The two-step noise reduction (TSNR) technique removes the annoying reverberation effect while maintaining the benefits of the decision-directed approach. However, classic short-time noise reduction techniques, including TSNR, introduce harmonic distortion in the enhanced speech. To overcome this problem, a method called harmonic regeneration noise reduction (HRNR) is implemented in order to refine the a priori SNR used to compute a spectral gain able to preserve the speech harmonics as proposed by Plapous et al.
("Improved Signal-to-Noise Ratio Estimation for Speech Enhancement", IEEE Transactions on ASLP, Vol. 14, Issue 6, pp. 2098 - 2108, Nov. 2006).