A method comprising: filtering , at a speech encoder, an audio signal into a first group of sub-band signals (L1, L2, ..., LM) within a first frequency range and a second group of sub-band signals (H1, H2, ..., HN) within a second frequency range; generating a first residual signal of a first sub-band (H1) in the second group of sub-bands by performing linear prediction analysis; generating a second residual signal of a second sub-band (H2) in the second group of sub-bands by performing linear prediction analysis; combining the first group of sub-band signals to generate a low-band signal and quantizing the low-band signal to generate a low-band excitation signal; generating a harmonically extended signal based on the low-band excitation signal and a non linear processing function; generating a third group of sub-band signals (HE1, HE2, ..., HEN) based, at least in part, on the harmonically extended signal , wherein the third group of sub-bands correspond to the second group of sub-bands; and determining a first adjustment parameter for a first sub-band (HE1) signal in the third group of sub-band signals and a second adjustment parameter for a second sub-band signal (HE2) in the third group of sub-band signals, wherein the first adjustment parameter adjusts a gain to substantially match an energy of the first residual signal with an energy of the first sub-band signal (HE1) of the third group of sub-band signals, and wherein the second adjustment parameter adjusts a gain to substantially match an energy of the second residual signal with an energy of the second sub-band signal (HE2) of the third group of sub-band signals. filtering , at a speech encoder, an audio signal into a first group of sub-band signals (L1, L2, ..., LM) within a first frequency range and a second group of sub-band signals (H1, H2, ..., HN) within a second frequency range; generating a first residual signal of a first sub-band (H1) in the second group of sub-bands by performing linear prediction analysis; generating a second residual signal of a second sub-band (H2) in the second group of sub-bands by performing linear prediction analysis; combining the first group of sub-band signals to generate a low-band signal and quantizing the low-band signal to generate a low-band excitation signal; generating a harmonically extended signal based on the low-band excitation signal and a non linear processing function; generating a third group of sub-band signals (HE1, HE2, ..., HEN) based, at least in part, on the harmonically extended signal , wherein the third group of sub-bands correspond to the second group of sub-bands; and determining a first adjustment parameter for a first sub-band (HE1) signal in the third group of sub-band signals and a second adjustment parameter for a second sub-band signal (HE2) in the third group of sub-band signals, wherein the first adjustment parameter adjusts a gain to substantially match an energy of the first residual signal with an energy of the first sub-band signal (HE1) of the third group of sub-band signals, and wherein the second adjustment parameter adjusts a gain to substantially match an energy of the second residual signal with an energy of the second sub-band signal (HE2) of the third group of sub-band signals. <EOS>