Abstract:
A flexible digital transmission system that improves upon the ATSC A/53 HDTV signal transmission standard. The system includes a digital signal transmitter for generating a first Advanced Television Systems Committee (ATSC) standard encoded 8-VSB bit stream and, for generating an encoded new robust bit stream for transmitting high priority information bits, wherein symbols of the new bit stream are capable of being transmitted according to a transmission mode including: a 2-VSB mode and a 4-VSB transmission mode. The standard 8-VSB bit stream and new bit stream may be simultaneously transmitted over a terrestrial channel according to a broadcaster defined bit-rate ratio. The transmission system includes a control mechanism for generating information needed for encoding robust packets at a transmitter device. It also includes a mechanism for encoding control parameters and multiplexes the generated information with the standard and robust bit-streams for transmission. A receiver architecture is additionally provided to decode standard and robust bit-streams transmitted by the transmitter device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    The present invention claims the benefit of commonly-owned, co-pending U.S. Provisional Patent Application Serial No. 60/295,616 filed Jun. 4, 2001. This patent application is additionally related to commonly-owned, co-pending U.S. Provisional Patent Application Serial No. 60/280,782 filed Apr. 2, 2001 entitled IMPROVED ATSC DIGITAL TELEVISION SYSTEM, the entire contents and disclosure of which is incorporated by reference as if fully set forth herein. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to digital transmission systems and particularly, to the Advanced Television Systems Committee (ATSC) Digital Television (DTV) standard (A/53). The invention describes a method for transmitting a robust bit-stream along with the standard bit-stream that is in compliance with the ATSC standard, an further describes a method for enabling identification of robust packets at a receiver device.  
           [0004]    2. Discussion of the Prior Art  
           [0005]    The ATSC standard for high-definition television (HDTV) transmission over terrestrial broadcast channels uses a signal that comprises a sequence of twelve (12) independent time-multiplexed trellis-coded data streams modulated as an eight (8) level vestigial sideband (VSB) symbol stream with a rate of 10.76 MHz. This signal is converted to a six (6) MHz frequency band that corresponds to a standard VHF or UHF terrestrial television channel, over which the signal is broadcast at a data rate of 19.39 million bits per second (Mbps). Details regarding the (ATSC) Digital Television Standard and the latest revision A/53 is available at http://www.atsc.org/.  
           [0006]    [0006]FIG. 1 is a block diagram generally illustrating an exemplary prior art high definition television (HDTV) transmitter  100 . MPEG compatible data packets are first randomized in a data randomizer  105  and each packet is encoded for forward error correction (FEC) by a Reed Solomon (RS) encoder unit  110 . The data packets in successive segments of each data field are then interleaved by data interleaver  120 , and the interleaved data packets are then further interleaved and encoded by trellis encoder unit  130 . Trellis encoder unit  120  produces a stream of data symbols having three (3) bits each. One of the three bits is pre-coded and the other two bits are produced by a four (4) state trellis encoder. The three (3) bits are then mapped to an 8-level symbol.  
           [0007]    As known, a prior art trellis encoder unit  130  comprises twelve (12) parallel trellis encoder and pre-coder units to provide twelve interleaved coded data sequences. In multiplexer  140  the symbols of each trellis encoder unit are combined with “segment sync” and “field sync” synchronization bit sequences  150  from a synchronization unit (not shown). A small in-phase pilot signal is then inserted by pilot insertion unit  160  and optionally pre-equalized by filter device  165 . The symbol stream is then subjected to vestigial sideband (VSB) suppressed carrier modulation by VSB modulator  170 . The symbol stream is then finally up-converted to a radio frequency by radio frequency (RF) converter  180 .  
           [0008]    [0008]FIG. 2 is a block diagram illustrating an exemplary prior art high definition television (HDTV) receiver  200 . The received RF signal is down-converted to an intermediate frequency (IF) by tuner  210 . The signal is then filtered and converted to digital form by IF filter and detector  220 . The detected signal is then in the form of a stream of data symbols that each signify a level in an eight (8) level constellation. The signal is then provided to NTSC rejection filter  230  and to synchronization unit  240 . Then the signal is subjected to equalization and phase tracking by equalizer and phase tracker  250 . The recovered encoded data symbols are then subjected to trellis decoding by trellis decoder unit  260 . The decoded data symbols are then further de-interleaved by data de-interleaver  270 . The data symbols are then subjected to Reed Solomon decoding by Reed Solomon decoder  280 . This recovers the MPEG compatible data packets transmitted by transmitter  100 .  
           [0009]    While the existing ATSC 8-VSB A/53 digital television standard is sufficiently capable of transmitting signals that overcome numerous channel impairments such as ghosts, noise bursts, signal fades and interferences in a terrestrial setting, there exists a need for flexibility in the ATSC standard so that streams of varying priority and data rates may be accommodated.  
           [0010]    It would thus be highly desirable to provide in an ATSC digital transmission system, a technique for transmitting new robust bit-streams along with the standard ATSC 8-VSB bit-stream, wherein the new bit-stream has a lower Threshold of Visibility (TOV) compared to the standard ATSC stream.  
