Abstract:
This disclosure describes a flexible digital transmission system that improves upon the ATSC A/53 HDTV signal transmission standard. The system includes a digital television signal transmitter for generating a first Advanced Television Systems Committee (ATSC) standard 8-VSB bit stream and, for generating an encoded new bit stream capable of transmitting high priority information bits, wherein symbols of the new bit stream are capable of being transmitted according to a transmission mode selected from group comprising: a 2-VSB mode, a 4-VSB mode, and a hierarchical-VSB (H-VSB) transmission mode. Each respective 2-VSB, 4-VSB, and H-VSB mode is characterized as having symbols mapped according to possible symbol values from an alphabet comprising respectively, {−7, −5, 5, 7}, {7, 3, −3, −7}, and {7, 5, 3, −3, −5, −7}. 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.

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. 60/280,782 filed Apr. 2, 2001. This patent application is additionally related to commonly-owned, co-pending U.S. patent application Ser. No. ______ [Atty. Docket 702230] entitled APPARATUS AND METHOD FOR GENERATING ROBUST ATSC 8-VSB BIT STREAMS, 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 using the ATSC standard in a backward compatible manner.  
           [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 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 as shown in unit  134  (FIG. 3).  
           [0007]    As known, 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 from synchronization unit  150 . A pilot signal is then inserted by pilot insertion unit  160 . 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]    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.  
           [0009]    It would thus 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.  
           [0010]    It would further be highly desirable to provide in an ATSC digital transmission system, an improved technique for transmitting a new bit-stream along with the standard ATSC bit-stream wherein the new bit-stream has a lower Threshold of Visibility (TOV) compared to the ATSC stream, and consequently can be used for transmitting high priority information bits (robust bit-stream).  
           [0011]    It would further be highly desirable to provide a terrestrial broadcast system that supports a flexible payload data-rate in a 6-Mhz channel with a flexible Carrier-Noise Ratio (CNR).  
           [0012]    It would be further highly desirable to incorporate within the existing ATSC digital transmission standard an improved technique for transmitting a new bit-stream along with the standard ATSC bit-stream wherein the new bit-stream includes high priority information bits, and such that the transmission is backward compatible with existing digital television receiver devices.  
         SUMMARY OF THE INVENTION  
         [0013]    Accordingly, it is an object of the present invention 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.  
           [0014]    It is a further object of the present invention to provide in an ATSC digital transmission system, an improved technique for transmitting a new robust bit-stream along with the standard ATSC bit-stream wherein the new bit-stream has a lower Threshold of Visibility (TOV) compared to the ATSC stream, and consequently can be used for transmitting high priority information bits (robust bit-stream). That is, the robust bit-stream may be decodable at a lower TOV and in severe multi-path channel, thus enabling more receivers to receive the signal (effectively increasing the coverage area).  
           [0015]    It is another object of the present invention to provide a terrestrial broadcast system that supports a flexible payload data-rate in a 6-Mhz channel with a flexible CNR.  
           [0016]    It is yet another object of the present invention to incorporate within the existing ATSC digital transmission standard an improved technique for transmitting a new bit-stream along with the standard ATSC bit-stream wherein the new bit-stream includes high priority information bits, and such that the transmission is backward compatible with existing digital television receiver devices.  
           [0017]    It is still another object of the present invention to provide an 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 wherein symbols belonging to the standard stream and the new stream are multiplexed and transmitted over a terrestrial channel by the transmitter according to a broadcaster defined bit-rate ratio.  
           [0018]    In accordance with the preferred embodiments of the invention, there is provided a digital signal transmission system and methodology comprising: a means for generating a first Advanced Television Systems Committee (ATSC) standard 8-VSB bit stream; a means for generating a new bit stream capable of transmitting high priority information bits, said new bit stream and said standard 8-VSB bit stream capable of being simultaneous transmitted over a fixed bandwidth communications channel to a receiver device capable of receiving said standard and new bit stream, wherein the data rate of the standard bit stream is inversely related to a degree of robustness of information bits comprising the new bit-stream.  
