Abstract: Audio signal bandwidth extension may be performed on a narrow bandwidth signal received from a remote source over the audio communication network. The narrow band signal bandwidth may be extended such that the bandwidth is greater than that of the audio communication network. The signal may be extended by synthesizing an audio signal having spectral values within an extended bandwidth from synthetic components. The synthetic components may be generated using parameters derived from original narrowband audio signal. The audio signal may be synthesized in the form of an excitation signal and vocal tract envelope. The excitation signal and vocal tract may be extended independently. In various embodiments, excitation components may be derived from constrained synthesis using a constraint filter with nulls in regions where the extension is desired.