           [0011]    It would further be highly desirable to provide a flexible ATSC digital transmission system and methodology that provides a mechanism for transmitting defined parameters used to correctly identify robust packets at a receiver device.  
           [0012]    It would further be highly desirable to provide a flexible ATSC digital transmission system and methodology that permits a trade-off of the standard bit-stream&#39;s data rate for the new bit-stream&#39;s robustness, and, further is such that the transmission is backward compatible with existing digital television receiver devices.  
         SUMMARY OF THE INVENTION  
         [0013]    It is thus an object of the present invention to provide in an ATSC digital transmission system, a technique for transmitting a new robust bit-stream along with the standard ATSC bit-stream.  
           [0014]    It is another object of the present invention to provide a flexible ATSC digital transmission system and methodology that provides a mechanism for generating and transmitting defined parameters used for correctly identifying robust packets at a receiver device.  
           [0015]    It is a further object of the present invention to provide a flexible ATSC digital transmission system that is backward compatible with existing digital television receiver devices.  
           [0016]    In accordance with the preferred embodiments of the invention, there is provided a digital signal transmission system and method comprising: a means for encoding packets to be transmitted as either 8-VSB modulated bit stream symbols or, encoding robust packets for transmission as robust bit stream symbols; a transmitter device for transmitting a robust bit-stream comprising the robust bit stream symbols separately, or in conjunction with a standard bit-stream comprising 8-VSB modulated bit stream symbols over a fixed bandwidth communications channel for receipt by a receiver device; and, a means for generating information needed for decoding robust packets at the receiver device, the transmitter device multiplexing the generated packet decoding information with the standard bit-stream and the robust bit-stream for enabling appropriate symbol decoding at the receiver device.  
           [0017]    To insure backward compatibility with existing receivers from various manufacturers, an optional non-systematic Reed-Solomon encoder is provided to add parity bytes to the robust bit-stream packets. The standard 8-VSB bit-stream will be encoded using the ATSC FEC scheme (A/53). Packets transmitted using the inserted new bit-stream will be ignored by the transport layer decoder of the existing receiver, thus reducing the effective payload that can be decodable by existing receivers. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]    Details of the invention disclosed herein shall be described below, with the aid of the figures listed below, in which:  
         [0019]    [0019]FIG. 1 illustrates a block diagram of an exemplary high definition television (HDTV) transmitter according to the prior art;  
         [0020]    [0020]FIG. 2 illustrates a block diagram of an exemplary high definition television (HDTV) receiver according to the prior art;  
         [0021]    [0021]FIG. 3 is a representative functional diagram of the improved digital transmission system  300  for robust (pseudo 2-VSB and 4-VSB) and standard bit streams according to the invention;  
         [0022]    [0022]FIG. 4 is an illustration depicting a data segment comprising the FEC encoded bit streams transmitted by the improved digital transmission system  300  of the invention;  
         [0023]    [0023]FIG. 5( a ) illustrates a trellis encoder block  350  employs trellis code intrasegment interleaving according to the prior art;  
         [0024]    [0024]FIG. 5( b ) illustrates a block diagram of one exemplary prior art trellis encoder and pre-coder unit (one of twelve such units shown in FIG. 5( a ) and an eight (8) level symbol mapper; and,  
         [0025]    [0025]FIG. 6 illustrates the result of a packet duplication process implemented in the packet formatter device for robust streams;  
         [0026]    [0026]FIG. 7( a ) depicts the process of packet duplication for packets formatted according to control parameter Mode=2,3 and for NRS=0, and  
         [0027]    [0027]FIG. 7( b ) depicts the process of packet duplication for packets formatted according to control parameter Mode=2,3 and for NRS=1;  
         [0028]    FIGS.  8 ( a ) and  8 ( b ) particularly depicts the bit rearrangement process performed by the packet formatter for control parameter MODE=1 and for NRS=0 (FIG. 8( a )) and for NRS=1 (FIG. 8( b ));  
         [0029]    [0029]FIG. 9( a ) illustrates a single MPEG field  400  comprising 312 packets;  
         [0030]    [0030]FIG. 9( b ) illustrates an example MPEG field  402  for the case when control parameter information includes NRP=“1101” and RPP=“00”; and,  
         [0031]    [0031]FIG. 9( c ) illustrates an example MPEG field for the case when control parameters NRP=“1100” and RPP=“01”; and,  
         [0032]    [0032]FIG. 10 illustrates a block diagram of a novel ATSC receiver  500  capable of decoding both the standard and new (robust) bit-streams. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    Co-pending, commonly-owned U.S. patent application Ser. No. ______ [US010173; D#15062] filed ______ entitled IMPROVED ATSC DIGITAL TELEVISION SYSTEM, the whole contents and disclosure of which is incorporated by reference as if fully set forth herein, describes a system that enables the transmission of more robust ATSC bit streams, e.g., those defined as pseudo 2-VSB and 4-VSB and hierarchical VSB or H-VSB modulated bit-streams, by a digital transmitter, or the embedding of robust bit streams in the standard ATSC 8-VSB bit stream. According to co-pending, commonly-owned U.S. patent application Ser. No. ______, each of the proposed new ATSC bit streams is robust (“Robust Streams”) in the sense that the error correcting capacity of bits in the Robust Stream is greater than the error correcting capacity of bits in the standard ATSC 8-VSB bit stream.  
