Abstract:
A method and system for generating a compressed domain, digital signal suitable for transmission, the method including: providing a first digital data stream signal including a first plurality of data bits and a second digital data stream signal including a second plurality of data bits; determining a threshold value for the first digital data stream signal; identifying select ones of the first plurality of bits dependent upon the threshold value; and, substituting data bits from the second plurality for the select bits of the first plurality to generate a composite digital data stream signal; wherein the composite digital data stream signal is adapted to be received and decoded by receivers adapted to receive the first digital data stream signal and receivers adapted to receive the composite digital data stream signal.

Description:
FIELD OF INVENTION 
   The present invention relates to compressed domain transmission and reception methods and systems, and more particularly to an improved method for combining a second compressed digital data stream signal with a first compressed digital data stream signal without degrading transmission/reception performance of the system for the first compressed digital signal data stream signal. 
   BACKGROUND OF INVENTION 
   The Advanced Television Systems Committee Digital Television standard (ATSC DTV) describes a system design standard for providing high-quality audio, video and ancillary data transmission and reception using a single 6 MHz channel. An ATSC DTV compliant system can reliably deliver approximately 19 Mbits/sec over a 6 Mhz terrestrial (8 VSB) broadcasting channel and approximately 38 Mbits/sec over a 6 Mhz cable television (16 VSB) channel. 
   Predictably, as the popularity of DTV systems grows, so does the demand for more efficient overall use of the allocated bandwidth and increased reliability. Accordingly, there is a need to more efficiently utilize the overall allocated bandwidth of an ATSC DTV signal in an ATSC system. 
   SUMMARY OF INVENTION 
   A method and system for generating a compressed domain, digital signal suitable for transmission, the method including: providing a first digital data stream signal including a first plurality of data bits and a second digital data stream signal including a second plurality of data bits; determining a threshold value for the first digital data stream signal; identifying select ones of the first plurality of bits dependent upon the threshold value; and, substituting data bits from the second plurality for the select bits of the first plurality to generate a composite digital data stream signal; wherein the composite digital data stream signal is adapted to be received and decoded by receivers adapted to receive the first digital data stream signal and receivers adapted to receive the composite digital data stream signal. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
       FIG. 1  is a block diagram illustrating some basic components of a conventional ATSC transmitter and receiver pair; 
       FIG. 2  is a block diagram of a transmitter according to a preferred embodiment of the present invention with a conventional receiver and receiver according to a preferred embodiment of the present invention; and, 
       FIG. 3  illustrates a conventional convolutional encoder. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will be described as it relates to an 8VSB terrestrial broadcast mode ATSC DTV system. However, it should be understood the present invention is equally applicable to other types of compressed domain signal transmission/reception systems as well though. 
   Referring now to the Figures, like numerals identify like elements of the invention.  FIG. 1  is a partial block diagram of the basic components of a conventional ATSC transmitter  10  and receiver  20  pair used for transmitting and receiving a single digital data stream signal  90 . Input data stream signal  30  which including MPEG compatible 188 byte packets to be transmitted are processed to provide Forward Error Correction (FEC) using a Reed-Solomon (R/S) encoder  40 , an interleaver  60  and a convolutional coder  80 . More particularly, the signal  30  is fed to the R/S encoder  40  to provide signal  50  which includes 20 parity bytes for each 188 byte packet. The signal  50  is then fed into the interleaver  60  which outputs an interleaved signal  70  having the characteristics of {fraction (1/12)} interleaving of the bits. The interleaved signal  70  is then fed to the convolutional coder  80  which provides ⅔ rate trellis coding for the signal  70 , to generate the coded signal  90  as is conventionally understood. The coded signal  90  is formatted into data frames and synchronization information is added using conventional processes and elements (not shown). The formatted signal  90  is then transmitted for reception by receiver  20 , using in this embodiment the 8 VSB terrestrial broadcast mode, this process being well understood by those possessing ordinary skill in the art as it relates to the discussed ATSC DTV standard. The received signal  90  is processed by the trellis decoder  100 , the deinterleaver  120  and R/S decoder  140  to provide decoded signal  150  as is also conventionally understood. 
   Generally, the method of the present invention combines a second data stream with an ATSC data stream signal  90 , and then modulates and transmits the combined data stream. The second data stream has a bit rate lower than that of the ATSC data stream. According to a preferred embodiment of the invention, the second data stream can be up to a 2.152 Mbps data stream and can be combined with the ATSC data stream using 2VSB processing techniques without degrading the performance of the ATSC DTV system. Further, the second data stream can be used as a continuous training sequence or as a second, low bit rate content provider data stream such as is commonly used in data-casting stock information. 
   In order to ensure that the present system is backward compatible, it is important that when the second stream is combined with the first stream (the conventional ATSC stream), that the error rate of the first stream remains sufficiently low to satisfy appropriate requirements. In the case of an ATSC stream, an appropriate requirement being Threshold Of Visibility (TOV) for example. The Threshold of Visibility (TOV) for an ATSC signal has been established by subjective viewing measurements to be a segment error probability of 1.93×10 −4  at the R/S decoder  140  output. Hence, a receiver according to the present invention should be capable of decoding both the first and second streams, while a conventional receiver continues to be able to decode the first stream without the second stream interfering with overall system performance. 
