An orthogonal search method for use in a CELP speech coding, by calculating a search reference value, the orthogonal search method comprising the steps of: generating an adaptive codevector representing a pitch component; generating a random codevector representing a random component; generating a synthetic speech signal by a synthesis filter being excited by the adaptive codevector and the random codevector; calculating a distortion between the input speech signal and the synthetic speech signal; and selecting one random codevector that minimizes the distortion by maximizing the search reference value, the following steps to calculate the search reference value, wherein the search reference value, E, is a ratio of N over D, N being the numerator; wherein the numerator is N = p t H t Hp x − x t Hp Hp Hc 2 where x is a target signal obtained by differentiating a zero input response of the synthesis filter from the input speech signal, the method is characterised by the steps of: computing the power, a = p t H t Hp, of a signal, Hp, obtained by synthesis in the synthesis filter using the adaptive codevector; computing an auto-correlation matrix, A = H t H, of filter coefficients of the synthesis filter; calculating a matrix, M, by providing a time reverse synthesis, r t = p t H t H, to the signal, Hp, obtained by synthesis in the synthesis filter using the adaptive codevector and by taking an outer product, M = rr t, of the resultant signal by the time reverse synthesis; calculating a value, D = c ( αA − t M) c, by using the power α, the auto-correlation matrix A, the matrix M and the random codevector c; and calculating the search reference value using D as the denominator, wherein p is the adaptive codevector, c is the random codevector, H is the synthesis filter coefficient matrix, t denotes transpose. generating an adaptive codevector representing a pitch component; generating a random codevector representing a random component; generating a synthetic speech signal by a synthesis filter being excited by the adaptive codevector and the random codevector; calculating a distortion between the input speech signal and the synthetic speech signal; and selecting one random codevector that minimizes the distortion by maximizing the search reference value, where x is a target signal obtained by differentiating a zero input response of the synthesis filter from the input speech signal, the method is characterised by the steps of: computing the power, a = p t H t Hp, of a signal, Hp, obtained by synthesis in the synthesis filter using the adaptive codevector; computing an auto-correlation matrix, A = H t H, of filter coefficients of the synthesis filter; calculating a matrix, M, by providing a time reverse synthesis, r t = p t H t H, to the signal, Hp, obtained by synthesis in the synthesis filter using the adaptive codevector and by taking an outer product, M = rr t, of the resultant signal by the time reverse synthesis; calculating a value, D = c ( αA − t M) c, by using the power α, the auto-correlation matrix A, the matrix M and the random codevector c; and calculating the search reference value using D as the denominator, wherein computing the power, a = p t H t Hp, of a signal, Hp, obtained by synthesis in the synthesis filter using the adaptive codevector; computing an auto-correlation matrix, A = H t H, of filter coefficients of the synthesis filter; calculating a matrix, M, by providing a time reverse synthesis, r t = p t H t H, to the signal, Hp, obtained by synthesis in the synthesis filter using the adaptive codevector and by taking an outer product, M = rr t, of the resultant signal by the time reverse synthesis; calculating a value, D = c ( αA − t M) c, by using the power α, the auto-correlation matrix A, the matrix M and the random codevector c; and calculating the search reference value using D as the denominator, p is the adaptive codevector, c is the random codevector, H is the synthesis filter coefficient matrix, t denotes transpose.