         [0034]    The present invention herein described in accordance with FIG. 3 is directed to a novel digital transmission system  300  that enables the flexible transmission and receipt of MPEG compatible packets as robust and standard digital bit streams for accommodating a large range of carrier-to-noise ratios and channel conditions. Moreover, the present invention herein described relates to a digital transmission system implementing novel methods for enabling backward compatibility with existing digital receiver devices, and, specifically, providing a mechanism for transmitting defined parameters used to correctly identify robust MPEG packets at a receiver device.  
         [0035]    A representative functional diagram of the improved digital transmission system  300  for transmitting robust pseudo 2-VSB or 4-VSB bit-streams (in addition to standard bit streams) according to the invention is now described with respect to FIG. 3. As shown in FIG. 3, the system  300  includes an input path  303  for receiving packets  307  and generating digital bit streams according to the existing ATSC 8-VSB standard or, as new (robust) pseudo 2-VSB, 4-VSB or H-VSB modulation bitstreams. Preferably, all robust packets, e.g., pseudo 2-VSB or 4-VSB modulated, are processed and transmitted by system  300  in a backward compatible manner, meaning that existing receivers will be able to identify the robust stream packets as valid RS (Reed-Solomon) code-words. It may do so, for instance, by enabling a packet identifier (PID) corresponding to the robust stream packets (for existing receivers) to comprise a Null packet header.  
         [0036]    As described in herein-incorporated, co-pending, commonly-owned U.S. patent application Ser. No. ______ [Atty Docket #702213; D#15062], several feature points of the new ATSC encoding system include, but are not limited to: 1) the Threshold of Visibility (TOV) for the new robust stream may be as low as 8.5 dB; 2) enablement of significant performance gains for the new robust stream in the presence of strong static multi-path and dynamic multi-path; 3) permit a trade-off of data rates for robustness; 4) three different system modes: pseudo 2-VSB, 4-VSB, and Hierarchical VSB (H-VSB); 5) an optional Reed-Solomon encoder to satisfy backward compatibility requirements; 6) the broadcaster may choose the amount of mix of the robust stream and the standard stream. The mix percentage may range from 0% (standard stream only) to 100% (robust stream only); and, 7) several options are provided for the placement of robust packets within a frame.  
         [0037]    Thus, in order to implement the improved ATSC digital transmission system, a number of selectable parameters must first be chosen and transmitted to the receiver device. An efficient method for conveying this information to the receiver device is now herein described. Particularly, some of the reserved Frame Sync bits defined in accordance with the A/53 ATSC standard may be used for this purpose. As shown in FIG. 4, according to the ATSC A/53 standard, Revision B (Aug. 7, 2001), a VSB transmitter organizes data transmission as a series of data frames (not shown) with each data frame comprising two data fields, each data field comprising 313 data segments. Each Data Field is periodic (approximately 24.2 ms) and starts with one complete Data Segment referred to as a Data Field Sync (not shown) which is a unique synchronizing signal. The remaining 312 data segments each carry the equivalent of data from one 188-byte transport packet plus its associated forward error correction (FEC) overhead (generated by the RS coder and the trellis coder). A data field sync segment  20  is illustrated in FIG. 4 which comprises 832 symbols including a Data Segment Sync  25  signal which comprises four (4) symbols transmitted in binary form and provides segment synchronization. According to the ATSC A/53 standard, the remaining 828 symbols of the data segment  20  comprises several symbol fields including pseudo-noise sequences  26   a , . . . ,  26   d  of varying lengths, and, a reserved symbol field  27  comprising 104 symbols. According to the invention, it is within this reserved symbol field  27  where the set of parameters for identifying the type of digital bit-stream transmission, including standard and new robust (pseudo 2-VSB, 4-VSB and H-VSB) bit-streams in the backward compatible manner, for a receiver are communicated.  
         [0038]    Table 1 indicates the parameters that have to be defined in order to correctly identify robust packets at a receiver. As these have to be interpreted at an equalizer device of the receiver, they are heavily protected using robust error correcting codes. The encoded code-word is preferably inserted in the 104 reserved symbol field  27  of a Data Field Sync segment  20 .  
                                   TABLE 1                                   MODE   NRS   NRP   RPP                           (2)   (1)   (4)   (2)                      
 
         [0039]    Table 1 particularly indicates the use of four parameters (and their respective number of bits) to identify robust packets. A first parameter “MODE” includes specification of the robust packets and is used in identifying the format of the robust packets. Two bits are used to identify four possible modes as now described with respect to Table 2: 
                           TABLE 2                                   MODE   Description                           00   Standard. No robust packets in the field           01   H-VSB mode           10   4-VSB mode           11   Pseudo 2-VSB mode                      
 
         [0040]    For instance, as shown in Table 2, the MODE 00 indicates a standard stream with no robust packets to be transmitted; MODE 01 indicates an H-VSB stream; MODE 10 indicates an 4-VSB stream; and MODE 11 indicates a pseudo 2-VSB stream is to be transmitted. If MODE=00 then rest of the parameters may be ignored.  
