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The actual hardware realization of real-time digital filters has been so far carried out from the frequency-domain viewpoint based on the conventional transfer function method. However, when the digital system with multiple inputs and outputs is represented by the state variable method, then it becomes difficult to construct generally the digital filters by the transfer function method. This paper presents a direct realization of the real-time digital filter with n inputs and n outputs based on the state variable method. In this construction n unit delays are required, while n3 unit delays are required in the construction based on the transfer function method. Errors of the digital filter obtained thus are calculated on the basis of a noise model. As a result the filter realized on the basis of the state variable method is preferable in that it is simple in construction and noiseless in error. The actual hardware realization of the filter with two inputs and two outputs is given. As an example, the real-time digital filter is used as a simulator.