A method of determining transmit line spectral frequency vectors representing filter coefficients for a time-varying filter for encoding speech according to a source-filter model, whereby speech is modelled to comprise a source signal filtered by the time-varying filter, the method comprising: receiving a speech signal comprising successive frames; for each of a plurality of frames of the speech signal, deriving a first line spectral frequency vector (LSFopt n,0) for a first portion of the frame, and a second line spectral frequency vector (LSFopt n,1) for a second portion of the frame, wherein the first and second line spectral frequency vectors are target line spectral frequency vectors converted respectively from linear prediction coefficients for the first and second portions of the frame; and for each current one of the plurality of frames, determining one of the transmit line spectral frequency vectors (LSF n,1) associated with the second portion of the current frame and determining an interpolation factor (i) associated with the first portion of the current frame, based on the first and second line spectral frequency vectors (LSFopt n,0; LSFopt n,1), and on the transmit line spectral frequency vector for a preceding one of the frames (LSF n-1,1); wherein the determining of the transmit line spectral frequency vector and the interpolation factor for each current frame comprises minimizing a full frame residual energy of the current frame, the full frame residual energy consisting of a) a difference between the second line spectral frequency vector of the current frame (LSFopt n,1) and the transmit line spectral frequency vector of the current frame (LSF n,1), and b) a difference between the first line spectral frequency vector for the current frame (LSFopt n,0) and an interpolated line spectral frequency vector (LSF n,0), wherein the interpolated line spectral frequency vector (LSF n,0) is interpolated from the transmit line spectral frequency vectors for the preceding and current frames (LSF n-1,1, LSF n,1) based on the interpolation factor (i). receiving a speech signal comprising successive frames; for each of a plurality of frames of the speech signal, deriving a first line spectral frequency vector (LSFopt n,0) for a first portion of the frame, and a second line spectral frequency vector (LSFopt n,1) for a second portion of the frame, wherein the first and second line spectral frequency vectors are target line spectral frequency vectors converted respectively from linear prediction coefficients for the first and second portions of the frame; and for each current one of the plurality of frames, determining one of the transmit line spectral frequency vectors (LSF n,1) associated with the second portion of the current frame and determining an interpolation factor (i) associated with the first portion of the current frame, based on the first and second line spectral frequency vectors (LSFopt n,0; LSFopt n,1), and on the transmit line spectral frequency vector for a preceding one of the frames (LSF n-1,1); wherein the determining of the transmit line spectral frequency vector and the interpolation factor for each current frame comprises minimizing a full frame residual energy of the current frame, the full frame residual energy consisting of a) a difference between the second line spectral frequency vector of the current frame (LSFopt n,1) and the transmit line spectral frequency vector of the current frame (LSF n,1), and b) a difference between the first line spectral frequency vector for the current frame (LSFopt n,0) and an interpolated line spectral frequency vector (LSF n,0), wherein the interpolated line spectral frequency vector (LSF n,0) is interpolated from the transmit line spectral frequency vectors for the preceding and current frames (LSF n-1,1, LSF n,1) based on the interpolation factor (i).