         [0041]    Referring back to Table 1, the second “NRS” (Non-systematic Reed-Solomon coder) parameter indicates whether the non-systematic RS coder is to be used to encode the robust packets. A single bit is used to identify the two possible NRS modes as now described with respect to Table 3: 
                           TABLE 3                                   NRS   Description                           0   Non-systematic RS coder is not used           1   Non-systematic RS coder is used                      
 
         [0042]    For instance, NRS=0, indicates that the non-systematic RS coder is not used and so one robust packet will be coded into two symbol segments by the FEC block. If NRS=1, then that indicates that the systematic RS coder is used and therefore a group of four robust packets will be coded into nine symbol segments by the FEC block. Tables 4 and 5 illustrate example ratios of the number of robust packets per frame (i.e., the number of Robust packets vs. the number of standard packets, per frame (mix) and, example corresponding bit-rates for NRS=0 and NRS=1, respectively.  
                                                               TABLE 4                           # of Robust/# of               standard packets,   Bit Rate            per frame (mix)   Robust   Standard                    0/312   (0%)   0       19.28           2/308       123.589   Kbps   19.033   Mbps       3/306   (2%)   185.385   Kbps   18.909   Mbps       4/304       247.179   Kbps   18.785   Mbps       6/300       370.769   Kbps   18.538   Mbps       8/296   (5%)   484.359   Kbps   18.291   Mbps       12/288       741.538   Kbps   17.797   Mbps       16/280   (10%)   988.718   Kbps   17.302   Mbps       20/272   (13%)   1.236   Mbps   16.808   Mbps       26/260   (16%)   1.606   Mbps   16.067   Mbps       32/248   (20%)   1.977   Mbps   15.325   Mbps       39/234   (25%)   2.410   Mbps   14.460   Mbps       52/208   (33%)   3.213   Mbps   12.853   Mbps       78/156   (50%)   4.820   Mbps   9.640   Mbps       104/104   (66%)   6.427   Mbps   6.427   Mbps       156/0   (100%)   9.640   Mbps   0                  
 
         [0043]    Table 4 particularly indicates the bit-rates of the respective robust and the standard bit-streams for different mix values, when NRS=0. It should be noted that the mix percentages indicated in Table 4 are rounded off values.  
                                                           TABLE 5                           # of Robust/# of               Standard packets,   Bit Rate            per frame   Robust   Standard                     0/312   0       19.28   Mbps        4/303   247.179   Kbps   18.724   Mbps        8/294   484.359   Kbps   18.168   Mbps       12/285   741.538   Kbps   17.612   Mbps       16/276   988.718   Kbps   17.055   Mbps       20/267   1.236   Mbps   16.499   Mbps       24/258   1.483   Mbps   15.943   Mbps       28/249   1.730   Mbps   15.387   Mbps       32/240   1.977   Mbps   14.831   Mbps       40/222   2.472   Mbps   13.718   Mbps       52/195   3.213   Mbps   12.050   Mbps       64/168   3.955   Mbps   10.382   Mbps       72/150   4.449   Mbps   9.269   Mbps       76/141   4.696   Mbps   8.713   Mbps       96/96    5.932   Mbps   5.932   Mbps       120/42    7.415   Mbps   2.595   Mbps                  
 
         [0044]    Table 5 particularly indicates the bit-rates of the robust and the standard bit-streams for different mix values when NRS=1.  
         [0045]    Referring back to Table 1, the third “NRP” parameter indicates the Number of Robust Packets in a frame. Table 6 may be used to map this 4 bit number to the number of robust packets in a frame.  
                                                       TABLE 6                                       Number of robust packets               before encoding            NRP   NRS = 0   NRS = 1                    0000   0   0       0001   2   4       0010   3   8       0011   4   12       0100   6   16       0101   8   20       0110   12   24       0111   16   28       1000   20   32       1001   26   40       1010   32   52       1011   39   64       1100   52   72       1101   78   76       1110   104   96       1111   156   120                  
 
         [0046]    Referring back to Table 1, the fourth “RPP” parameter indicates the Robust Packets&#39; Position in a frame. Robust packets may be either distributed uniformly within a frame or arranged contiguously within a frame starting from an initial position. Note that uniform distribution is not possible for all values of NRP. Table 7 describes the various types of robust packet distributions within a frame.  
                   TABLE 7                       RPP   Robust packets&#39; position                   00   Distributed uniformly within a frame with a granularity of one       01   Distributed uniformly within a frame with a granularity of two       10   Distributed uniformly within a frame with a granularity of four       11   Arranged contiguously within a frame starting from position one                  
 
         [0047]    As described herein, robust symbol mapping techniques are utilized to get performance advantage for the new robust bit-stream. This necessitates a control mechanism to track bytes belonging to the robust bit-stream and the standard bit-stream through the FEC section of the transmitter. The transmitter also implements the ‘Packet Formatter’ block  330  (FIG. 3) in the data-path to re-format data bytes belonging the robust bit-stream, as will be explained in greater detail.  
