Journal of the Society of Motion Picture and Television Engineers (1950-1954)

about the general scheme of receivers for use with the NTSG signal may, however, be appropriate. Figure 8 shows the elements of one form of receiver for the proposed NTSC signal. In this receiver a three-gun picture tube has been used; other forms of display device are entirely possible.10 The receiver includes all of the usual elements of a monochrome television receiver such as radio and intermediate frequency circuits, detector, video frequency circuits to feed the picture tube grid, and the usual scanning and high voltage supply circuits for the picture tube. The color circuits are energized from a tap off the main video circuit of the receiver through a filter which passes only the portion of the video spectrum containing the color components. This selected signal includes the modulation sidebands which were generated in the transmitter by application of the original color difference signals as modulation to the color subcarrier; it also includes the components of the luminance signal which appear in the selected frequency band. The luminance components are, of course, even harmonics of half the line frequency while the color sidebands are odd harmonics. A local oscillator in the receiver reproduces the subcarrier frequency which was suppressed at the transmitter; it is kept accurately in phase by reference to the periodic "burst" of the color subcarrier frequency which is transmitted during the synchronizing interval. The local oscillator signal and the color signal selected from the receiver output are both applied to the red color-difference demodulator; its output contains the beat between these, and jt is found that one component of that complex beat is the original videofrequency, color-difference signal. The frequency conversion occurring in the production of this beat signal has transformed all of the color modulation sidebands which were odd harmonics of onehalf the line frequency as transmitted into even harmonics as the signal appears in the output of the demodulator. These even harmonics are, of course, suitable for producing a visible image on the cathode-ray tube and we therefore apply them to the red gun of that tube. Similar procedure is followed with respect to the blue color-difference signal and the blue gun, the only difference being that the phase of the locally generated subcarrier as applied to the blue demodulator is 90° away from the phase in which that signal is applied to the red demodulator. The green color-difference signal is obtained by a proper addition of the red and blue signals, giving due regard to the algebraic signs of the signals and the required output; alternatively, it may be obtained by the use of a third demodulator if the phase of the local subcarrier applied to that demodulator is properly chosen. V. CONCLUSION In its work on color television the National Television System Committee has now formulated a proposal for a colortelevision broadcasting signal.* The design of the signal is based upon careful study of the information need of the human viewer, and it is believed that the signal is capable of adequately satisfying that need. The signal is transmitted * See the Appendix. in the same 6-mc channel as our present signals and has the incidental but important feature of being compatible with present monochrome television signals. Transmitting, networking, and receiving apparatus suitable for use with the signal is now becoming available in sufficient quantity to permit thorough field testing of the proposal. It is my expectation that the field test will show conclusively the suitability of the signal for color-television broadcasting. 332 AprU 1953 Journal of the SMPTE Vol. 60