Processing of n.t.s.c. color television signals

Abstract

Digital sampling of the luminance component of an N.T.S.C. color television signal is achieved by sampling with a mean frequency of twice the color subcarrier frequency to produce a sub-Nyquist sampled luminance signal. The sampling phase is changed between each pair of lines by an amount equal to half the interval between samples. The digital sampling can be applied to separate luminance and chrominance signals, to encoded N.T.S.C., and to the conversion of N.T.S.C. signals into signals of other form including PAL. Further, phase-perturbed sampling techniques permit the construction of an advantageous N.T.S.C. coder and de-coder system. The coder operates to cause spectrum folding of a luminance input signal by multiplying by a phase perturbed signal of twice the color subcarrier, and adding, the resultant being comb filtered to select frequencies which are multiples of the line frequency. The chrominance signals I, Q are combined into I.+-.Q and are modulated onto phase-perturbed subcarrier, the resultant being comb filtered to select frequencies which are of integral multiples of half the line frequencies. The two signals are then added. The comb filters mentioned can be based on one line delays or on odd multiples of one line, particularly 263 or 525 lines. A converse decoder filters then spectrum folds the luminance component, and filters and multiplies by subcarrier to derive the chrominance. Once an N.T.S.C. signal has been encoded by use of the encoder, subsequent decoding and recoding produces no theoretical impairment into the signal.