1. A method of encoding an audio signal, the method comprising: receiving, in an audio encoder, a time domain signal segment originating from the audio signal; performing, in the audio encoder, a frequency analysis of the time domain signal segment so as to obtain a frequency domain representation of the signal segment; searching an adaptive spectral code book of the audio encoder for an adaptive spectral code book vector which provides a first approximation of the frequency domain representation, the adaptive spectral code book comprising a plurality of adaptive spectral code book vectors; selecting the adaptive spectral code book vector providing a first approximation; generating a residual frequency representation from a difference between the frequency domain representation and the selected adaptive spectral code book vector; searching a fixed spectral code book of the audio encoder for a fixed spectral code book vector which provides an approximation of the residual frequency representation, the fixed spectral code book comprising a plurality of fixed spectral code book vectors; selecting the fixed spectral code book vector providing an approximation of the residual frequency representation; updating the adaptive spectral code book of the audio encoder by including a vector obtained as a linear combination of the selected fixed spectral code book vector and the selected adaptive spectral code book vector; and generating, in the audio encoder, a signal representation of the received time domain signal segment, the signal representation being indicative of an index referring to the selected adaptive spectral code book vector and an index referring to the selected fixed spectral code book vector, the signal representation to be conveyed to a decoder.