Abstract:
Robust and existing standard bit streams are mixed in a backward compatible manner for forming enhanced modes for better reception of ATSC DTV signals. This is achieved by an enhanced coding block provided at the input of a conventional ATSC trellis encoder unit. The enhanced coding block comprising a trellis encoder encodes only the robust stream while passing the normal standard stream unaltered.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   Applicants claim the priority benefits under 35 U.S.C. §119(e) of U.S. Provisional Application Ser. No. 60/348,723, filed on Oct. 23, 2001, entitled TRELLIS ENCODER FOR RATE 1/4 AND 1/2 ROBUST CODING FOR A BACKWARD COMPATIBLE ATSC DTV TRANSMISSION STANDARD, the entire disclosure and content of which is incorporated herein by its reference. 
   Additionally, co-assigned, co-pending U.S. patent application Ser. No. 10/078,933, filed Feb. 19, 2002, entitled ENHANCED ATSC DIGITAL TELEVISION SYSTEM, and co-assigned, co-pending U.S. patent application Ser. No. 10/142,585, filed May 9, 2002, entitled A DIGITAL TELEVISION (DTV) TRANSMISSION SYSTEM USING ENHANCED CODING SCHEMES are incorporated by reference in their entirety as if fully set forth herein. 

   FIELD OF INVENTION 
   The present invention generally relates to the field of High Definition Television (HDTV). More specifically, the present invention is directed to a system and apparatus for encoding a digital television robust bit stream for enhanced signal to noise ratio and in a backward compatible manner. 
   BACKGROUND 
   In the television broadcasting industry, an attempt is being made to enhance high definition television (HDTV) systems to account for problems associated with broadcasting broadband signals over existing television frequency channels. Some of the problems encountered when broadcasting any signal over existing television frequencies include phase interference caused by multipath signal reception, signal fading, and signal attenuation caused by atmospheric conditions, terrain conditions, distance from the transmission source, and the like. These problems are exacerbated when an attempt is made to send and recover a digital signal in an error free manner, such as required by the current HDTV standard for non-degraded HDTV reception. 
   The current HDTV standard as promulgated by the Advanced Television Systems Committee (ATSC) employs many features to make use of the limited bandwidth, i.e., 6 MHz, available for HDTV transmission on air and in cable. For example, the HDTV baseband signal undergoes the known MPEG 2 compression, and a known type of forward error correction (FEC) Reed Solomon (RS) encoding in order to compress and error correct the data payload associated with HDTV transmission. The HDTV standard also calls for randomization of the bit stream by injection of a pseudorandom code to ensure that the signal is evenly distributed across the allocated spectrum, i.e., channel. As is known in the art, data interleaving is also implemented to scramble the sequential order of the data stream and to disperse the MPEG 2 packet data throughout time in order to minimize the transmitted signal&#39;s sensitivity to burst type interference. To further accommodate the 6 MHz allotted to the transmission channel, the interleaved HDTV bit stream of packets is further encoded to limit the number of amplitude levels required in transmission. 
   The current ATSC standard calls for vestigial sideband transmission with 8 discrete amplitude levels (8-VSB), i.e., channel symbols. One encoding method that implements additional FEC while achieving the 8-VSB requirement is known as the trellis code. Trellis codes, which are convolutional, i.e., serial and dependent on previous information bit values, are a known method of data encoding. The current ATSC specification calls for a 2/3 trellis code for 8-VSB. As is known, the 2/3 ratio specifies that the trellis code will encode 3 bits for every 2 bits input to the encoder, i.e., a 2/3 rate encoder is specified. According to the present state of the art, existing receivers are implemented to decode the 2/3 encoding of the specified 2/3 trellis encoder. 
     FIG. 1  illustrates an exemplary prior art HDTV transmission encoding system. As shown, a standard packet stream  110  is input into the system where the stream is first randomized by data randomizer block  120  and encoded by Reed Solomon (RS) encoder  130 . For the reasons discussed above, the randomized and encoded packet stream is then interleaved by data interleaver  140 . Trellis encoder  150 , typically running at a 2/3 encoding rate, then prepares the packet stream for 8-VSB modulation by encoding the bit stream to eight discrete levels. The Trellis encoded packet stream  180  is then routed to an 8-VSB modulator (Not Shown) for transmission of the HDTV signal. 
