Bandwidth Extension of Telephone Speech to Low Frequencies Using Sinusoidal Synthesis and a Gaussian Mixture Model

The quality of narrowband telephone speech is degraded by the limited audio bandwidth. This paper describes a method that extends the bandwidth of telephone speech to the frequency range 0-300 Hz. The method generates the lowest harmonics of voiced speech using sinusoidal synthesis. The energy in the extension band is estimated from spectral features using a Gaussian mixture model. The amplitudes and phases of the synthesized sinusoidal components are adjusted based on the amplitudes and phases of the narrowband input speech, which provides adaptivity to varying input bandwidth characteristics. The proposed method was evaluated with listening tests in combination with another bandwidth extension method for the frequency range 4-8 kHz. While the low-frequency bandwidth extension was not found to improve perceived quality, the method reduced dissimilarity with wideband speech.