The invention describes a method and a system for generating training data (DT) for an automatic speech recogniser (2) for operating at a particular first sampling frequency (fH), comprising steps of deriving spectral characteristics (SL) from audio data (DL) sampled at a second frequency (fL) lower than the first sampling frequency (fH), extending the bandwidth of the spectral characteristics (SL) by retrieving bandwidth extending informationOBE) from a codebook (6), and processing the bandwidth extended spectral characteristics (SLE) to give the required training data (DT). Moreover a method and a system (5) for generating a codebook (6) for extending the bandwidth of spectral characteristics (SL) for audio data (DL) sampled at a second sampling frequency (fL) to spectral characteristics (SH) for a first sampling frequency (fH) higher than the second sampling frequency (fL) are described.