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IIIHIIIHHHHHHHUSOOS283650A United States Patent (19) 11 Patent Number: - 5,283,650 Koo Et Al

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IIIHIIIHHHHHHHUSOOS283650A United States Patent (19) 11 Patent Number: - 5,283,650 Koo Et Al IIIHIIIHHHHHHHUSOOS283650A United States Patent (19) 11 Patent Number: - 5,283,650 Koo et al. 45 Date of Patent: Feb. 1, 1994 SYSTEM FOR GHOST CANCELLATION 4,393,397 7/1983 Holmes ............................... 358/905 (54) 5, 17,291 5/1992 Ardekani et al. ................... 358/87 COMPRISING AN IMPROVED GCR SIGNAL 5,121,211 6/1992 Koo ...................... ... 358/87 SEQUENCE 5,184,221 2/1993 Nishi et al. .......................... 358/167 75 Inventors: David Koo, Briarcliff Manor; Craig Primary Examiner-James J. Groody B. Greenberg, Mohegan Lake, both Assistant Examiner-Glenton B. Burgess of N.Y.; Takashi Sato, Tokyo, Japan Attorney, Agent, or Firm-Michael E. Marion 73) Assignee: North American Philips Corporation, New York, N.Y. 57) ABSTRACT A system for improved ghost cancellation for use in (21) Appl. No.: 941,356 particular in television receivers. The system featuring a 22 Filed: Sep. 4, 1992 superior ghost cancellation reference (GCR) signal which exhibits improved performance in noisy environ Related U.S. Application Data ments and which exhibits the flat and wide bandwidth (63) Continuation-in-part of Ser. No. 676,927, Mar. 8, 1991, necessary for effective channel characterization which Pat. No. 5,161,017, and a continuation-in-part of Ser. also exhibiting a higher and more evenly distributed No. 83,600, Feb. 5, 1992, Pat. No. 5,179,444, which is amplitude versus time characteristic than that provided a continuation of Ser. No. 698,521, May 10, 1991, Pat. by known, non-cyclic ghost cancellation signals. The No. 5,12,211. .* GCR signal is encoded a selected television line of an (51) Int, C. ........................ H04N 7/08; H04N 5/14; eight field sequence of lines however the polarity of the H04N 5/208; H04N 5/213 : GCR signal is reversed selectively from field to field (52) U.S. C. ...................................... 348/614; 375/13 prior to inserting it in respective television lines. At a (58) Field of Search ............... 358/167, 166, 187, 905, decoder or television receiver incorporating the inven 358/36, 37; 455/65;375/13, 19; H04N 5/14, tion, the selected television lines are combined in a 5/208, 5/213, 5/38, 7/08 manner which provides for both a robust received GCR value and cancellation of the effects of pair-wise con (56) References Cited stant television line signals on the GCR signals. U.S. PATENT DOCUMENTS 4,047,224 9/1977 Yamaguti............................ 358/166 4 Claims, 2 Drawing Sheets NEN LINE OUT O O SYNC DETECTOR U.S. Patent Feb. 1, 1994 Sheet 1 of 2 5,283,650 NEN LINE OUT FIG. U.S. Patent Feb. 1, 1994 Sheet 2 of 2 5,283,650 VIDEOIN VIDEO OUT O C MEMORY 75 FIG.2 5,283,650 1. 2 ual field are minimized and a more accurate representa SYSTEM FOR GHOST CANCELLATION tion of the channel can be provided. COMPRISING AN MPROVED GCR SIGNAL The path characterization process can be hindered SEQUENCE due to the presence of sync and color burst signals 5 present on a television line in close proximity to the This application is a continuation-in-part of U.S. Ser. GCR signal. Ghosted versions of these signals can over No. 676,927, filed on Mar. 8, 1991, now U.S. Pat. No. lap the portion of the received television line containing 5,161,017 and a continuation-in-part of U.S. Ser. No. the GCR. DClevel shifts can cause further distortion to 831,600, filed on Feb. 5, 1992 now U.S. Pat. No. the received GCR signal. The BTA of Japan has devel 5,179,444 which is a continuation of 698,521, filed May 10 oped a method which seeks to eliminate the effects of 10, 1991 which issued as U.S. Pat. No. 5,12,211. The these signals and their ghosts on the received GCR contents of these applications are incorporated by refer signal. U.S. Pat. No. 5,130,799 describes a system and ence herein. method for ghost cancellation using this type of eight field sequence. BACKGROUND OF THE INVENTION 15 Since the color burst signal changes phase by 180" In 1979, the IEEE published an article which has from frame to frame, the color burst signal will be at the become a basic reference in the field of television echo same phase in lines four fields, or two frames, apart. The (or "ghost”) elimination. The article is entitled "A Tu BTA method suggests using an eight field sequence torial. On Ghost Cancellation. In Television Systems' wherein four fields are transmitted with a television line and was written by Walter Ciciora, Gary Sgrignoli and 20 comprising a GCR signal, and four fields are transmit William Thomas and it is incorporated by reference ted with a television line without the GCR signal. At herein. the receiver, lines