Investigating the use of asymmetric windows in CELP vocoders

The behavior of several windows in relation to time delay and spectral estimation is analyzed in its application to CELP (code excited linear prediction) coders, with emphasis on time delay considerations. From the data, the importance of asymmetric windows in such a context becomes evident. In order to obtain objective conclusions, a perceptually weighted residual energy was used as the comparison criterion. Experimental data show that an appropriately chosen window can reduce the time delay by up to 50% of the analysis frame. Typical reductions in delay will be about 6-8 ms. It is observed that this delay reduction is obtained with no cost in complexity or speech quality, and that in many algorithms it can be implemented in the coder without affecting compatibility with previous versions of the same algorithm. As an example of a specific implementation, an asymmetric window has been introduced in the Federal Standard 1016 CELP algorithm (4.8 kbit/s CELP). It has been shown to reduce the implicit delay of that algorithm by 5.6 ms with no loss in quality.<>