     FIG. 2  depicts an exemplary prior art trellis encoder, shown in more detail. A precode sequence is generated by an interference filter pre-coder  210  from the X 2  input, which is the most significant part of a randomized packet stream containing HDTV information. The filter output is then routed to trellis encoder  215  where it is fed through without any further processing to the Z 2  output. The X 1  input represents the least significant part of a randomized packet stream containing HDTV information. The trellis encoder feeds X 1  through to the Z 1  output. The Z 0  output is a function of the X 1  input and historical (two 12 symbol delay blocks, D) X 1  values. The three bit Z 2 Z 1 Z 0  combination is mapped by an 8-level symbol mapper  220  to the R channel symbols representing the eight discrete levels for 8-VSB modulation. It should be noted that in addition to utilizing a 2/3 encoding rate with one bit pre-coded so as to maintain an effective encoding rate of 1/2, the existing A/53 prior art, i.e., legacy trellis encoder for HDTV transitions through 4 states, i.e., the A/53 trellis encoder is a four state machine. A complete discussion of 4 state trellis encoding may be found in U.S. patent to Csajka et al (U.S. Pat. No. 4,077,021), and is incorporated herein by reference. 
   Under ideal reception conditions, i.e., no multipath signal interference, atmospheric or other type interference, the above known transmission encoding scheme is adequate to deliver the payload, i.e. HDTV signal, to a receiver with no degradation in picture quality. However, terrestrial, i.e., on-air television broadcasting may encounter many types of interference and disturbance between the transmission system and the reception system. Signal strength loss and interference contribute to a minimum threshold of visibility (TOV), typically measured in decibels (DB) also measured as carrier to noise ratio (CNR) above a noise/interference level, under which the HDTV signal simply cannot be recovered. 
   While the above discussed prior art system is beneficial in the capability to transmit an HDTV signal over an existing legacy on air channel such as the 6 MHz wide channel discussed above, there remains the drawback of maintaining a relatively high TOV over interference and noise for successful reception of the HDTV signal. In real world applications, however, HDTV signal attenuation and degradation due to a variety of factors, such as those discussed above, are likely, when encountered, to cause an 8-VSB signal to fall below its required TOV, resulting in disruption of reception at an HDTV receiver. While legacy analog systems may degrade slowly under interference, i.e., the picture may gradually fade away into a noise signal (snow), the digital HDTV signal will suddenly freeze at the below TOV level. While a robust data stream solves the problem of reducing the TOV in order to maintain HDTV viewing under less than ideal conditions, there remains the problem of maintaining compatibility with legacy, i.e., prior art HDTV transmission and reception systems. The prior art simply has a need for a more robust and backwards compatible HDTV system. 
   SUMMARY OF THE INVENTION 
   Therefore, it is an object of the present invention to provide a system and apparatus for increasing the robustness of an HDTV transmission stream while maintaining backward compatibility with existing HDTV reception systems by using an enhanced coding scheme concatenated with the legacy encoding scheme. 
   This and other objects and advantages may be obtained in the present invention by providing a system, apparatus and method that implements a backwards compatible, robust HDTV bit stream of packets that significantly reduces TOV/CNR requirements for successful HDTV reception. Specifically, there is provided an apparatus for HDTV packet transmission that comprises an enhanced coding block which includes a trellis encoder having a plurality of multiplexers for multiplexing robust and standard bit streams, the enhanced coding block being connected to an A/53 trellis encoder comprising a pre-coder, the A/53 trellis encoder, and a symbol mapper, wherein the enhanced coding block further comprises a feedback input for accepting signals from the pre-coder of the A/53 block; and the symbol mapper including a subset of the A/53 symbol set associated with the standard bit stream of packets in conjunction with the robust bit stream for modulating a transmitter in a backward compatible manner. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The present invention will now be described in more detail by referring to the drawings that accompany the present application. It is noted that in the accompanying drawings like reference numerals are used for describing like and corresponding elements thereof. 