         [0048]    [0048]FIG. 3 is a block diagram depicting the ATSC transmitter  300  for transmitting robust bit streams according to the invention. For purposes of description, the ATSC transmitter  300  is described without the non-systematic RS coder (i.e., NRS=0). It is understood that a further embodiment of the ATSC transmitter that includes the optional non-systematic RS coder (i.e., NRS=1), is modified to take into account the additional complexity. As shown in FIG. 3, the ATSC transmitter  300  according to the invention implements a randomizer element  310  for first changing the input data byte value according to a known pattern of pseudo-random number generation. According to the ATSC standard, the data randomizer XORs all the incoming data bytes with a 16-bit maximum length pseudo random binary sequence (PRBS) which is initialized at the beginning of a data field. The output randomized data is then input to an Reed Solomon (RS) encoder element  320  which operates on a data block size of 187 bytes, and adds twenty (20) RS parity bytes for error correction to result in a RS block size total of 207 bytes transmitted per data segment. It is these bytes that will then be post processed and sent using robust constellations. After the RS encoding, the 207 byte data segment is then input to the packet formatter  330  which functions to re-format the data bytes belonging to the robust bit-stream accordingly. The packet formatter  340  essentially buffers and groups the incoming robust bit-stream into groups of 207 bytes and passes the standard bit-stream bytes without any modification. In general, only 4 bits of each byte at the packet formatter output, the LSBs ( 6 , 4 , 2 , 0 ), correspond to the incoming stream. The other 4 bits of each byte, the MSBs ( 7 , 5 , 3 , 1 ), may be set to any value for reasons as will be explained in greater detail herein. After byte re-formatting in the packet formatter  330 , the data is input to the convolutional interleaver mechanism  340  for scrambling the sequential order of the data stream according to the ATSC A/53 standard. As will be explained in greater detail, the tracking of bytes associated with each robust packet or standard packet is performed in a concurrent processing control path  304  as shown in FIG. 3. As further shown in FIG. 3, the interleaved, RS-encoded data bytes are then trellis coded by the trellis encoder device  350  which employs ⅔ rate trellis code with one unencoded bit which is precoded, i.e., one input bit is encoded into two output bits using a ½ rate convolutional code while the other bit is precoded. As shown in FIG. 5( a ), the trellis encoder  350  employs trellis code intrasegment interleaving and symbol mapping, and comprises, for example, twelve identical trellis encoders and precoders  351   1  to  351   12  operating on interleaved data symbols.  
         [0049]    Preferably, a more robust trellis encode mapping scheme (pseudo 2-VSB or 4-VSB) system is implemented for tracked robust symbols as compared to the standard 8-VSB symbol mapping scheme that is implemented for tracked normal (standard) symbols. It should be understood that for the trellis encoding of robust symbols, a ⅓ trellis encoding is implemented such that one bit of input is mapped into three bits which is mapped into one symbol for robust streams. For standard streams, two bits are mapped into three bits according to the conventional 8-level symbol mapping technique for standard packets. For conventional bytes belonging to the standard stream (SS), all 8-bits of each byte carry information. For the robust stream (NS), it is desirable that only four bits of each byte carry information. More particularly, as shown in FIG. 5( b ), according to the invention, for the robust bit-stream, the trellis encoder  350  receives a byte, of which only 4-bits (LSBs) contain valid information. When a byte that belongs to the robust stream is received by the trellis encoder  350 , the information bits (e.g., LSBs bits ( 6 , 4 , 2 , 0 )) are placed on X1, and X2 is subsequently determined to obtain the particular symbol mapping scheme, e.g., pseudo 2-VSB. Once X2 is determined, the 4-MSBs of the byte, e.g., bits ( 7 , 5 , 3 , 1 ) will be replaced by these values. When all the bits of a byte are determined, a new byte will then have been formed containing the LSBs and the MSBs. This byte may then be passed to the “non-systematic” Reed-Solomon encoder  375  when NRS=1. As described in greater detail in co-pending U.S. patent application Ser. No. ______, [US010173, Atty Dkt. #15062], the parity bytes of the “non-systematic” Reed-Solomon encoder and the PID bytes will however be encoded using the 8-VSB encoding scheme. The symbol mapping techniques for each mode are now described as follows:  
         [0050]    Pseudo 2-VSB Mode  
         [0051]    The 2-VSB mode is obtained by making Z2 and Z1 equal to the information bit X1 (i.e., LSB bits ( 6 , 4 , 2 , 0 )) in the trellis encoder unit  352  of FIG. 5( b ). The X2 is then calculated such that, when precoded, it results in Z2. This operation is nothing other than X2=X1+Y2d mod 2, where Y2d is the content of the register  356  of the pre-coder unit  353  of FIG. 5( b ). This operation, combined with the existing symbol mapping scheme implemented at the 8-level symbol mapper  354 , results in symbols from the alphabet {−7,−5,5,7}. This is essentially a pseudo 2-VSB signal in the sense that the information bit is transmitted as the sign of this symbol. The actual symbol is a valid trellis coded 4-level symbol which can be decoded by existing trellis decoder devices.  
