Algorithm Description

To alter the pitch in the voiced speech case, an LPC analysis and reconstruction method is used, Figure 1. The speech is first partitioned into frames consististing of a single pitch period starting at a zero crossing associated with glottal closure appended to a brief (2.0 msec. in the current study) ``overlap'' segment of prior signal context. The overlap segment is used during signal reconstruction for inter-frame smoothing to reduce transients. A low order (8'th order filter and 20.050 kHz speech in the present study) LPC model is fitted to the Hanning windowed, first differenced, pitch frame (including overlap interval) using the autocorrelation method. The residual error signal is then computed from the unwindowed pitch frame. The low-order LPC model extracts just the most prominent spectral characteristics of each frame, leaving the excitation (presumably) and fine spectral detail of each frame to be represented in the residual. Regardless of the model order, the initial part of each pitch period is reconstructed exactly from the RELP synthesis.

Figure 1: Block diagram of HYBRID-LPC pitch modifier for the case of voiced speech.

F0 modification is achieved by exciting the LPC model with a duration adjusted residual. The duration of the residual is adjusted by windowing, and for the case of decreased pitch, zero padding. The window applied to the residual is defined by:

where T is the desired output pitch period duration, or the full duration of the original (input) pitch period, whichever is smaller.

For increased F0 (T less than original pitch period duration), the windowed (and shortened) residual is used to excite the synthesis filter. To decrease F0, the windowed residual error is zero padded to the desired duration and then used to excite the synthesis filter.

The pitch in voiced speech is thus modified by altering the duration of the LPC model residual error and exciting the model with the altered residual. Additionally, the LPC parameters and residual are inter-frame smoothed through linear interpolation during the overlap interval prior to the nominal pitch period onset. This smoothing functions to decrease ringing and other artifacts at pitch period boundaries.