A method for quantizing audio data, comprising: - indexing an input vector representing audio data contained in a set of vectors of a plurality of sets of vectors, wherein said sets of vectors are leader classes defining a lattice codebook, and wherein each leader class comprises a different leader vector and permutations of said leader vector, wherein said leader classes are signed, and wherein said indexing yields a position index representative of the positions of values in said input vector and a sign index representative of the signs of the components of said input vector, wherein said indexing comprises: - performing, in case that said input vector is contained in a set of vectors of a pre-defined group of one or more sets of vectors of said lattice codebook, a specific processing that is adapted to a characteristic of said sets of vectors in said pre-defined group of sets of vectors and is only applicable in case of input vectors contained in sets of vectors with said characteristic, wherein in said specific processing, said sign index is determined together with a count of the components of the leader vectors which have a component value less than zero in a loop, which loop comprises checking the sign of each vector component and computing an array p consisting of index values representing the position of the vector component in the input vector for each vector component having value different than zero, wherein said position index is determined based on the count, array p and the lattice dimension once the loop has been completed and wherein said characteristic to which said specific processing is adapted is that said pre-defined group of leader classes only contains leader classes with leader vectors the components of which have two different values with even components, and - performing, in case that said input vector is not contained in a set of vectors of said pre-defined group of sets of vectors of said lattice codebook, a general processing in which said position index and said sign index are determined one after the other. - indexing an input vector representing audio data contained in a set of vectors of a plurality of sets of vectors, wherein said sets of vectors are leader classes defining a lattice codebook, and wherein each leader class comprises a different leader vector and permutations of said leader vector, wherein said leader classes are signed, and wherein said indexing yields a position index representative of the positions of values in said input vector and a sign index representative of the signs of the components of said input vector, wherein said indexing comprises: - performing, in case that said input vector is contained in a set of vectors of a pre-defined group of one or more sets of vectors of said lattice codebook, a specific processing that is adapted to a characteristic of said sets of vectors in said pre-defined group of sets of vectors and is only applicable in case of input vectors contained in sets of vectors with said characteristic, wherein in said specific processing, said sign index is determined together with a count of the components of the leader vectors which have a component value less than zero in a loop, which loop comprises checking the sign of each vector component and computing an array p consisting of index values representing the position of the vector component in the input vector for each vector component having value different than zero, wherein said position index is determined based on the count, array p and the lattice dimension once the loop has been completed and wherein said characteristic to which said specific processing is adapted is that said pre-defined group of leader classes only contains leader classes with leader vectors the components of which have two different values with even components, and - performing, in case that said input vector is not contained in a set of vectors of said pre-defined group of sets of vectors of said lattice codebook, a general processing in which said position index and said sign index are determined one after the other.