A new expression of the variance of roundoff noise in 2-D separable denominator digital filters described by Roesser's local state-space model is presented. The covariance matrices and noise matrices necessary to analyze the variance of roundoff noise under the l//2 norm scaling are obtained as the solutions of Lyapunov equations. The synthesis problem of 2-D separable denominator digital filters with minimum roundoff noise is formulated under l//2 norm scaling. The synthesis method of 2-D minimum noise realizations under the l//2 norm scaling is proposed by applying the minimization technique of roundoff noise in the 1-D case. Furthermore, it is shown that 2-D minimum noise realizations are rotated and scaled balanced realizations. Using this property, 2-D minimum noise realizations are proved to be free of overflow oscillations under zero input conditions, if their second-order modes are distinct.
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