       FIG. 1  shows a prior art HDTV transmission system packet flow; 
       FIG. 2  shows packet flow through a prior art configured filter, trellis encoder, and 8 level symbol mapper; 
       FIG. 3  shows a block diagram illustrating a trellis encoder with a preferred concatenated encoding for a robust stream; 
       FIG. 4  shows a block diagram illustrating a preferred upper coding scheme in the trellis encoder to achieve 2-VSB mapping or enhanced 8-VSB encoding; 
       FIG. 5  depicts a truth table representing a preferred control bit generating means for the enhanced coding block; 
       FIG. 6  is a table of simulated performance results for an Added White Gaussian Noise (AWGN) channel for various mix ratios of the normal stream with the corresponding robust stream, wherein NS means not simulated, and NM means Not Measured, and shows TOV advantages of this invention for various mixes of standard and robust streams; and 
       FIG. 7  is a simplified top-level diagram of the enhanced ATSC transmitter, and shows the inventive transmission system with trellis encoder and non-systematic RS encoder feedback for standard stream compatibility. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Although this invention is applicable to numerous and various types of data transmission encoding systems, it has been found particularly useful in the environment of HDTV systems. Therefore, without limiting the applicability of the invention to HDTV systems, the invention will be described in such an environment. Such an HDTV system is described in co-pending U.S. Patent Applications, Ser. No. 60/295,616 entitled PACKET IDENTIFICATION MECHANISM AT THE TRANSMITTER AND THE RECEIVER FOR AN ENHANCED ATSC 8-VSB SYSTEM; 60/301,559 entitled PROPOSAL FOR A POTENTIAL REVISION OF THE ATSC DTV STANDARD USING ENHANCED CODING SCHEMES; 60/324,482 entitled KEY ELEMENTS FOR THE NEWLY PROPOSED ATSC STANDARD; 60/280,782 entitled PROPOSAL FOR AN IMPROVED ATSC DTV STANDARD; the entire disclosures and content of which are incorporated herein by their reference. 
   A preferred implementation of the current invention will support a flexible payload data-rate in, preferably, a 6 MHz channel and with a flexible CNR. In one aspect of this invention, as shown in  FIG. 3 , combined robust and standard bit streams X′ 1  and X′ 2  are input to enhanced coding block  305  which comprises a first trellis encoder device described in detail with respect to  FIG. 4 . 
     FIG. 4  illustrates the enhanced coding block  305  which comprises a trellis encoder providing an upper coding scheme to achieve 2-VSB mapping or enhanced 8-VSB encoding with a 16-state rate 1/3 trellis encoder to achieve a rate 1/4 or 1/2 robust stream encoding. It should be understood that this encoder may additionally be converted to a 32 state or higher state to provide even greater error correction capability. As shown in  FIG. 4  the X′ 1  bit stream is input to the 1/2 input of a first stage multiplexer  410 . The contents of two twelve symbol delay registers, D  405 , comprising historical values of the bit stream are combined with the X′ 1  signal and input to the 1/4 code rate select input of the multiplexer  410 . 
   The first stage multiplexer  410  output is both combined with pre-coder data from the prior art A/53 trellis encoder ( FIG. 2 ,  210 ) and input to the “2”, i.e., 2-VSB modulation select input, of a second stage multiplexer  415 . Multiplexer  410  functions in a known 2 to 1 manner, which means that either the 1/2 or the 1/4 coded input is selected for output, according to the control bit m 1 . 
   The second stage multiplexer  415  additionally includes an “8”, i.e., 8-VSB modulation select, input from delay block  404 . An output of the second stage multiplexer  415  is then fed to the R, i.e., standard stream select, input of third stage multiplexer  420  where its second input, the N, i.e., robust stream select input, comprises the original X′ 1  bit stream. Multiplexer  415  functions in a known 2 to 1 manner, which means that either the 2-VSB or the 8-VSB coded input is selected for output, according to the “mod type” control bit. 
   The output X 1  of the third stage multiplexer  420  corresponds to the X 1  input of the prior art A/53 trellis encoder ( FIG. 2 ,  215 ). Multiplexer  420  functions in a known 2 to 1 manner, which means that either the Normal stream, “N” or the Robust Stream, “R” input is selected for output, according to the “N/R” control bit. 