           [0019]    In accordance with the preferred embodiments, the two bit-streams are multiplexed at the packet level. The new stream uses robust symbol mapping schemes to transmit one bit of information (robust) per symbol. Three such methods are described including: a pseudo 2-VSB, 4-VSB and hierarchical-VSB (H-VSB). In the pseudo 2-VSB and H-VSB modes, the sign of the symbol indicates the robust information bit. In the 4-VSB mode, the information bit is trellis coded using the existing trellis encoder.  
           [0020]    Transmission of a new bit-stream along with the standard ATSC bit-stream using pseudo 2-VSB, 4-VSB, and hierarchical VSB (H-VSB) symbol mapping techniques requires the mapping of symbols, respectively, from the alphabet [7, 5, −5, −7], [7, 3, −3, −7], and [7, 5, 3, −3, −5, −7].  
           [0021]    To insure backward compatibility with existing receivers from various manufacturers, an optional “non-systematic” Reed-Solomon encoder may be used 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 new bit-stream will be ignored by the transport layer decoder of the existing receiver. Thus, the effective payload that can be decodable by existing receivers is reduced due to the insertion of the new bit-stream. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    Details of the invention disclosed herein shall be described below, with the aid of the figures listed below, in which:  
         [0023]    [0023]FIG. 1 illustrates a block diagram of an exemplary high definition television (HDTV) transmitter;  
         [0024]    [0024]FIG. 2 is a simplified top-level diagram of the improved digital broadcast system  300  for pseudo 2-VSB and 4-VSB according to the present invention;  
         [0025]    [0025]FIG. 3 is a block diagram of one exemplary prior art trellis encoder and pre-coder unit (one of twelve such units shown in FIG. 2) and an eight (8) level symbol mapper; and,  
         [0026]    [0026]FIG. 4 illustrates a simplified diagram of the H-VSB system mode for HDTV  400  in accordance with the principles of the present invention.  
         [0027]    [0027]FIG. 5 is a simplified block diagram of the modified trellis encoder and modified mapping scheme for the hierarchical mode according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    Co-pending, commonly-owned U.S. patent application Ser. No. ______ [Atty Docket #702230; please insert if known] filed entitled APPARATUS AND METHOD FOR GENERATING ROBUST ATSC 8-VSB BIT STREAMS, 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 a more robust ATSC 8-VSB stream, i.e., a hierarchical VSB or H-VSB, by a digital transmitter. According to this reference, a new ATSC 8-VSB bit stream (“New Stream”) is provided in which each bit in a first portion of the bits (e.g., half of the bits) is robust (the “Robust Stream”). The error correcting capacity of bits in the Robust Stream is greater than the error correcting capacity of bits in a standard ATSC 8-VSB bit stream. The present invention herein described in accordance with FIG. 2 improves upon the digital transmission system described in herein incorporated, commonly-owned U.S. patent application Ser. No. ______ [Atty Docket #702230] by enabling flexible transmission rates for Robust and Standard streams for accommodating a large range of carrier-to-noise ratios and channel conditions.  
         [0029]    A representative functional diagram of the improved digital transmission system  300  for pseudo 2-VSB and 4-VSB according to the invention is now described with respect to FIG. 2. As shown in FIG. 2, the system  300  includes two packet stream paths: a first path  303  corresponding to receipt and processing of the existing ATSC standard bitstream  302  and a second path  306  corresponding to the new (robust) bitstream  307 . Preferably, the standard data and robust stream inputs  302 ,  307  to the system comprise MPEG compatible packets.  
         [0030]    All packets sent via the first path (i.e., the standard or 8-VSB stream  302 ) are sent using the existing 8-VSB coding scheme. Thus, as shown in FIG. 2, a regular ATSC 8-VSB bit stream (Standard Stream) is input to the Reed Solomon (RS) encoder unit  110 . The output  310  of the RS Encoder  110  is input to a packet multiplexer (MUX)  320 , which, as will be described, multiplexes both standard stream and robust packets from the respective first and second paths.  
