limit the information which can be carried. The NTSC has compromised on a subcarrier frequency which is 3.579545 megacycles above the video carrier. Since this frequency is approximately .6 Mc from the edge of the pass band (see Fig. 3 App. B), if the blue and red chroma signals were transmitted they would be limited to .6 Mc. Resolution being a function of bandwidth, this would limit the resolution of color to very coarse detail. This limitation to a .6 Mc bandwidth applies only when two sets of information such as the two color difference signals must be modulated on a single subcarrier. The reasons for this is that in the quadrature method of modulating the subcarrier, both upper and lower side-bands of each color difference signal must be equal. It is, however, possible to send one set of information by using a single side-band. Thus, it would be possible to send two sets of information up to .6 Mc and continue to a higher modulating frequency with a single set of information, e.g. a single color difference signal, using only one sideband. This is the method used in the NTSC system with the result that coarse color detail m the scene being televised which produces signals of frequency less than .6 Mc is reproduced in approximately the original color. (The third color difference signal is recreated at the receiver.) Semi-fine color detail in the scene being televised which produces signals of frequencies greater than .6 Mc and less than 1.5 Mc is reproduced in hues which are contaminated. (With only one color difference signal being transmitted the primaries cannot properly combine at the receiver.) Very fine color detail in the scene being televised which produces signals of frequency greater than 1.5 Mc is reproduced in monochrome. The NTSC has made a variation in the method of sending the red minus brightness and blue minus brightness signals. Instead of sending the blue minus brightness and the red minus brightness over the subcarrier, each of these signals is mixed with the other so that the blue minus brightness contains some red and the red minus brightness contains some blue.' Corresponding 5 A sine wave subcarrier can carry two sets of information by splitting the sine wave into two components in quadrature and amplitude modulating each component with one set of information. 6 The line frequency is chosen as 1/286 times the frequency difference between the sound and picture carriers (4.5 mc/sec) or 15,734.26 cycles/ second. Since there are 525 lines per frame the frame frequency becomes 29.97 cycles per second and the field frequency 59.94 cycles per second. The subcarrier frequency is chosen as an odd multiple of one half the line frequency which in this case was chosen as 455/2 of the line frequency or 3.579545 mc/sec. It will be noted that the line, field and frame frequencies are very close to the nominal values used for monochrome namely, 15,750, 60 and 30 cycles/ sec. thus existing monochrome sets will be able to respond to such scanning rates. The above combination will result in the beat note between the quiescent sound carrier and the color subcarrier being an odd multiple of one half the line frequency. It has been determined that such a relationship results in a minimum visibility, on the received picture, of such interaction as well as a minimum visibility of the subcarrier itself due to a similar relationship of its frequency to that of the line scanning rate. 7 The amplitude of these two orthorgonal components of the chrominance signals can be expressed in terms of color difference signals, as follows : Eq' = .41 (Eb' — Et') + .48 (Er' — Et') Et' = —.27 (Eb' — Et') + .74 (Er' — Et') Where Eq' = narrow-band component of the color signal Ei' = wide-band component of the color signal Broadcasting • Telecasting December 21, 1953 • Page 58-E