Audio decoder and method for transforming a digital audio signal from a first to a second frequency domain

A method in an audio decoder for transforming a digital audio signal from a first frequency domain to a second frequency domain, comprising:

• receiving subsequent frames of a digital audio signal being represented in a first frequency domain, the digital audio signal having a Nyquist frequency which is half of an original sampling rate of the digital audio signal,
• for each frame of the digital audio signal:
• identifying an upper limit of a frequency range of said frame of the digital audio signal by analyzing spectral contents of said frame of the digital audio signal, wherein the upper limit is determined as the highest frequency having a non-zero spectral content within said frame,
• if the upper limit of the frequency range is below the Nyquist frequency by more than a threshold amount, lowering the Nyquist frequency of said frame of the digital audio signal from its original value to a reduced value by removing spectral bands of said frame of the digital audio signal above the identified upper limit of the frequency range,
• transforming said frame of the digital audio signal from the first frequency domain to a second frequency domain via an intermediate time domain, wherein said frame of the digital audio signal has a sampling rate in the intermediate time domain which is reduced in relation to the original sampling rate by a sub-sampling factor defined by a ratio between the original value of the Nyquist frequency and the reduced value of the Nyquist frequency, and
• appending spectral bands to said frame of the digital audio signal in the second frequency domain above the reduced value of the Nyquist frequency so as to restore the Nyquist frequency to its original value.