         [0031]    As will be described, all packets sent via the robust path  306  are sent using the pseudo 2-VSB or 4-VSB coding scheme in a backward compatible manner. Particularly, the second path  306  includes a new stream processing block  350  including Reed-Solomon encoder device  330  and a packet formatter  340  for processing the input robust stream as will be described in greater detail herein.  
         [0032]    Generally, the Standard Stream from Reed Solomon (RS) encoder  110  and the New Stream  325  from the new stream processing block  350  are multiplexed on a packet basis in multiplexer (MUX)  320 . The multiplexed Standard Stream and New Stream pass through convolutional data interleaver  360  and then trellis encoded in trellis encoder unit  130 . Data interleaver  360  rearranges the bytes of the multiplexed Standard Stream and New Stream during the interleaving process. Trellis encoder  130  encodes bytes belonging to the multiplexed Standard Stream  310  and New Stream  325  and maps the symbols generated by the Standard Stream bytes and by the New Stream bytes in a trellis encode symbol mapper unit as will be described. Additionally, as shown in FIG. 2, there is provided a “non-systematic” Reed Solomon (RS) encoder unit  370  which is a device that is used to satisfy backward compatibility requirements in existing ATSC DTV receivers. An advantageous embodiment of Reed Solomon (RS) encoder  330  is set forth and described in U.S. patent application Ser. No. 09/781,486 entitled “System and Method for Sending Low Rate Data on a Packet Basis in an 8-VSB Standard Data Packet Stream” filed on Feb. 12, 2001. The disclosures of U.S. patent application Ser. No. 09/781,486 are hereby incorporated by reference in the present patent application as if fully set forth herein. Generally, according to the invention, the “non-systematic” RS encoder  370  ensures that the existing receivers will identify the robust stream packets as valid RS code-words, particularly, by enabling a packet identifier (PID) corresponding to the robust stream packets (for existing receivers) to comprise a Null packet header.  
         [0033]    The symbol mapping schemes acording to the invention are now described. Typical VSB receiver devices such as shown in FIG. 2 typically make use of the trellis decoder to aid channel equalization. Thus, it is imperative that the trellis decoder continues to function and decode symbols corresponding to the new stream with reasonable accuracy, so that the performance of the existing receiver won&#39;t deteriorate when the new robust stream is transmitted. The proposal uses the existing trellis encoding scheme without any change in the way bytes are read from the data interleaver  360  into the trellis encoder. There is also no change in encoding the packets corresponding to the standard stream.  
         [0034]    A detailed functional diagram of an exemplary prior art trellis encoder  130  is shown in FIG. 3. As shown in FIG. 3, the trellis encoder  133  and pre-coder unit  132  provide their outputs to an eight (8) level symbol mapper  134 . A multiplexer for coupling the trellis encoder 133 and pre-coder unit  132  to eight (8) level symbol mapper  134  is not shown in FIG. 3. According to the transmission scheme of the invention, each data symbol to be encoded comprises two bits, X 1  and X 2 . Bit X 2  is pre-coded by pre-coder  132  which comprises a one bit register  136  to derive pre-coded bit Y 2 . The Bit Y 2  is not altered further by trellis encoder  130  and is output as bit Z 2 -The other input bit, X 1 , does not pass through pre-coder  132  but does pass through trellis encoder  133  as bit Y 1 . Trellis encoder  133  encodes bit X 1  in accordance with a rate-½ convolutional code utilizing one bit data registers,  138 ,  139 . The result is output as bit Z 0  and bit Z 1 . Therefore, three bits (i.e., bit Z 0 , bit Z 1 , and bit Z 2 ) are output by trellis encoder  130  to eight (8) level symbol mapper 134. The eight (8) level symbol mapper  134  converts the three bits Z 0 , Z 1  and Z 2  to a value “R” in an eight (8) level constellation of permissible code values. The permissible code values for R are −7, −5, −3, −1, +1, +3, +5, and +7. These values correspond with the three bit combinations shown in eight (8) level symbol mapper  134 . Further details regarding the trellis-encoder unit  130  may be found in herein-referenced, commonly-owned, co-pending U.S. patent application Ser. No. ______ [Atty Docket #702230].  
