llllllllllllllllllllllllllllllllIllllllllllllllllllllllllllllllllllllllllll USOO5572249A Unlted States Patent [19] [11] Patent Number: 5,572,249 Ghosh [45] Date of Patent: Nov. 5, 1996

[54] METHOD AND APPARATUS FOR OPTIMAL 5,214,501 5/1993 Cavallerno et a1...... 348/488 NTSC REJECTION FILTERING AND 5,263,051 11/1993 Eyuboglu ...... 375/58 TRANSMITTER AND RECEIVER 5,272,533 12/1993 Akiyama et a1. .. 348/624 COMPRISING SAME 5,282,019 1/1994 Basile et al...... 348/473

. 5,282,023 l/1994 Scarpa ...... 348/624 [751 1mm Monisha Ghosh, Mohegan Lake’ N-Y- 252118)? 31331 $383‘. Til; ...... " 3121213 [73] Assignee: Philips Electronics North America 5,386,239 1/1995 Wang et al, ...... 348/470 Corporation, New York, N .Y. OTHER PUBLICATIONS Lee-Fang Wei, “Precoding Technique For Partial-Response [211 APPl- NO-I 271,810 Channels with Applications to HDTV Transmission”, IEEE [22] Filed: JuL 7 1994 Journal on Selected Areas in Communications, vol. 11, No. ’ 1, Jan. 1993, pp. 127_135. [51] Int. Cl.6 ...... H04N 7/00; H04N 7/13 “VSB Transmission System”, Zenith Electronics Corpora [52] US. Cl...... 348/21; 348/470; 348/613; tion, Dec. 17, 1993, Technical Details. 348/608; 348/725; 375/346 Zenith “Technical Details”, Digital Spectrum Compatible, [58] Field of Search ...... 348/21, 470, 487, 36P- 23, 1991 348/426, 608, 611, 612, 613, 614, 624, . . _ . . 606, 571, 708, 711, 725427, 728, 909, 121$”; Marion 607, 627, 723; 375/103, 99, 58, 60, 350, ’ ’ ‘ 285, 296, 346; 364/724.01, 724.12; 455/295, [57] ABSTRACT 296’ 307; H04N 7/00’ 7/13 The invention comprises an optimal causal, monic (?rst R f -t d coe?icient of ?lter is 1) NTSC rejection ?lter for use at an [56] e erences Cl 8 ATV receiver which is designed to optimally process the US. PATENT DOCUMENTS interference caused by an NTSC co-channel signal while keeping the noise enhancement to a desirably low value. In 22251313551 """""""""""""""" " other words, the design method gives the ?lter with the 5:086:340 2/1992 Citta et a1. .1“: ....:"35s/141 BEST NTSC rejection Capability for a give" noise enhance 5,0s7,975 2/1992 Citta B1211...... 358/183 m?m- ' 5,121,203 6/1992 Citta ...... 358/141 ' 5,162,900 11/1992 Citta ...... 348/607 2 Claims, 5 Drawing Sheets

SAMPLING AT 80 85 ‘ 6(1) 55\ SYMBOL RATE , .... i IF FILTER AND NO -' INTERFERENCE I' EQUALIZEH TUNER > DOWNCONVERTER ‘ CONVERTER >'5 REJECTIONHum E'—> ANDTRACKER PHASE mJ M ____ ‘all ____ n A

i 110

TIMING

' v r ‘ Y > 308113160680“ = TRELLIS A BYTE 4 “S __> To MPEG DEINTERLEAVEH DECODER DEINTERLEAVER DECODER 05000511

90 95 100 105

US. Patent Nov. 5, 1996 Sheet 3 of 5 5,572,249

I I I I I l l I I I 6:’ I l l l I I I I I I I

I I T I I I I I I I I I I I I I I I I I I I I I \JHvitEtwnvr I I 5 I I I I I U.S. Patent Nov. 5, 1996 Sheet 4 of 5 5,572,249

-10 . I‘ .. A A i "H I" '