   Assuming the first data stream (data stream  1 ) is a conventional ATSC stream for sake of explanation, it has a 19.28 Mbps data rate. Assuming also that the transmission channel is ideal, it has been discovered that at certain times, certain ones of the of the symbols to be transmitted from data stream  1  can be replaced with symbols from data stream  2  (the additional data stream) thereby forming a combined data stream. 
   In a first embodiment, bits from the data stream are replaced with bits from data stream  2  before R/S encoding (e.g. prior to processing by R/S encoder  40 ). In such a case, to satisfy the TOV requirements for the ATSC signal, the segment error rate at the output of the R/S decoder  140  (of the signal  150 ) and hence at the R/S decoder  140  input (of signal  130 ) has been experimentally determined to be approximately 1.93×10 −4 . Assuming that the R/S decoder  140  can decode  10  symbol errors per segment, the TOV is equivalent to a maximum SER at the RS decoder output of 
         max   ⁢           ⁢   SER     =         [       (       (       1     Seg   .   ER       -   1     )     ×   10     )     +     (     1   ×   11     )       ]     /     1     Seg   .   ER         ×   Length   ⁢           ⁢   of   ⁢           ⁢   Segment         
 
where it is assumed that the R/S decoder  140  can decode  10  symbol errors per segment and the Length of a segment is 828 data symbols. The maximum SER is 0.012. This means that 1 in 83 symbols from Data Stream  1  can be replaced by symbols from Data Stream  2  at the R/S encoder  40  input while keeping Data Stream  1  above the TOV for all ATSC receivers. Thus, the maximum data rate for Data Stream # 2  can be defined as R 2 =max SER×19.28 Mbps where 19.28 Mbps is the payload data rate before coding, resulting in a 0.23 Mbps 8-VSB signal as Data Stream  2 . This results in data stream  2  essentially being transmitted as a 0.23 Mbps 8VSB signal within the ATSC DTV signal  1 .
 
   According to a preferred embodiment of the present invention, first and second data streams are conventionally provided. Bits from the first data stream (an ATSC data stream) are replaced with bits from a second data stream after convolutional encoding. In such a case, to satisfy the TOV requirements for an ATSC system, the SER at the Trellis decoder  100  input (signal  90 ), and hence at the output of the convolutional coder  80 , has been found to be approximately 0.2. This means  1  of every 5 symbols can be an error, while still satisfying the TOV requirements. Therefore, 1 of every 5 symbols at the output of the convolutional coder  80  (signal  90 ) can be from the second data stream while still ensuring that conventional ATSC receivers continue to receive the first data stream at a rate which satisfies the TOV requirements. This results in the second data stream advantageously being approximately transmitted as a 6 Mbps 8VSB signal within the ATSC DTV signal (32.28 MHz/5). The invention will be further discussed as it relates to this preferred embodiment. 
   Referring now also to  FIG. 2 , therein is illustrated a transmitter  10 ′ according to the present invention, a conventional ATSC receiver  20  such as is illustrated in  FIG. 1 , and an ATSC receiver  20 ′ according to the present invention. The transmitter  10 ′ includes conventional R/S encoder  40 , interleaver  60  and convolutional encoder  80  serially coupled to provide FEC for data stream  1  (the standard ATSC stream). The transmitter  10 ′ further includes, according to a preferred form of the invention, a second R/S encoder  40 ′, a second interleaver  60 ′ and a second convolutional encoder  80 ′ serially coupled to provide FEC for data stream  2  (the additional, second data stream to be combined with the ATSC data stream). Therefore, data streams  1  and  2  each are separately coded and interleaved forming signals  150  and  150 ′ independent of one another. Signals  150  and  150 ′ are combined to form a composite data stream by replacing symbols from data stream  1  (signal  150 ) with symbols from data stream  2  (signal  150 ′) at selected intervals using switch  160  responsively to controller  170 . 
   Again for purposes of explanation, assuming the maximum transmission rate across an ideal channel is 10.76 MHz, this results in stream  2  being transmitted at a rate of 2.152 MHz (10.76/5). It should of course be understood that to adjust for a non-ideal channel, the rate at which symbols from data stream  1  are replaced by symbols from data stream  2  can be changed. For example, if an SER of 0.15 at the input of the trellis decoder  100  (signal  90 ) results from interference such as channel noise, 1 out of every 20 symbols of data stream  1  (signal  150 ) can be replaced with a symbol from data stream  2  (signal  150 ′) resulting in data stream  2  being transmitted at a rate of 0.538 MHz (10.76/20). 