         [0035]    According to the invention, for the robust stream  325 , the trellis encoder  130  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  130 , the information bits (i.e., LSBs bits ( 6 , 4 , 2 , 0 )) are placed on X1, and X2 is subsequently determined to obtain the particular symbol mapping scheme. 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 is then passed to the “non-systematic” Reed-Solomon encoder  370  of FIG. 2. As will be described in greater detail, the parity bytes of the “non-systematic” Reed-Solomon encoder  370  and the PID bytes (not shown) will however be encoded using the 8-VSB encoding scheme. The symbol mapping techniques for each mode are now described as follows:  
         [0036]    Pseudo 2-VSB Mode  
         [0037]    In view of FIG. 3, 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 )). The X2 is then calculated such that, when pre-coded, it results in Z2. This operation is nothing other than X2=X1+Y2d mod 2, where Y2d is the content of the register  136  of the pre-coder  132 . This operation, combined with the existing symbol mapping scheme, 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.  
         [0038]    4-VSB Mode  
         [0039]    In view of FIG. 3, the 4-VSB mode is obtained by making Z1 equal to the information bit. 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  136 . 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.  
         [0040]    Referring back to FIG. 2, the new stream processor block  350  is primarily composed of a Reed-Solomon encoder  330  and a packet formatter  340 . The Reed-Solomon encoder  330  adds parity bytes to the robust packets in the same manner as the Reed-Solomon encoder  110  of the standard stream. The incoming 187-byte packet (e.g., of an incoming MPEG-II packet) is appended with 20 parity bytes to result in 207 bytes. These bytes will then be post processed and sent using robust constellations. The packet formatter  340  essentially buffers and groups the incoming bit-stream into groups of 207 bytes. 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.  
         [0041]    Referring back to FIG. 2, if the “non-systematic” Reed-Solomon encoder is used, then only 187 bytes will be created in this way 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 special way 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, will be permuted in such a way that after the data interleaver, 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 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.  
         [0042]    In addition to the above, the packet formatter 340 may additionally add an optional PID, e.g., 3-bytes, such that the existing receivers will process packets containing the robust stream as null packets. This PID will typically be set to “null packet”. When is the PID bytes are inserted, the information bits of the robust stream will be included in 184 bytes only. This mode implies the use of the “non-systematic” Reed-Solomon encoder. This mode, incorporated for the purpose of improving backward compatibility, will reduce the effective data rate as 23 bytes (20 parity bytes and 3 PID bytes) have to be added per packet by the packet formatter.  
         [0043]    Referring again to FIG. 2, the present invention provides various options as to how the new packets will be processed by existing receivers. The first option is one for which the new packets will not be correctly decoded by the Reed-Solomon decoders of existing receivers. The second option is one for which the new packets will be decoded correctly by the Reed- Solomon decoders of existing receivers. However, the existing receivers will not be able to decode the information from these packets. This option is proposed to provide the flexibility to cover the widest possible set of the existing receivers from different manufacturers. The use of the additional (“non-systematic”) Reed-Solomon encoder to insure backward compatibility, however, reduces the total payload by an amount equal to the parity bytes. When all of the existing receivers are phased out, broadcasters may choose not to use this option, to increase the effective payload of the robust stream.  
         [0044]    The Reed-Solomon encoder defined in the existing ATSC A/53 standard appends parity bytes at the end of the 187-byte packet to yield a 207-byte codeword. This encoding scheme is known as a systematic code. However, Reed-Solomon codes need not be systematic. Given a particular application, the encoding may be performed in such a way that the parity bytes are placed in arbitrary positions in the total 207 available byte positions. The resulting codeword is a valid Reed-Solomon codeword from the systematic code family. A Reed-Solomon decoder does not need knowledge of the parity byte positions. Thus, an unmodified Reed-Solomon decoder that decodes the systematic code will additionally decode this code.  