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

FIG. 4 US. Patent I Nov. 5, 1996 Sheet 5 0f 5 5,572,249

MDDIIID-II/I ADDITION EFY‘I‘I'IQOI ’’ f ‘20 ITI‘RE INTERLEAVER \/ MUX

+ ~ -

NTSC PREDICTION H5__/__ FILTER

FIG. 5 5,572,249 1 2 METHOD AND APPARATUS FOR OPTIlVIAL of Digital Communication and Coding” authored by A. J. NTSC REJECTION FILTERING AND Viterbi and J. K. Omura and published by McGraw Hill in TRANSMITTER AND RECEIVER 1979, which is incorporated by reference herein. COMPRISING SAME For a comb-?lter with a delay of 12 symbols, the number of trellis states are extremely large. To simplify their design, BACKGROUND OF THE INVENTION Zenith converts the MPEG coded and RS coded and inter leaved data-stream from serial to parallel, then uses 12 The Federal Communications Commission and cable tele parallel trellis encoders followed by a parallel to serial vision testing organizations such as CableLabs have been converter at the transmitter. The trellis decoder for the case evaluating digital television delivery systems in order to 10 when the comb ?lter is used, implements Viterbi decoding choose a new television “standard” which someday will on a trellis with the number of states equal to two or four replace NTSC in the United States. These systems all times the number of states of the trellis encoder. This is involve digital coding and data compression techniques, for described in detail in “VSB Transmission System: Technical example those utilizing the MPEG algorithms or variations Details”. thereof. For the case when co-channel conventional television The FCC plans to test and approve an advanced television interference is absent, Viterbi decoding is implemented on a (ATV) standard comprising for example, high de?nition trellis with the number of states equal to the number of states television (HDTV) and standard de?nition (SDTV) digital of the trellis encoder. This is possible since pre-coding is not signals for terrestrial broadcasting in 1995, and although the used in the transmitter. speci?cs of the-standard are yet to be fully tested and agreed The choice between the path afforded by simple trellis upon, the FCC has indicated that the system will initially decoding or of that using the comb ?lter and the expanded take the form of a so called “simulcast” approach. trellis at the receiver is decided by the measured error-rate The new ATV signals will have to ?t into currently unused of the periodically sent data ?eld sync symbols at the outputs television channels (so-called “taboo” channels) and ini of the post-comb ?lter and with no post-comb ?lter. tially co-exist with conventional analog television signals 25 When both co-channel and AWGN (additive white Gaus without co-channel interference. sian noise) are present however, the performance of the comb ?lter degrades dramatically. This is because the NTSC will be used hereinafter to represent one example AWGN after the comb ?lter does not remain white, but gets of conventional television broadcasting. Other examples “colored”, in other words the noise samples are not inde would be SECAM and PAL. Although NTSC is exempli?ed pendent of each other. This affects the performance of the - herein, it is not meant to be construed as a limitation and will 30 trellis decoder which is optimized for performance in an be used herein synonomously with “conventional” to rep AWGN channel. Since the co-channel conventional televi resent conventional television in general. sion interference is maximum at the fringe area where the In 1995 the FCC will test the so-called “Grand Alliance” signal power is small and hence the AWGN is large, this is system which is a proposed system being cooperatively indeed a scenario which must be taken into account. A ?rst developed by the corporate sponsors which developed the objective of the instant invention is therefore to improve the ?rst round of individual proposals which were tested by the performance of an ATV receiver when co-channel interfer FCC in 1991 and 1992. This Grand Alliance system pro ence and a high AWGN level are present. poses to take the best features from those systems already The number of states of the trellis encoder is limited by tested in order to present a single optimum system for FCC the fact that the Viterbi decoder for the comb-?lter path must approval as the US. standard. operate on a trellis with at least double the number of states The Grand Alliance has already decided on a coding of the trellis encoder. This limits the AWGN performance of algorithm which will comply with the source coding stan the trellis encoder/decoder when co-channel television inter dards proposed by MPEG (Motion Pictures Experts Group). ference is not present. A second object of the instant inven In addition, the RF transmission scheme selected by the tion therefore is to improve the AWGN performance of the 45 Grand Alliance is the trellis coded 8VSB system designed trellis encoder/decoder in an ATV receiver when co-channel and built by Zenith Electronics. Details of the Zenith VSB television interference is not present. system are described in “Digital Spectrum Compatible The comb ?lter method of NTSC rejection requires the Technical Details”, Sep. 23, 1991 and more recently modi ATV spectrum to be shifted 45.8 khz with respect to the ?ed and described in “VSB Transmission System: Technical NTSC spectrum in order to align the nulls of the comb ?lter Details”, Feb. 18, 1994, which are incorporated by reference with the picture and color carriers as described in “VSB herein. Transmission System: Technical Details“. This causes the The technique used in the Zenith SVSB modem to combat digital spectrum to spill over into the adjacent 6 Mhz co-channel interference is as follows. A comb ?lter is used channel which is undesirable for adjacent channel rejection. in the digital (e.g. HDTV) signal receiver to introduce nulls 55 Another object of the invention is to do away with this in the digital spectrum at the frequency locations of the frequency offset. conventional (e.g. NTSC) picture, color and the sound Finally, the switching between the use of a comb ?lter in carriers. This provides a signi?cant improvement in perfor the receiver or not, suggested by Zenith is cumbersome. A mance when conventional television, e.g. NTSC, is broad signi?cant number of computations must be performed to cast on a co-channel. determine whether the comb ?lter should be used or not. When co-channel interference from an NTSC signal is Furthermore the use of the comb ?lter also speci?es the use present at the HDTV receiver, the comb filter at the receiver of 12 parallel encoders and correspondingly 12 parallel is treated as a partial response channel in cascade with the decoders which also is cumbersome. Another object of the trellis coder. An optimum decoder can then be developed invention therefore is to avoid the use of a comb ?lter at the which uses Viterbi decoding on an expanded trellis, the 65 receiver. states of which correspond to the cascade of the states of the In addition to other documents cited herein, this applica' comb-?lter and the trellis coder as described in “Principles tion incorporates by reference the following documents: 5,572,249 ' 3 4 US Pat. Nos. 5,086,340, 5,087,975 and 5,121,203 and DETAILED DESCRIPTION OF A PREFERRED US. Ser. Nos. 08/170,471, ?led Dec. 20, 1993 and 08/197, EMBODIMENT OF THE INVENTION 773, ?led Feb. 10, 1994. FIG. 1 describes a Zenith VSB transmitter (as described in “VSB Transmission System: Technical Details” which SUMMARY OF THE INVENTION has been modi?ed in accordance with the invention. The invention comprises an optimal causal, monic (?rst The MPEG coded data (or, more generally, digital tele coe?icient of ?lter is 1) NTSC rejection ?lter for use at an vision data)is processed by the Reed-Solomon (RS) encoder 5, the byte interleaver 10 and then the trellis encoder 15. ATV receiver which is designed to optimally process the These three blocks are well-known and described, for interference caused by an NTSC co-channel signal while example, in “VSB Transmission System: Technical Details”. keeping the noise enhancement to a desirably low value. In other words, the design method gives the ?lter with the The symbol interleaver 20 randomizes the trellis-coded BEST NTSC rejection capability for a given noise enhance symbols before they are processed by the pre-coder 25. The symbol interleaver 20 can be implemented as a block ment. interleaver or as a convolutional interleaver; both imple The invention described in US. application Ser. No. mentations are described in US. Pat. No. 3,652,998 which 08/170,471 (assigned to the instant Assignee), provides a is incorporated by reference herein. design method that attenuates just the picture and sound The implementation of the pre-coder 25 is described in carriers of the NTSC interference. The method however is the patent application U.S. Ser. No. Oil/197,773 which is not necessarily optimal in reducing NTSC interference. One incorporated by reference herein. A design for pre-coder 25 feature of the instant invention is that it utilizes knowledge 20 is shown in FIG. 5. The output of the symbol interleaver 20 of the entire interference spectrum in order to design the’ is passed through a modulo-M adder 120, which adds its rejection ?lter. The