   Pre-coder multiplexer  425  includes as its R input, the combined pre-coder data  427  from the prior art A/53 trellis encoder ( FIG. 2   210 ) and first stage multiplexer  410  output  428 . The pre-coder multiplexer  425  generates an output  429  that corresponds to the X 2  input of the prior art A/53 trellis encoder ( FIG. 2   215 ). Multiplexer  425  functions in a known 2 to 1 manner, which means that either the Normal stream, “N” or the Robust Stream, “R” input is selected for output, according to the “N/R” control bit. Details of the prior art A/53 trellis encoder are discussed in the  ATSC Standard: Digital Television Standard, Revision B , Document A/53B, dated 7 Aug. 2001, which contents in its entirety is incorporated herein by reference. 
   It should be understood that the combination of prior art trellis encoder  215 , pre-coder  210  and enhanced coding block  305  comprising trellis encoder  400  may provide a 16 state encoding process with the prior art 8 symbol levels compatible with existing 8-VSB modulation. The 8-VSB symbol mapping scheme used in conjunction with a 16 state trellis encoder for encoding a robust bit stream, comprises an enhanced 8-VSB mode. Additionally, the control bit generator  402  which applies control bits to delay registers  405 , multiplexers  410 ,  415  and  420  allows for selection between normal, i.e., standard stream bits for 8-VSB and robust stream bits for enhanced 8-VSB, as well as enhanced 2-VSB modes. 
   Using a subset of the 8-VSB symbol set, the pre-coding for 2-VSB mode is obtained by, preferably, equating Z 2  and Z 1  to the information bit, i.e., robust data stream bit. Pre-coder input X 2  is calculated by control bit generator  402  such that the pre-coder output Z 2  is the information bit. Correspondingly, X 1  is equated to the information bit. The operation performed is X 2 =X 1 +Y 2d  mod  2 , where Y 2d  is the register contents of pre-coder  310 . The resultant symbol mapping at the output of the trellis encoder then becomes {−7, −5, 5, 7}. Symbol values from the foregoing alphabet {−7, −5, 5, 7} are used for 2-VSB modulation. 
   Since the information bit has been encoded as the sign of the symbols, i.e., either a positive symbol or a negative symbol, the number of symbols carrying information is therefore 2, resulting in 2-VSB, i.e., pseudo 2-VSB, modulation at symbol mapper output R. 
   Thus, robust packets on the receiver side may be decoded without being error flagged. It should be noted that the term “robust stream” refers to any data stream with HDTV content that is transmitted at a reduced bit rate from the nominal standard stream rate of 19.39 Mbps. 
   It is understood that both the standard stream and the robust stream are preferably comprised of 188-byte MPEG-compatible data packets which include a sync byte and 187 bytes of data. With 20 RS parity bytes added by the RS encoder block  130  upstream from the trellis encoder, the MPEG compatible data packets comprise 207 bytes of data. 
   As an example, the 1/4 trellis encoding scheme which adds 4 encoding bits for every 1 robust stream bit results in four 207 byte length packets for transmission downstream. It is also known that the standard stream sends a 32 bit Program Identifier (PID) in a Program Map Table (PMT) as specified by the MPEG 2 systems standard in ISO/IEC 13818-1. A preferable implementation of the current invention involves generating robust packets with a PID, preferably null, that is different from the PID as specified by the existing MPEG 2 systems standard. As a consequence, backwards compatibility is achieved because existing, i.e., legacy, receivers will ignore the transmitted robust packets encoded with PID&#39;s that are not a member of the PMT. 