         [0052]    4-VSB Mode  
         [0053]    In view of FIG. 5( b ), the 4-VSB mode is obtained by making Z1 equal to the information bit in the trellis encoder unit  352 . The X2 is then calculated such that when pre-coded, Z2 equals Z0. This operation is nothing other than X2=Z0+Y2d mod 2, where Y2d is the content of the pre-coder register  356 . These operation and the use of the existing symbol mapping results in symbols from the alphabet {−7,−3,3,7} which is essentially a trellis coded 4-VSB symbol. The actual 4-level symbol is a valid trellis coded symbol that can be decoded by existing trellis decoders.  
         [0054]    Thus, according to the invention, packets are formatted such that only the information is placed at the bit location suitable for processing by the trellis encoder. For robust streams, the information bit need only be placed in the robust byte at the desirable bit position for robust trellis encoding and symbol mapping. With greater particularity, at the MPEG packet level, for each robust packet carrying information, two packets are generated: one being the information carrier packet, and the other functioning as a placeholder packet. In the packet formatter  330 , only the information carrier packet (not the placeholder packet) is processed. Particularly as shown in FIG. 6, the packet formatter generates two robust bytes (packets)  332   a,    332   b  for each byte  331  of each packet received from the robust stream. The packet formatter  330  will generate two identical bits, e.g., bits  333 ,  334  corresponding to each information bit  335  of each input byte processed. That is, every two bits  333 ,  334  of each byte  332   a,    332   b  corresponds to the information carrying bits  335  for input to the trellis encoder as the X1 and X2 bits, e.g., when pseudo 2-VSB mapping is employed (Z2=Z1) as shown in FIG. 5( b ). Thus, the robust packet formatter ensures that the information bits is provided at the desired bit position X1, X2 for appropriate robust mapping at the trellis encoder  352  for forming the Z0-Z2 inputs in accordance with the desired robust symbol mapping scheme employed.  
         [0055]    Referring back to FIG. 3, if the “non-systematic” Reed-Solomon encoder  375  is used, then only 187 bytes will be created to carry 4*187 bits of the robust stream. The remaining 20 bytes will be determined after these 187 bytes are trellis coded in a fashion to obtain (pseudo) 2-VSB and 4-VSB symbols. In creating the 207 bytes, the 187 bytes containing the information stream and the other 20 bytes, the specific values of which are at this processing stage yet to be determined, are permuted in such a way that after the data interleaver  340 , these 20 bytes will appear at the end of the 187 bytes. At this new stream processing stage, the values of the 20 bytes can be set to any value. If, however, the “non-systematic” Reed-Solomon encoder  375  is not used, then all the LSBs of the 207 bytes will correspond to 207*4 bits from the incoming robust bit-stream. In this case, the 187-byte MPEG compliant packet will be transmitted using 828*2 symbols.  
         [0056]    Table 8 summarizes the packet formatter  330  functionality for different combinations of the MODE and the NRS parameters.  
                               TABLE 8                               Number of   Number of                   input   output       NRS   MODE   packets   packets   Functionality                   0   2, 3   1   2   Byte duplication       0   1   2   2   Rearrange bits       1   2, 3   4   9   Byte duplication,                       Insert “place holders”       1   1   8   9   Rearrange bits,                       Insert “place holders”                  
 
         [0057]    In view of Table 8, the packet formatter  330  comprises three functional units: a basic formatter unit, parity byte location calculator unit and ‘place holder’ inserter unit. For instance, when NRS=0, it transforms each robust information byte  331  into two bytes  332   a,    332   b.  This is depicted in FIG. 7( a ) whereby a robust information packet input is transformed into two packets when NRS=0. The packet formatter&#39;s functionality particularly depends on the MODE and NRS control parameters. If NRS=0, then the packet formatter basically performs the function of byte duplication or byte rearrangement, as depicted in FIG. 7( a ). However, if NRS=1 (i.e., non-systematic RS-encoding is employed for backwards compatibility at existing receiver devices) then, the packet formatter additionally inserts ‘place holders’ for the additional header and parity bytes. A more detailed discussion regarding the parity byte ‘place holder’ insertion mechanism is described in commonly-owned, co-pending U.S. patent application Ser. No. ______ [US010464/702790, Docket No. 15059], the whole contents and disclosure of which is incorporated by reference as if fully set forth herein.  
         [0058]    As mentioned, the basic packet formatter function duplicates the bytes of a packet, as now shown in FIG. 7( a ), if MODE=2 or 3 (4-VSB, pseudo 2-VSB), NRS=0 and, in FIG. 7( b ), for the case when MODE=2 or 3 (4-VSB, pseudo 2-VSB) and NRS=1. If MODE=1 (H-VSB modulation employed), the bits of the input packet are rearranged as shown in FIGS.  8 ( a ) and  8 ( b ).  
