United States Patent (19) 11) 4,287,530 Gay et al. (45) Sep. 1, 1981 54 DEMODULATOR SYSTEM INCLUDING A nance information signals from the frequency modu TUNABLE DISCRIMINATOR SUITABLE lated Secam RF subcarrier frequencies without the FOR USE IN A SECAM TELEVISION necessity for precise and stable tuned circuits. The de RECEIVER modulating system includes a single discriminator hav 75 Inventors: Michael J. Gay, Chancy; Johannes A. ing an electronically tunable center frequency, a Gutmann, Geneva, both of remodulator, for producing a reference frequency am Switzerland plitude modulated by the chrominance information (73) Assignee: Motorola Inc., Schaumburg, Ill. signals which are supplied to a commutator as known, and a feedback circuit coupled between the discrimina 21 Appl. No.: 148,347 tor and remodulator. The discriminator is selectively 22 Filed: May 9, 1980 tuned to a reference center frequency during each pic ture line clamp period to produce a zero level output 51) Int. Cl. ............................................... H04N 9/50 signal. During the chrominance information portion of I52) U.S. Cl. ...................................................... 358/23 each line, the center frequency of the discriminator is 58) Field of Search ............................. 358/23; 329/50 alternately offset by first and second control signals to 56 References Cited center frequencies coinciding with the two reference U.S. PATENT DOCUMENTS frequencies to produce a zero output level whenever the Secam signals are at the respective RF subcarrier 3,806,632 4/1974 Okada .................................... 358/23 frequencies. The feedback circuit ensures that the zero 4,232,268 ll/1980 Hinn ....................................... 358/23 output levels obtained during the information carrying Primary Examiner-Richard Murray portion of each line is identical with the zero output Attorney, Agent, or Firm-Michael D. Bingham level set during the clamp period. 57) ABSTRACT A demodulating system to be utilized in Secam televi sion receivers for deriving the R-Y and B-Y chromi 4 Claims, 3 Drawing Figures COLOR PROCESSING TOMATRIX CIRCUIT U.S. Patent Sep. 1, 1981 Sheet 1 of 2 4,287,530 LWW01XIÈJ AOLWLWWOO LWTTIOSO>JO U.S. Patent Sep. 1, 1981 Sheet 2 of 2 4,287,530 as: s ESTnaP ESTI?ld96? ?|20|80p=|=== == + S0|||5 -1-5+9 l 698| 298/?88 LWTTIOSO96>JO 09 ENITTWN9IS 1. 4,287,530 2 DEMODULATOR SYSTEM INCLUDING A SUMMARY OF THE INVENTION TUNABLE DISCRIMINATORSUTABLE FOR USE In accordance with the foregoing there is provided a IN A SECAM TELEVISION RECEIVER demodulator system for use in a Secam television re ceiver including a single frequency locked loop discrim BACKGROUND OF THE INVENTION inator of which the center frequency is electronically This invention relates to color television demodulator tuned in sequence with the Secam reference frequencies systems and more particularly to a chroma demodulator to provide an output of precisely zero (or of a chosen section of a television receiver for receiving Secam reference level) during the appropriate lines. broadcast signals wherein a tunable discriminator is 10 provided for tuning the demodulator to the Secam ref DESCRIPTION OF THE DRAWINGS erence frequencies. FIG. 1 is a simplified block diagram of a Secam tele In Secam color television receiver systems the chro vision receiver system; minance signals R-Y and B-Y are transmitted sequen FIG. 2 is a more detailed block diagram schematic tially line by line by frequency modulation of an RF 15 illustrating the chroma demodulator section of the subcarriers as is known. To allow correct identification Secam television receiver system of FIG. 1 including of these signals in the receiver the R-Y subcarrier has a the discriminator and remodulator portion of the pre frequency of 4.40625 megaHertz and the B-Y subcarrier ferred embodiment of the present invention; and a frequency of 4.250 megaHertz. These two reference FIG. 3 is a partial schematic and block diagram of the frequencies are commonly referred to as fR and fB 20 discriminator-remodulator section of the chroma de respectively. To permit correct hues to be regenerated modulator stage of the present invention. in the television receiver the frequency discriminator DESCRIPTION OF THE SPECIFIC section of the chroma demodulator must provide a zero EMBODIMENT (or a precisely known) output at both the frequency for 25 Turning to FIG. 1 there is shown a simplified sche of the R-Y line and at the frequency fB of the B-Y line. matic of a Secam television receiver which is well Contemporary Secam television receivers usually known to those skilled in the art. As illustrated, the meet this requirement by using two frequency discrimi television receiver 10 includes an antenna 12 coupled to nators, e.g., Foster-Seeley discriminators, each tuned T.V. tuner 14. The output of tuner 14 is coupled to an respectively to the center frequencies fR and