     FIG. 7  illustrates a top level diagram of an enhanced ATSC transmitter, the details of which are discussed in co-assigned, co-pending U.S. patent application Ser. No. 10/127,531, filed Apr. 22, 2002, entitled AN IMPROVED DIGITAL TRANSMISSION SYSTEM FOR AN ENHANCED ATSC 8-VSB SYSTEM, co-assigned, co-pending U.S. patent application Ser. No. 10/142,585, filed May 9, 2002, entitled A DIGITAL TELEVISION (DTV) TRANSMISSION SYSTEM USING ENHANCED CODING SCHEMES, and co-assigned, co-pending U.S. patent application Ser. No. 10/118,876, Apr. 9, 2002, entitled PACKET IDENTIFICATION MECHANISM AT THE TRANSMITTER AND RECEIVER FOR AN ENHANCED ATSC 8-VSB SYSTEM, and are incorporated by reference in their disclosures and content as if fully set forth herein. Additionally, an optional RS encoder which is, preferably, “non-systematic” ( FIG. 7  at  705 ) and includes a feedback path  715  from the enhanced coding block, i.e. trellis encoder  305  to an interleaver  710  upstream from the enhanced coding block may be included to provide parity byte generation that is backward compatible with legacy receivers. This compatibility is provided by assuring that the 20 Reed Solomon (RS) parity bytes are placed at the end of all bytes containing standard 8-VSB information bits, as RS decoders in legacy receivers expect this positioning for RS parity bytes as described in commonly-owned, co-pending U.S. patent application Ser. No. 10/142,585, filed May 9,. 2002, entitled A DIGITAL TELEVISION (DTV) TRANSMISSION SYSTEM USING ENHANCED CODING SCHEMES. 
   In a preferred implementation of the invention, the trellis encoder  400  processes a byte by encoding two consecutive bits at a time to convert an 8 bit input into 4 symbols. The control bits  401  preferably are specified for each two bit group within a byte. Therefore, to convert a byte, the value of each control bit may change four times in accordance with the 2 bits to be encoded. Referring to  FIG. 5 , a representative truth table of the control bits for byte encoding is illustrated. 
     FIG. 5  illustrates a truth table for generating the control bits required to encode a byte according to a desired signal modulation, symbol encoding scheme and rate. Preferably, the information specified by a broadcaster, for example, and included in the packet streams, the “Byte stream type”, “N/R”, i.e. standard stream/robust stream, “Mod type”, i.e., modulation type, “rate”, i.e., encoding rate, provides control inputs  403  which may be tracked by control unit  402  to determine “en”, i.e., symbol delay enable, and “m1”, i.e., first stage multiplexer select output values of control unit  402 . 
   The particular control bit patterns are for illustration purposes, and do not preclude other bit patterns, e.g., using positive or negative logic, and/or other hexadecimal values defined with the same logical effect. As an example, using the control scheme depicted in  FIG. 5 , the 1111, i.e., hexadecimal (hex) pattern “F” in the N/R field  515  designates that a normal, i.e., full rate standard stream symbol is currently being processed. Note that in this event all other fields except for the en field  530  which also has a hex value of “F”, are “don&#39;t cares”. 
   Continuing with the current example, and referring to  FIGS. 4 and 5 , it is observed that the “en” input of “F” to symbol delay registers  405  inhibits shifting the current byte into the registers  405 . Additionally, the “N/R” input of “F” to pre-coder multiplexer  425  and third stage multiplexer  420  select the “N”, i.e., standard stream data path for throughput through the output of the trellis encoder  400 . In a similar manner, other data stream symbols belonging to other permutations of “byte type”, “N/R”, “Mod type”, “rate”, “en”, and “m1” control the trellis encoder of enhanced coding block  305  according to truth table  501  of  FIG. 5 . 
   Referring to  FIG. 6 , it is shown that for various mixes of standard and robust streams produced by a preferred implementation of this invention, signal decoding is enhanced over the prior art standard stream approach. As shown, simulated tests with added white gaussian noise (AWGN) have indicated that this invention can provide a signal which can be decoded with a CNR of only 2.0 db, as opposed to the 15 db requirement when a 100% standard stream of the prior art is used. The actual TOV advantage is a function of the stream type mix ratio, encoder rate, and modulation type. A preferred mix ratio  605  of robust stream to standard stream for 2-VSB mode encoding ranges from 5% robust to 50% robust. A preferred mix ratio  610  of robust stream to standard stream for 8-VSB mode encoding ranges from 5% robust to 25% robust. 
   Now that the invention has been described by way of a preferred embodiment, various modifications and improvements will occur to those of skill in the art. For example, the output processor block  430  of the trellis encoder  400  can alternatively be designed with a single logic unit that achieves the same functionality. Thus, it should be understood that the preferred embodiment is provided as an example and not as a limitation. The scope of the invention is defined by the appended claims.