prior art comb ?lter or the rejection ?lter output passed through an NTSC prediction ?lter 115 like the described in the ’471 application both basically use the one at the receiver. The operation of the modulo-M adder knowledge of the position of the picture, sound and color 120 is described in “New Automatic Equalizer Employing carriers only. 25 Modulo Arithmetic”, Electronic Letters, pages 138—l39, in The invention provides the optimum ?lter performance in March 1971, where the value of M is chosen to be greater terms of NTSC rejection for a given amount of noise than or equal to the peak value of the incoming symbol. enhancement. The embodiment described herein provides an The output of the pre-coder 25 is passed through a NTSC rejection of about 12.06 db for a noise enhancement multiplexer (MUX) 40, which multiplexes with the output of of about 0.25 db with the NTSC color bar signal. The pre-coder 25, any training signals that are used. preferred embodiment described herein is also relatively A pilot signal is inserted into the multiplexed data stream insensitive to the nature of the input NTSC signal and gives by pilot insertion means 45 and the multiplexed data stream good NTSC rejection for signals other than the color bar signal. with pilot is then passed through a VSB modulator 50 and 35 RF upconverter 55, after which it is transmitted over a Another feature of the invention is that since the ?lter is communications path, for example a terrestrial broadcast formulated as a predictor, the output will be maximally channel. white. The ?lter output can be further “whitened” by use of The blocks 40, 45, 50, 55 are described in detail in “VSB an interleaver as described in the ’471 application incorpo Transmission System: Technical Details” and “Digital Spec rated by reference herein. ’ 40 trum Compatible-Technical Details” which are both incor A further feature of the ?lter designed in accordance with porated by reference herein. the invention is that it is causal and monic and can be used FIG. 2 is a block diagram of a VSB receiver comprising to precode the data at the transmitter as suggested in the ’773 patent application. the invention. Tuner 60 selects the channel of interest and converts the Still another feature of the invention is that the design 45 received signal from RF to IF, where the IF ?lter and down method it comprises is independent of the modulation for converter 65 can be, for example, a Surface Acoustic Wave mat of the digital signal and could be used for non-VSB (SAW) ?lter (which shapes the signal spectrum according to transmission (QAM) modalities, for example quadrature the ?lter used in the VSB modulator 50) followed by a nrixer amplitude modulation transmission as well. which down converts the shaped signal to a baseband signal Another feature of the invention is that the digital signal 50 as explained in greater detail in “Digital Spectrum Compat need not be olTset by 45.8 kHz as is presently required in the ible—Technical Details”. 8VSB system designed by Zenith. Analog-to-digital (A/D) converter 70 samples the down converted signal from IF ?lter and down-converter 65 at BRIEF DESCRIPTION OF THE DRAWINGS 55 symbol-rate which is, for example, 10.76 MHz. The inter ference rejection ?lter 80, which is described in more detail FIG. 1 describes a block diagram of a VSB transmitter for in FIG. 3, then processes the sampled signal and passes it to use with the invention; the equalizer and phase tracker 85. FIG. 2 describes a block diagram of a VSB receiver for The soft-decision symbol interleaver 90 essentially per use with the invention; forms the inverse operation of the symbol interleaver 20, FIG. 3 shows the structure of an NTSC interference except that the symbols have “soft” decision information as rejection ?lter comprising the invention; and described in “Principles of Digital Communication and FIG. 4 graphically compares the frequency response of Coding” which is incorporated by reference herein. the NTSC rejection ?lter of FIG. 3 and the NTSC color bar The output of the symbol deinterleaver 90 is then passed spectrum. 65 through a trellis decoder 95, a byte deinterleaver 100 and the FIG. 5 describes the pre-coder located at the transmitter RS decoder 105 which are described in “VSB Transmission illustrated in FIG. 1. System: Technical Details”. Sync and timing block 110 5,572,249 5 controls the clocking for each of the digital signal processing blocks and also the A/D converter 70. An optimal NTSC interference rejection ?lter in accor r1; 1 dance with the invention for use in a VSB receiver (for example interference rejection ?lter 80 of FIG. 2) is shown where b is a L>