of the sequence received four fields Although the Ciciora article described the fundamen apart, and having color burst signals of opposite phase, tal. principles, apparatus and algorithms applicable to are subtracted from each other. This combination yields ghost cancellation, the state of the art has only recently 25 a GCR signal value averaged over the eight fields, progressed to the point of providing practical ways to unaffected for the most part by the ghosts caused by the implement and improve these basic concepts. color burst signal. In addition, this subtraction elimi There are two main steps to the ghost cancellation nates the DC drift, sync signals and other types of pair process. First the characteristics of the communications wise constant reference or test signals present on one or path or channel (which include the echo artifacts, if 30 more lines adjacent to the line chosen to provide the any) must be determined at the receiver. Once the chan GCR signal. The term "pair-wise constant” will be used nel characteristics are calculated, filters are used to herein to refer to signals which are constant on lines a implement the inverse channel characteristics to sub fixed number of fields apart, for example four fields stantially eliminate the ghosts. apart. A straight forward approach to determining the char 35 In accordance with the BTA sequence, the GCR acteristics of a transmission path is to transmit a ghost signal is, actually transmitted only in four of the eight cancellation reference signal (GCR) over the path. The television lines of the sequence and thus the average originally transmitted GCR signal is compared with the GCR signal value which is used to characterize the path received, and "ghosted", GCR signal, and a model of at the receiver is limited somewhat in robustness. In the characteristics of the communications pathis devel 40 addition, the sequence and apparatus described by the oped based on the comparison. 799 patent is not useful in cancelling the effects of test U.S. Pat. No. 5,121,211 describes an improved ghost signals which are not pair-wise constant, in otherwords cancellation reference signal and a system utilizing such signals which change in between fields, or signals which a signal for cancelling ghosts. The GCR signal de do not appear in pairs of lines within the sequence, for scribed in the 211 patent is transmitted on a single line 45 example teletext and SID signals. of the vertical blanking interval of the television signal. It is therefore an object of the instant invention to One feature of this GCR signal is that it has a higher provide a method and apparatus for minimizing or elim energy level with peaks more evenly distributed in time inating the effects of pair-wise constant television line than other GCR signals which have been proposed such signals as well as those non-pair-wise line signals which as the BTA GCR signal. The 211 GCR thus provides 50 may appear in within each sequence of fields. the flat, wide frequency spectrum necessary to fully Another object of the invention is to provide a more characterize the path and has a high energy level (am robust GCR signal thus improving the signal to noise plitude) and a more even distribution of energy over a ratio of the ghost cancellation system. time interval. Parent application '927, describes an improved ghost 55 SUMMARY OF THE INVENTION cancellation circuit which comprises FIR and IIR fil The instant invention comprises a method and appa ters which operate at a higher speed by reducing the ratus for transmitting and processing at a receiver, a time in which it takes to generate the appropriate filter multifield sequence of television lines, each field com coefficients. As discussed in that application, proposals prising a GCR signal. The use of all of the fields of the have been made to transmit a GCR signal in one line of 60 eight field sequence of the preferred embodiment to each field in an eight field sequence. The received GCR transmit the GCR signal described in the 211 patent, signals from the eight fields are averaged at the receiver yields as a net result at the receiver or decoder, a re and the average value of the received GCR is then ceived GCR signal value with at least about a 26 dB compared with the stored reference GCR in the receiv improvement in robustness over the average GCR er's ghost cancellation circuit in order to derive the 65 value calculated using the four field BTA approach. necessary filter coefficients for characterizing the chan A feature of one embodiment of the instant invention nel. By using a value avereaged over a given number of is that the sequence utilizes a pattern whereby GCR fields, variations which take place within each individ signals on different television lines are encoded and/or 5,283,650 3 4.
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