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, one representing the real part, the other the imaginary part of a complex number, thereby defining at least one input vector, achieving filtering on said at least one input vector by passing through a Fourier space, thereby generating at least one complex output vector, the real portion and the imaginary portion of said at least one output vector representing one of said Q output signals respectively, and wherein said at least one output vector 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 at least one output vector, and wherein: 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 ⁡ ( t ) e 2 ⁡ ( t ) ) wherein is the convolution operator, e0(t) is said input vector such that 
 e 0( t )= e 1 ( t )+ j·e 2 ( t ), s0(t) is said output vector such that s0(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(e0(t)), S(f)=FFT(s0(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.