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 coders, with emphasis on time delay considerations. From the data, the importance of Asymmetric Windows in such context becomes evident. In order to get objective conclusions, we have used a perceptually weighted residual energy as the comparison criteria. It is shown that a reduction in delay of up to 8 ms can be obtained with no penalty in quality, simply with the correct choice of the window. Application to the military CELP standard, and considerations about the choice of asymmetric windows for other CELP implementations are also included.