Speech enhancement using STC-based bandwidth extension

Telephone speech is typically bandlimited to 4 kHz, resulting in a ‘muffled’ quality. Coding speech with bandwidth greater than 4 kHz reduces this distortion, but requires a higher bit rate to avoid other types of distortion. An alternative to coding wider bandwidth speech is to exploit correlation between the 0-4 kHz and 4-8 kHz speech bands to resynthesize wideband speech from narrowband speech. This paper presents a method for re-synthesizing narrowband coded speech using sinusoidal transform coding (STC), modified codebook mapping and a novel method for the synthesis of highband unvoiced components. Informal listening test results indicate that this method produces a significant quality improvement in speech which has been coded using narrowband standards.