A method for quantising a vector of line spectral frequency coefficients for an audio frame subsequent to a first audio frame comprising: quantising a vector of line spectral frequency coefficients of the first frame using a predictive mode of operation of a vector quantiser by referring to a past quantised vector of line spectral frequency coefficients, wherein the vector quantiser can operate in either a predictive mode of operation or a non-predictive mode of operation; determining a vector of recovered line spectral frequency coefficients which correspond to the vector of line spectral frequency coefficients of the first frame when subjected to a frame erasure event, wherein the vector of the plurality of recovered line spectral frequency coefficients is generated by a frame error concealment process, wherein the frame error concealment process is equivalent to a frame error concealment process used in a decoder; determining a distortion measure between the vector of line spectral frequency coefficients of the first frame and the vector of recovered line spectral frequency coefficients; comparing the determined distortion measure to a pre-determined threshold value, wherein the method is characterised by: determining the mode of operation of the vector quantiser for a vector of line spectral frequency coefficients for a subsequent frame of audio samples, wherein the mode of operation is dependent on the comparison such that when the determined distortion measure is greater than the pre-determined threshold value the non-predictive mode of operation is selected, and quantising the subsequent audio frame according to the determined mode of operation. quantising a vector of line spectral frequency coefficients of the first frame using a predictive mode of operation of a vector quantiser by referring to a past quantised vector of line spectral frequency coefficients, wherein the vector quantiser can operate in either a predictive mode of operation or a non-predictive mode of operation; determining a vector of recovered line spectral frequency coefficients which correspond to the vector of line spectral frequency coefficients of the first frame when subjected to a frame erasure event, wherein the vector of the plurality of recovered line spectral frequency coefficients is generated by a frame error concealment process, wherein the frame error concealment process is equivalent to a frame error concealment process used in a decoder; determining a distortion measure between the vector of line spectral frequency coefficients of the first frame and the vector of recovered line spectral frequency coefficients; comparing the determined distortion measure to a pre-determined threshold value, wherein the method is characterised by: determining the mode of operation of the vector quantiser for a vector of line spectral frequency coefficients for a subsequent frame of audio samples, wherein the mode of operation is dependent on the comparison such that when the determined distortion measure is greater than the pre-determined threshold value the non-predictive mode of operation is selected, and quantising the subsequent audio frame according to the determined mode of operation. determining the mode of operation of the vector quantiser for a vector of line spectral frequency coefficients for a subsequent frame of audio samples, wherein the mode of operation is dependent on the comparison such that when the determined distortion measure is greater than the pre-determined threshold value the non-predictive mode of operation is selected, and quantising the subsequent audio frame according to the determined mode of operation.