         [0059]    As shown in FIGS.  8 ( a ) and  8 ( b ), rearranging of bits is performed in H-VSB mode (MODE=1) to ensure that the ‘robust stream’ bits from a robust packet  338  always go into MSB bit positions and the ‘embedded stream’ bits from embedded packet  339  always go into LSB bit positions of the reformatted packets  336 ,  337 , respectively. FIGS.  8 ( a ) and  8 ( b ) particularly depicts the bit rearrangement process performed by the packet formatter for control parameters MODE=1 and NRS=0 (FIG. 8( a )) and, for MODE=1, NRS=1 (FIG. 8( b )) when non-systematic RS-encoding is employed for backwards compatibility.  
         [0060]    In sum, the input to the transmission subsystem from the transport subsystem is a 19.39 Mbps serial data stream comprising 188-byte MPEG compatible data packets. These MPEG packets are organized as groups of 312 packets to comprise a single MPEG field  400  as shown in FIG. 9( a ). Each packet is classified as belonging to either a standard or robust bit-stream depending on the control information (MODE, NRP and RPP). For instance, the parameters NRP and RPP determine which packets in the group of 312 packets (MPEG field) belong to the robust bit-stream. NRP as defined above in Table 6 determines the number of robust packets in an MPEG field, while RPP (Table 7) identifies the position of robust packets within that field. The MODE parameter is used by the trellis encoder for encoding robust packets. It should be understood that the above condition implies that the control parameters may be changed only every 312 packets (i.e. once for each MPEG field).  
         [0061]    Providing a processing example at the transmitter system  300 , if mode parameters NRP=“1011” and RPP=“00” and NRS=0, are received, then, from Table 6, it may be determined that there are 78 robust packets (39*2 after encoding) in an MPEG field, for this value of NRP. RPP=“00” indicates that these packets are distributed uniformly in an MPEG field starting with the first packet. The spacing between the robust packets is determined by the factor (312/78). So, for these set of parameters every fourth packet in an MPEG field starting from the first packet is a robust packet. It has to be noted that because of the additional processing done for robust packets, only some of them (50% for NRS=0), actually carry payload while the remaining robust packets are place-holders. FIG. 9( b ) illustrates an example MPEG field  402  for NRP=“1101” and RPP=“00”.  
         [0062]    In another processing example, with NRP=“1100” and RPP=“01”, from Table 6, it is determined that there are 52 robust packets in a group of 312 packets, for this value of NRP. With an RPP value of “01”, this indicates that these packets are distributed uniformly in an MPEG field with a granularity of two (2) starting with the first packet. The spacing between the robust packet pairs is determined by the factor (312/2*52). So, for this set of parameters two packets every six packets in an MPEG field  404  starting from the first packet are robust packets. FIG. 9( c ) illustrates an example MPEG field  404  for NRP=“1100” and RPP=“01”.  
         [0063]    As mentioned, in a concurrent processing path  304 , the MODE, NRP and RPP parameters associated with each group of 312 packets of a received MPEG field is implemented for robust packet identification. As such, the control parameters the MODE, NRP and RPP parameters may only be changed every 312 packets, i.e., one MPEG field. In system operation, the parameters are particularly input to a Generate ‘hd_sd_in’ processing block  315  which implements logic for generating control information at the packet level based on MODE, NRP and RPP parameters. As described, the output  325  of this block is a bit value (e.g., ‘1’) if the packet currently being processed belongs to the new robust stream (NS) or, is another bit value (e.g., ‘0’), if the packet received belongs to the standard stream (SS). More specifically, the output  325  of the Generate ‘hd_sd_in’ processing block  315  generates a bit for each byte present in each packet of the current MPEG field, e.g., 312 packets. Once the hd_sd generation block  315  identifies each packet, it will output a ‘1’ for each robust byte and a ‘0’ for each standard byte.  
         [0064]    Preferably, according to this scheme, the coding gain is obtained by using different trellis encoding schemes for bytes belonging to different bit-streams. However, as the bytes of the bit-streams are rearranged by the data interleaver  340  and trellis interleaver  350  (of FIG. 5( a )) corresponding tracking bits  325  generated by block  315  are accordingly rearranged by the convolutional bit interleaver  341  and by the trellis interleaver blocks  345  by the time they are encoded by the trellis coder  350 . The convolutional bit interleaver block  341  is similar in function to the convolutional byte interleaver  340  specified in the ATSC A/53 standard, except that the memory element is 1 bit instead of 1 byte. This block is used to track bytes through the convolutional interleaver  340 . That is, in a synchronized fashion, each interleaved byte output from the convolutional interleaver block  340  is tracked by the convolutional bit interleaver  341  so that the integrity of the tracking bits in the control path  304  that correspond to each of the bytes to be transmitted, is preserved.  