fB. 30 IF amplifier 16 an output thereof being provided via However, to maintain visibly neutral gray tones, the line 18 to audio system 20 which processes the signal tuning accuracies must be typically --5 KHz on each of provided thereto to provide audio signals to speaker 22. the two discriminators. This accuracy must be main There also appears at the output of IF amplifier the tained during the life of the receiver and over the full composite video signal of the form shown which in ambient temperature range in which the television re-3 5 cludes a sync pulse, the identification burst signal and ceivers exist. Such performance is not attainable with the video information signal. This composite signal is components and present day manufacturing techniques. applied via lead 24 to the luminance channel 26 which To overcome the above problems, various attempts has a chroma trap 28 provided so that at the output of have been made to use the burst of the for and fB luminance channel 26 there appears the luminance sig frequencies, which are transmitted at the beginning of 40 nal Y which is provided over lead 30 to the matrix 32. the R-Y and B-Y lines respectively, as a precise fre The video composite signal including the modulated quency reference. A number of technical difficulties Secam sequential identification signals is provided via however have made this impractical. lead 34 to "Cloche' filter 36 and then to chroma de Another prior art system for demodulating the Secam modulator 38. Chroma demodulator 38 processes the chrominance signals uses a single frequency discrimina 45 modulated Secam signals to provide the R-Y and B-Y tor to which are fed the R-Y and B-Y lines in sequence, chrominance signal information signals which are then yielding demodulated outputs of the R-Y and B-Y infor supplied to matrix 32. The two chrominance signals are mation in sequence. These signals are then passed via then summed with the luminance channel information direct and delayed paths to a commutator operating at to provide the red, green and blue hue control signals at half the line frequency such that at one output of the 50 the outputs of matrix channel 32 which are fed to the commutator there appears the R-Y signal and at the television cathode ray tube, as is known, to provide the other output appears the B-Y signal. A problem exists in color information control signals. these system due to the fact that the delay lines, which Turning to FIG. 2, chroma demodulator section 38 is are almost universally employed today, are glass delay illustrated in more detail and includes discriminator 40 lines and operate only at RF frequency. Therefore, the 55 having an input which is generally coupled to the out signal to be delayed must first be amplitude modulated put of "Cloche" filter 36 at terminal 42 wherein the onto a suitable subcarrier then demodulated after pas modulated Secam input signals are operated on to pro sage through the delay line. This subcarrier frequency vide a linear output voltage carrying the color informa must be precise and typically is provided by a reference tion of each picture line to be displayed across the dis oscillator incorporating a crystal. play tube. The output of the discriminator is fed via a It is an aspect of the present invention to provide an low pass filter to remove the residual high frequency improved demodulator system suitable for use in Secam components and is then de-emphasized and supplied to color television receivers. a remodulator 42 wherein the linear output of discrimi Another aspect of the present invention is to provide nator 40 amplitude modulates a subcarrier frequency a demodulator for a Secam color television receiver 65 signal provided by oscillator 44. The frequency of oscil including a single tunable discriminator for setting the lator 44, for convenience, may be at 4.4336 MHz; the demodulating system to the sequential reference fre PAL crystal frequency. It should be noted, that al quencies, though the present invention refers to the reference 4,287,530 3 4. signal provided from oscillator 44 as being equal to the operating potential at conductor 107. The capacitance Pal crystal frequency, any frequency within an accept of the LC tuned circuit is provided by reactance stage able range may be provided. The output of remodulator 108 of which the value is varied by the control signal 42 is provided to color processing circuit 46 which may appearing at the output of summing circuit 110. The include the three control stages; automatic color con output of phase comparator 102 is applied to the output trol (ACC), contrast control and saturation control as is of the frequency locked loop discriminator at lead 68 known to those skilled in the art. The output of color through low pass filter 112, the output of which is also processing circuit 46 is processed directly, via lead 50 connected to one input of summing circuit 110.