         [0045]    In the encoding process, the “non-systematic” Reed-Solomon encoder collects all the 187 bytes corresponding to the robust stream. The trellis encoder produces these bytes as described herein. Given the positions of the parity bytes, the Reed-Solomon encoder then produces 20 parity bytes corresponding to this packet. The parity bytes will then be appropriately placed in the data interleaver at the positions corresponding to the last 20 bytes of the 207 byte packet.  
         [0046]    The hierarchical VSB (H-VSB) mode transmits 2-bits of information per symbol with different degrees of priority. In a similar manner to the DVB-T hierarchical mode, the sign of the symbols corresponds to the high-priority bit while the level within a symbol corresponds to the low-priority bit.  
         [0047]    In this H-VSB mode, the overall payload of the system is not significantly reduced, unlike for the pseudo 2-VSB and 4-VSB modes. The broadcaster will be able to transmit three bit-streams: one stream being the regular ATSC bit-stream (Standard Stream, SS) and the other two new bit-streams being (New Stream, NS, comprised of a “robust” stream (high priority bit-stream)) and an “embedded” stream (low priority bit-stream). A new receiver would be able to decode all the bit-streams while an existing receiver will be able to decode SS packets and dump NS packets. The broadcaster has the ability to transmit different services at different levels of robustness.  
         [0048]    [0048]FIG. 4 illustrates a simplified diagram of the H-VSB system mode of transmission  400 . As shown in FIG. 4, the two bit-streams SS  302  and NS  308 ,  309  are multiplexed on packet basis by the packet mux  320  at the input to the FEC block (not shown). The multiplexed bit-streams are then passed through the convolutional interleaver  360  before being encoded by a trellis encoder  380 . The hierarchical block  350  processes the two streams belonging to the high priority stream  308  and the embedded low priority stream  309 . Respective Reed-Solomon encoder devices  330 ,  331  each encodes packets corresponding to a respective stream  308 ,  309 . The packet formatter  340  then combines these streams such that the MSBs of a byte, (i.e., bits  7 , 5 , 3 , 1 ), correspond to the high-priority stream and the LSBs, bits ( 6 , 4 , 2 , 0 ), of a byte correspond to the embedded stream. In general, after Reed-Solomon encoding, two 207-byte composite packets are formed from two 187-byte MPEG compliant input packets.  
         [0049]    In this mode, the trellis encoder  380  encodes bytes belonging to the two bit-streams and maps the symbols generated by standard stream bytes and new stream bytes using the mapping table illustrated in FIG. 5 with a =0 and a =2, respectively. As the bit X2 may be derived from the sign information of the received symbol, increasing the distance between +1 and −1 levels improves the reliability of this bit at the decoder. Using this new system, high priority bits of the new stream may be sent to X2 input and low priority bits of NS may be sent to X1 input of the trellis encoder. In a similar manner as for pseudo 2-VSB, X2 may additionally be computed such that the output of the pre-coder, Z2, equals the high priority bit. Subsequently, the “non-systematic” Reed-Solomon encoder  370  may then be used to add 20 parity bytes. These operations ensure that, given adequate SNR, the new packets will not be flushed as error by the Reed-Solomon decoders of existing receivers.  
