The standard in the United States for the transmission of digital television signals is known as 8 VSB data (ATSC Digital Television Standard A/53). This 8 VSB data has a constellation consisting of eight possible symbol levels. In a VSB system, the eight possible symbol levels are all in the same phase. In a QAM system, however, the symbols are transmitted in phase quadrature relationship.
The standard referred to above specifies the formatting and modulation of digital video and audio data. The transmitted data is in the form of symbols with each symbol representing two bits of data that are trellis encoded into three bits of trellis encoded data. Each three bits of trellis encoded data are mapped into a symbol having a corresponding one of eight levels. Reed/Solomon encoding and interleaving are also provided to increase the robustness of the transmitted information.
Auxiliary data (data other than digital video or audio data) are also permitted to be transmitted in a digital television channel. These data are formatted and modulated according to the standard in the same manner as video and audio data. Receivers made in accordance with the 8 VSB standard are able to read packet identifications (PIDs) which allow the receivers to differentiate between audio, video, and auxiliary data.
However, while the robustness of the transmitted digital television signals is sufficient for digital television reception, this robustness may not be sufficient for the transmission of auxiliary data, particularly where the auxiliary data are critical. Accordingly, one of the applications of the present invention is the transmission of auxiliary data in a VSB format with outer encoding for added robustness. The auxiliary data transmitted in accordance with the application of the present invention are referred to herein as robust VSB data (RVSB).