Methods and apparatus for preventing rounding errors when transform coefficients representing a motion picture signal are inversely transformed

Abstract From the Canadian patent

Mismatch error occur in the local decoder of a MPEG-standard compressor for a motion picture signal, and in an MPEG-standard expander for a compressed motion picture signal. This is because of the lack of a standard for rounding results of *.5 (* is an integer) obtained when a set of transform coefficients resulting from compressing the motion picture signal is inversely orthogonally transformed. The invention prevents mismatch errors by preprocessing the set of transform coefficients prior to the inverse othogonal transform (15). The transform coefficients in the set are summed (23A) and the parity (odd or even) of the sum is judged (21). When the parity of the sum is judged to be …