Digital Television at 45 Mbits/Sec

Abstract

An algorithm and a real-time hardware implementation are described for coding composite NTSC signal sampled at 4× color subcarrier into 45 Mbits/sec while preserving network quality. Adaptive differential pulse-code modulation and variable word-length coding are used to minimize the data source rate for the channel, requiring a buffer to smooth the data rate. Predictors and predictor-selection techniques are described. Variable word-length code sets were designed for each. Resultant data rates were used as a measure to determine the best predictor-selection technique for use. The coder switches between various modes of operation depending on buffer content or fullness. When the buffer fullness increases sufficiently, a mode with a coarser quantizer is used. For higher values, 4:3 and 2:1 subsampling is used. Tests and demonstrations are done on a combination hardware/software test-bed.