| 摘要 |
In the standard format now adapted by FCC for a digital HDTV signal, the video data symbols are interleaved and trellis encoded in accordance with a 4-state trellis code and the interleaving is of sequences of every 12 successive symbols. At the receiver the trellis decoder is therefore projected to consist of 12 respective decoder stages for the 12 interleaved sequences, each decoder stage having a branch metric calculator unit (BMC), and add-compare-select (ACS) unit and a path memory unit (PMU). The present invention separates the path memory requirements for the 12 interleaved sequences from the requisite BMC and ACS functions, so that the latter two units can provide those functions for all of the 12 interleaved sequences. A single extended PMU provides for storage of pointers to possible predecessor states of the trellis code corresponding to a present state thereof, going back to a predetermined number (such as 16) of sequentially preceding received symbol values. The PMU also provides, in each of the storage stages, sequential storage elements corresponding to the sequential interleaved symbol values. This makes it possible for the PMU to be realized as a single integrated RAM by appropriate grouping of the trellis code states. Also, the ACS function is performed by two separate ACS units for two mutually independent groups of trellis code states.
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