   The receiver  20  of  FIG. 2  receives, decodes and deinterleaves the received signal  90 ′ conventionally, as the TOV for the ATSC DTV signal  1  has not been violated and the symbols inserted from data stream  2  are merely identified as errors in data stream  1  and corrected for. The receiver  20 ′ includes conventional trellis decoder  100 , deinterleaver  120  and R/S decoder  140  for conventionally decoding and deinterleaving data stream  1  which represents the ATSC DTV data stream. The receiver  20 ′ includes trellis decoder  100 ′, deinterleaver  120 ′ and R/S decoder  140 ′ for decoding and deinterleaving data stream  2  (the second data stream combined with the ATSC DTV data stream  1 ) according to the preferred embodiment. 
   Transmitter  10 ′ and receiver  20 ′ further preferably respectively include switches  160 ,  180  and controllers  170 ,  190 . The switches  160 ,  180  are respectively responsive to the controllers  170 ,  190  which are synchronized using conventional ATSC DTV methodology. The switch  160  is operable to receive the signals  150 ,  150 ′, and at predetermined intervals selectively switch there between to effectuate selective replacement of symbols from data stream  1  (signal  150 ) with symbols from data stream  2  (signal  150 ′). For example, if 1 of every 5 symbols from data stream  1  (signal  150 ) is to be replaced with a symbol from data stream  2  (signal  150 ′), the controller  170  causes the switch  160  to feed signal  150  as output signal  90 ′ for four symbols, and then switch to feed signal  150 ′ for one symbol, and then back to feed signal  150  for four more symbols, and so on. Analogously, the switch  180  of receiver  20 ′ receives the signal  90 ′ and selectively provides it to trellis decoders  100  and  100 ′ responsively to controller  190 . Again, assuming the 1 out of 5 example discussed, the first four symbols received are fed to trellis decoder  100 , the fifth symbol is fed to both trellis decoders  100  and  100 ′, then four more to trellis decoder  100 , and so on. The controllers  170 ,  190  include counters in a particularly preferred embodiment of the invention for respectively tracking the number of symbols passed, in order for appropriate action of the switches  160 ,  180  to be effected. This switching can be accomplished using conventional switches, or a software switching algorithm as is well known in the art. 
   The ATSC 8VSB system uses a ⅔ rate trellis code, where one input bit is encoded into two output bits, the other input bit is precoded, and 12 identical Trellis coders are utilized. Referring now also to  FIG. 3 , there is illustrated a block diagram of conventional ⅔ rate convolutional encoder suitable for use as convolutional coder  80 . The encoder  80  receives 2 bits X 1 , X 2  and provides three output bits Z 0 , Z 1  and Z 2 , where the delay D is  12  as is well understood. Assuming bits X 1  and X 2  are provided as part of data stream  1 , it has been found that the SER of the method according to the present invention is sensitive to bits Z 2  and Z 1  being replaced with bits from data stream  2 . However, it has also been found that replacing the Z 0  bit from data stream  1  signal  150  with the 2VSB (and hence 1 bit) data stream  2  signal  150 ′ did not reduce the SER of data stream  1 . Table 1 illustrates simulation results for data stream  1  having an 8VSB data rate of 10.76 MHz and data stream  2  having a 2VSB symbol rate of 2.152 MHz, and the identified bit of each fifth symbol of data stream  1  being replaced by a bit from data stream  2 . 
                               TABLE 1                       BIT   SER                           Z2   0.99           Z1   0.99           Z0   &lt;2 × 10 −4                          
These results clearly indicate that bits Z 2  and Z 1  are not well protected, but that bit Z 0  is. It has also been found desirable to insert the bit from data stream  2  every X number of symbols, where X is an odd number and not a factor of 12 (as the delay in the convolutional encoder is 12). For the identified ATSC example, it is also preferable that X be equal to or greater than 5 as has been discussed. These precautions ensure that the inserted data from data stream  2  (signal  150 ′) is spread over all 12 Trellis decoders and does not interfere with continued conventional reception and decoding of data stream  1  by receiver  20 .
 
   Therefore, according to a preferred form of the present invention, data stream  2  can be transmitted as a 2VSB signal in the Z 0  position at a bit rate of 2.125 Mbps without degrading the performance of the ATSC DTV system. Data from stream  2  is substituted into the Z 0  bit position of data stream  1  every fifth Z 0  bit, while maintaining the SER for the data stream  1  below the TOV. The Trellis decoder  100 , deinterleave  120  and R/S decoder  140  conventionally compensate for these errors in a conventional receiver  20 . 
   It should of course be understood that as the quality of the transmission channel deteriorates, the bit rate of data stream  2  can be reduced to maintain the SER of data stream  1  below the TOV and hence not interfere with conventional ATSC DTV system operation. Data stream  2  (signal  150 ′) can be used as a low bit-rate content provider such as in data-casting stock information for example in good transmission/reception situations, and used as a continuous training signal in poor reception conditions to improve overall reliability of the ATSC DTV system. 
   Although the invention has been described and pictured in a preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form, has been made only by way of example, and that numerous changes in the details of construction and combination and arrangement of parts may be made without departing from the spirit and scope of the invention as hereinafter claimed. It is intended that the patent shall cover by suitable expression in the appended claim, whatever features of patentable novelty exist in the invention disclosed.