BANDWIDTH EXTENSION OF TELEPHONY SPEECH - Adaptive Signal Processing: Next Generation Solutions - page 369

Figure 7.11 The effect of nonlinear characteristics (half-way rectification).
(a) Harmonic (bandlimited) signal e nb (n). (b) Harmonic (extended) signal ˆ bb (n).
domain processing. Furthermore, the sine generators do not need an estimated value
for the fundamental frequency with every sample or frame but, for example, whenever
such an estimate is very reliable. 4
7.5.2 Vocal Tract Transfer Function Estimation
Besides the generation of the excitation signal ˆ bb ( n ) the wideband spectral envelope
needs to be estimated. Several classes of estimation techniques have been suggested.
† One of the simplest approaches is linear mapping. In this case the feature vector
containing the parameters of the bandlimited envelope is multiplied with a
4 If the pitch estimation is performed by searching the short-term autocorrelation of a signal for a maximum
in a predefined range [13] the reliability can be checked by comparing the maximum value with the short-
term power of the signal (the autocorrelation value at lag zero). If this ratio is close to one, the signal shows a
clear periodicity and, thus, the estimation can be classified as reliable.