         [0065]    As mentioned, according to the ATSC standard, a further block, the trellis encoder  350  implements twelve identical trellis encoders employing intrasegment interleaving, thus further affecting the order of the symbols in the output stream. In order to continue identifying the bytes in the trellis encoder  350 , a trellis interleaver control block  345  is provided so that each input to each trellis encoder is tracked. Tracking of information bytes through the control path  304  results in the generation of a ‘td_hd_sd’ bit associated with each symbol which identifies the symbol at the trellis encoder  350 . Depending on this bit, the trellis encoder  350  uses either robust encoding or standard encoding in the manner as explained in greater detail herein. For example, the output ‘td_hd_sd’  355  of control block  345  is equal to 1 when the trellis encoder output symbol belongs to a new (robust) stream (NS) and, is equal to 0 when the output symbol belongs to the standard stream (SS). The trellis encoder uses this information during symbol mapping. More particularly, each ‘td_hd_sd’ output  355  corresponds to a symbol generated at each of the twelve trellis encoders. The trellis interleaver block  345  thus tracks the corresponding symbols (robust or standard) output of the trellis encoder, and not bytes as in the other control blocks of processing path  304 . It should be understood that, in view of FIG. 3, in accordance with the ‘td_hd_sd’ output  355  indicating symbols belonging to robust or normal (standard) packets, the trellis encoder  350  will map the symbols according to the associated modulation schemes.  
         [0066]    Referring back to FIG. 3, as the receiver needs MODE, NRS, NRP and RPP information in order for it to properly decode both the bit-streams, the parameters themselves have to be robustly encoded so that they can be decoded even in severe multi-path channels. The encode sync header block  360  performs this function and, after encoding, the encode sync header block  360  places the encoded code-word in a fixed location (reserved bits) in the Frame Sync segment  370 . These control parameters are extracted from the detected frame synch signal at the receiver device. The output of the trellis encoder  350 , and frame synch signal  370  including the encoded control parameters is then multiplexed by multiplexor unit  365  to form a multiplexed signal  380  which is subject to the pilot insertion and RF up-conversion (FIG. 1).  
         [0067]    [0067]FIG. 10 illustrates a block diagram of a novel ATSC receiver  500  capable of decoding both the standard and new (robust) bit-streams. The embodiment of the receiver  500  depicted in FIG. 10 exemplifies the case when non-systematic RS encoder is not used, i.e., the control parameter NRS=0. As in the transmission system of FIG. 3, the receiver device  500  is provided for decoding the two types of bit streams and, particularly employs an extensive control mechanism  550  to properly track the symbols (bytes) belonging to the two symbol streams. It also implements a packet formatter to reformat the new (robust) NS packets.  
         [0068]    As shown in FIG. 10, after carrier demodulation and received signal equalization  502  are performed, a sync detect block  505  detects the frame sync signal present in the received signal  370 ′ that includes the encoded control parameter information associated with the received packets. A Decode sync header block  510  is provided to decode the Frame Sync header information and extract the MODE, NRS, NRP and RPP control parameters. These parameters are then sent to a ‘Generate hd_sd_in’ block  515  and ‘Generate ps_hd_sd’ block  520 . Particularly, as shown in FIG. 10, the Generate ‘hd_sd_in’ block  515  generates control information at packet level based on MODE, NRP and RPP parameters. For example, the output of this block is equal to ‘1’ if the packet belongs to NS (new stream) and is equal to ‘0’ if the packet belongs to SS (standard stream). This block only starts when a back-end lock (not shown) is obtained. The Generate ‘ps_hd_sd’ block  520  is similar to the Generate ‘hd_sd_in’ block  515  except that it is synchronized with the de-interleaver output sync and start up signals when the de-interleaver output start signal (not shown) toggles high. The Convolutional bit interleaver block  520  is similar to convolutional byte interleaver specified in the ATSC standard, except that the memory element is 1 bit instead of 1 byte. This block  520  is used to track bytes through the convolutional de-interleaver  540 . Likewise, the Trellis interleaver block  525  implements the 12-symbol trellis interleaver. The output of this block ‘td_hd_sd’ signal  526  will be greater than 0 (e.g., 1 for H-VSB, 2 for 4-VSB or 3 for pseudo 2-VSB) when the trellis decoder input symbol (or equalizer output symbol)  390  belongs to NS and is equal to 0 when the trellis decoder input symbol  390  belongs to SS. Functionally, the blocks  515 ,  520  and  525  in the receiver are similar to the corresponding blocks  315 ,  341  and  345  in the transmitter. The equalizer  502  additionally uses this signal  526  to get a better estimate of the symbol and the trellis decoder  530  uses this signal in metric calculation. The Packet Formatter block  555  reformats the robust bit-stream packets. When NRS=0, it transforms two NS packets into one packet for input to the RS decoder block  560 .  
         [0069]    While there has been shown and described what is considered to be preferred embodiments of the invention, it will, of course, be understood that various modifications and changes in form or detail could readily be made without departing from the spirit of the invention. It is therefore intended that the invention be not limited to the exact forms described and illustrated, but should be constructed to cover all modifications that may fall within the scope of the appended claims.