1. A method for processing a digital audio signal for achieving spatialization of sound, said method being applied by a digital signal processor (DSP) having libraries for calculating Fourier transforms from complex number space to complex number space, for digital processing of P input signals, P being an integer equal to 2, for filtering said P input signals by the convolution of sampled fast Fourier transforms (FFT), thereby obtaining Q output signals, Q being an integer equal to 2, wherein the method comprises at least the following steps: grouping said P input signals by twos, one representing the real part, the other the imaginary part of a complex number, thereby defining one or more input vectors, achieving filtering on said input vector(s) by passing through the Fourier space, thereby generating one or more complex output vectors, the real portion and the imaginary portion of said or each of said output vectors representing one of said Q output signals respectively, and wherein said or each of said output vectors is obtained in the following way: a filtering operator is applied on said input vector thereby giving a result, the result is subject to an inverse fast Fourier transform, thereby obtaining said output vector, and wherein: P=Q=2, e 1 (t) and e 2 (t) are said input P input signals, s 1 (t) and s 2 (t) are said Q output signals, a(t), b(t), c(t) and d(t) are filters defined such that: ( s 1 ( t ) s 2 ( t ) ) = ( a ( t ) d ( t ) b ( t ) c ( t ) ) ⊗ ( e 1 e 2 ) wherein is the convolution operator. eO(t) is said input vector such that eO(t)=e 1 (t)+j·e 2 (t), sO(t) is said output vector such that sO(t)=s 1 (t)+j·s 2 (t), said filtering operator is such that: S = F + H 2 · E + F - H 2 · E ~ * wherein: * is the conjugate complex operator, is the operator for inverting indices and E(f)=FFT(eO(t)), S(f)=FFT(sO(t)), F(f)=FFT[a(t)+j·b(t)], H(f)=FFT[c(t)=j·d(t)], wherein FFT designates the fast Fourier transform operator.