         [0050]    [0050]FIG. 5 is a simplified block diagram of the modified trellis encoder  380  and modified mapping scheme for the hierarchical mode according to the invention; further details regarding operation of the trellis-encoder may be found in herein-referenced, co-pending U.S. patent application Ser. No. ______ [Atty Docket #702230?]. Generally, as described in co-pending U.S. patent application Ser. No. ______ [Atty Docket #702230?], a control block (not shown) is provided to generate the appropriate control signals necessary for setting the value of α=0 in the symbol mapper unit, for example, when Standard Stream bytes are present in trellis encoder  380 ; and, setting the value of α=2 when New Stream bytes are present in the symbol mapper unit  134 ′. As shown in FIG. 5, if the value of a is set equal to one (1) then the R value for the bit combination zero one one (011) is minus two (−2) and the R value for the bit combination one zero zero (100) is plus two (+2). Because the X 2  bit can be derived from the sign information of the received symbol, increasing the distance between the −1 level and the +1 level improves the reliability of the X 2  bit at the decoder. When the value of a is set equal to zero (0) the distance between the −1 level and the +1 level is two (2) units. When the value of a is set equal to two (2) the distance between the −3 level and the +3 level is equal to six (6) units. In this alternate advantageous embodiment of the present invention, the Z 2  Z 1  Z 0  bits “010” and the Z 2  Z 1  Z 0  bits “011” both give the same R value of minus three (−3). This is not a problem because for the Robust Stream only the sign of the received symbol is needed. So for both “010” and “011” the sign is negative and the decoded bit X 2  is zero (0). Similarly, the Z 2  Z 1  Z 0  bits “100” and the Z 2  Z 1  Z 0  bits “101” both give the same R value of plus three (+3). For both “100” and “101” the sign is positive and the decoded bit X 1  is one (1). Using the apparatus and method of the present invention, the high priority bits in a New Stream (i.e., the Robust Stream) can be sent to the X 2  input of trellis encoder  630  and the low priority bits in a New Stream (i.e., the Embedded Stream) may be sent to the X 1  input of trellis encoder  380 . Packet multiplexer  320  ensures that the X 2  input of trellis encoder  380  receives the Robust Stream bits and that the X 1  input of trellis encoder  380  receives the Embedded Stream bits.  
         [0051]    In an alternative embodiment, the control information for setting the values of the a in the symbol mapper unit  134 ′ for pseudo 2-VSB, 4-VSB and H-VSB modes, is communicated at the packet level according to the method described in commonly-owned, co-pending U.S. Provisional Patent Application Ser. No. 60/295,616 [Atty. Docket 702430/010278P] filed Jun. 4, 2001 and entitled PACKET IDENTIFICATION MECHANISM AT THE TRANSMITTER AND THE RECEIVER FOR AN ENHANCED ATSC 8-VSB SYSTEM, the entire contents and disclosure of which is incorporated by reference as if fully set forth herein.  
         [0052]    An intended receiver, depending on its state (location, mobility, etc.) may decode the high priority information (robust stream) and if the SNR is sufficient may additionally decode the low priority information (standard and embedded streams). In an existing receiver, the new stream will cause some errors during the trellis decoding process. As a result of this, the receiver will use some of its error correcting capability to correct these errors. This will cause the TOV to increase by a small amount. Once all the packets are corrected by the FEC, the transport layer (or the MPEG decoder) will discard the packets belonging to the new stream NS and will only use packets belonging to the standard stream for source decoding.  
         [0053]    According to the invention, in the manner described in commonly-owned, co-pending U.S. patent application Ser. No. ______ [Atty. Docket 702430/010278 entitled PACKET IDENTIFICATION MECHANISM AT THE TRANSMITTER AND THE RECEIVER FOR AN ENHANCED ATSC 8-VSB SYSTEM, the transmitter specifies, on a data field basis (313 segments), the placement of the new robust packets within a field. To simplify receiver implementation, the first symbol of a specified segment in a field may correspond to the first symbol of the robust packet. The number and mode of the robust packets are specified in the reserved bits of the field sync segment. All the information needed to decode the robust packets are specified in the reserved bits. This information includes: 1) the Mode (i.e., pseudo 2-VSB, 4-VSB, H-VSB); 2) whether the optional “non-systematic” RS encoder is used; and, 3) the number of robust packets that start in the following field.  
         [0054]    The improved digital transmission system of the invention incorporates a built-in flexibility enabling adaptation to different types of channels, while at the same time providing flexibility in the overall payload distribution. It is capable of operating at low CNR in different channel impairment environment viz. Gaussian, Ricean and Rayleigh. The system may withstand strong (up to 0 dB), long delay static multi-path interference and also dynamic multi-path interference.  
