Outline of the Algorithms
HVXC
A basic blockdiagram of HVXC is depicted in Fig. 8. HVXC first performs LP analysis to find the LP coefficients.
Quantized LP coefficients are supplied to the inverse LP filter to find the prediction error. The prediction
error is transformed into a frequency domain and the pitch and the envelope of the spectrum are analyzed. The
envelope is quantized by weighted vector quantization in voiced sections. In unvoiced sections, closed-loop
search of an excitation vector is carried out.
Figure 8: HVXC
CELP 
Fig. 9 shows a blockdiagram of CELP. The LP coefficients of the input signal
are first analyzed and then quantized to be used in an LP synthesis filter
driven by the output of the excitation codebooks. Encoding is performed in
two steps. Long-term prediction coefficients are calculated in the first step.
In the second step, a perceptually weighted error between the input signal
and the output of the LP synthesis filter is minimized. This minimization
is achieved by searching for an appropriate codevector for the excitation
codebooks. Quantized coefficients, as well as indexes to the codevectors of
the excitation codebooks and the long-term prediction coefficients, form the
bitstream. The LP coefficients are quantized by vector quantization and the
excitation can be either MPE [13] or regularpulse
excitation RPE [14].
Figure 9: CELP
MPE and RPE both model the excitation signal by multiple pulses, however,
a difference exists in the degrees of freedom for pulse positions. MPE allows
more freedom on the interpulse distance than RPE which has a fixed interpulse
distance. Thanks to such a exible interpulse distance, MPE achieves better
coding quality than RPE [15]. On the other hand,
RPE requires less computations than MPE by trading off its coding quality.
Such a low computational requirement is useful in the wideband coding where
the total computation should naturally be higher than in the narrowband coding.
The excitation signal types of MPEG-4/CELP are summarized in Tab. III.
| Excitation | Bandwith | Features |
|---|---|---|
| MPE | Narrow, Wide | Quality, Scalability |
| RPE | Wide | Complexity |