Masking is provided for any block of a transmitted digital signal which includes error signals caused by noise in the transmission channel. The digital signal may be an analog signal which has been digitized. When an erroneous block is detected, a masking signal is developed which is substituted for at least the erroneous block. The masking signal starts with the final value of the last error-free block and decays exponentially from that value until no longer needed. The substitution will continue through the erroneous block and may continue until the value of the substitute signal approaches the concurrent value of the subsequent signal block or until the values of the two signals actually crossover, or until there is a crossover with the slopes of the two signals having the same direction, i.e., rising or falling.