         [0055]    The system further features a number of selectable parameters that accommodate a large range of carrier-to-noise ratios and channel conditions. It allows fixed, portable and/or mobile reception by trading off the data rates for robustness. This wide range of parameters allows the broadcasters to select a mode appropriate for current applications or for some anticipated future applications. For instance, a moderately robust mode (with a correspondingly lower data rate) is needed to ensure reliable portable reception with a simple set-top antenna. A less robust mode with a higher data rate can be used where the service can be received at relatively higher TOV. The less robust modes with larger payloads may be used for fixed reception while the robust modes with the smaller payloads may be used for mobile and portable reception.  
         [0056]    Table 1 presents representative payload (Mb/s) and performance parameters for different bit-rate ratios of the new stream to the standard 8-VSB stream. An intended representative receiver is assumed for all the performance parameters.  
                                                                                           TABLE 1                           Mix ratio   Mix with 2-VSB or 4-VSB   Mix with Hierarchical VSB            (New/8-   Robust   Standard       Robust-   Embedded   Standard           VSB).   stream   stream   Total   stream   stream   stream   Total                    100/0   8.7-9.6   —   8.7-9.6   8.7-9.6   8.7-9.6   —   17.4-19.3        50/50   4.3-4.8   9.6   13.9-14.4   4.3-4.8   4.3-4.8   9.6   18.2-19.3        20/80   1.7-1.9   15.4   17.1-7.3   1.7-1.9   1.7-1.9   15.4   18.8-19.3        10/90   0.9-1.0   17.3   18.2-18.3   0.9-1.0   0.9-1.0   17.3   19.1-19.3        0/100   —   19.3   19.3   —   —   19.3   19.3                  
 
         [0057]    [0057]                                                                             TABLE 2                                           8-VSB-   8-VSB-   8-VSB                       H-VSB:   with   with   with       Mix ratio   2-VSB   4-VSB   H-VSB: robust   embedded   2-VSB   4-VSB   H-VSB                                100/0   8.5   9.8   —   —   —   —   —        50/50   8.5   12.0   12.5   19.1   16.3   15.6   15.5        20/80   8.6   14.0   12.2   16.4   15.3   14.6   15.4        10/90   8.7   14.5   12.0   16.2   15.0   14.6   15.1        0/100   —   —   —   —   14.9   14.9   14.9                    
         [0058]    Advantageously, new receivers will be able to decode robust packets without errors even under severe static and dynamic multi-path interference environments at reduced CNR. As can be seen, the TOV (in additive white noise) is also reduced to as little as 8.5 dB. This significant improvement in performance is achieved with little performance penalty to packets encoded using the existing 8-VSB standard. Moreover, the system does not require a change in the power mask. The average power required to transmit bit-streams in different modes is almost equal to the existing levels.  
                                                                             TABLE 3                                           8-VSB   8-VSB   8-VSB                       H-VSB:   with   with   with       Mix ratio   2-VSB   4-VSB   H-VSB: robust   embedded   2-VSB   4-VSB   H-VSB                                100/0   11.4   15.2   17.0   —   —   —   —        50/50   10.5   14.6   16.7   20.3   20.5   20.1   —        20/80   12.3   18.5   17.0   —   19.0   18.7   —        10/90   —   —   —   —   —   —   —        0/100   —   —   —   —   19.1   19.1   19.1                  
 
         [0059]    [0059]                                                                             TABLE 4                                           8-VSB   8-VSB   8-VSB                       H-VSB:   with   with   with       Mix ratio   2-VSB   4-VSB   H-VSB: robust   embedded   2-VSB   4-VSB   H-VSB                                100/0   13.0   20.2   —   —   —   —   —        50/50   —   —   —   —   —   —   —        20/80   12.0   &gt;22   —   —   —   —   —        10/90   —   —   —   —   —   —   —        0/100   —   —   —   —   23   23   23                    
         [0060]    Exemplary pseudo 2-VSB and 4-VSB system modes of operation are now described. As depicted in Tables 1-4, two different bit-streams (standard bit-stream and robust bit-stream) may be transmitted using the same physical channel. As an example, according to Table 2, in pseudo 2-VSB mode, for a 20/80 (approx.) mix, robust bit-stream has a TOV at 8.6 dB while the standard bit-stream has a TOV at 15.3 dB (in AWGN environment). In 4-VSB mode, for 20/80 (approx.) mix, the robust bit-stream has a TOV at 14.0 dB while the standard bit-stream has a TOV at 14.6 dB (in AWGN environment). The performance of the standard bit-stream is affected due to the effective power reduction of the 8-VSB symbols in pseudo 2-VSB mode, while 4-VSB mode doesn&#39;t affect the standard bit-stream&#39;s performance significantly. It should be understood that any value of bit-rate ratio may be used, however, increasing the percentage of robust bit-stream will reduce the data rate of standard bit-stream. If the “non-systematic” RS encoder is used as in FIG. 2, then the system is backward compatible up to the transport layer, but the useable data rate of the robust stream is reduced. If the “non-systematic” RS encoder is not used, then the RS decoder in the existing receiver will set the error flag for the new packets. According to the 2/4-VSB systems, modifications to the ATSC FEC block include the provision of one pre-processor including an RS encoder and a packet formatter. Use of a “non-systematic” RS encoder within the FEC block is optional.  
         [0061]    The proposal also provides a flexible scheme that allows a trade-off to be made between the payload data rates sent using the two streams. For example, using pseudo 2-VSB mode, a 2 Mbps robust bit-stream can be transmitted together with a 15.3 Mb/s standard bit-stream. In this mode, the (Additive White Gausian Noise) AWGN CNR (TOV) for the robust bit-stream and the standard bit-stream will be 8.5 dB and 15.1 dB respectively.  
         [0062]    Exemplary H-VSB system mode of operation is now described. As depicted in Tables 1-4, two different bit-streams (standard bit-stream and robust bit-stream) may be transmitted using the same physical channel. However, the new stream in turn may comprise a robust stream and an embedded low-priority stream. The H-VSB mode of operation implements a hierarchical system in the sense that different bits have varying levels of error correcting capabilities. High priority bits of the new stream are more robust compared to the standard 8-VSB stream. The broadcaster may choose the mix of standard and new stream packets. The new stream packets may be time-multiplexed with the standard packets allowing attractive high/low priority payload tradeoffs. The total payload of the system is higher than that of 2/4-VSB systems. As in the 2/4-VSB systems, modifications to the ATSC FEC block: include the addition of one pre-processor comprising an RS encoder and a packet formatter. The “non-systematic” RS encoder within the FEC block is optional. Symbol levels are changed based on control information.  
         [0063]    Using H-VSB mode, a 2 Mbps robust bit-stream can be transmitted along with a 15.3 Mb/s standard bit-stream and a 2 Mb/s embedded stream. The TOV in this mode is about 12 dB and 15.4 dB for the robust bit-stream and standard bit-stream respectively.  
         [0064]    Due to various receiver implementations of different manufacturers, it is rather difficult to predict their behavior for the non standard bit-stream packets. Therefore, a flexible approach is proposed that will enable the broadcasters to adjust the system parameters to get maximum possible coverage. These include the use of an optional Reed-Solomon encoder to insure that new bit-stream packets will not be flagged as error packets by the Reed-Solomon decoder of existing receivers. The use of this optional encoder reduces the data rate of the robust bit-stream by 4*20 bits per packet, but might be useful during the initial transition period.  
         [0065]    The 2/4-VSB and H-VSB modes of the invention as described herein, may be supported by changes in a modem part of the system. Little change is assumed on the transport layer, however, some control will be needed at the transport layer level to make sure that the modem gets the needed packet rate for each stream path. This operation, in principle, is not different from known system operation at the transport layer level.  
         [0066]    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.