Multimode Speech Coding - Digital Speech: Coding for Low Bit Rate Communication Systems - page 278

distribute the bits among coder parameters in a given framewhilemaintaining
a constant bit rate, by classifying each frame into a certain mode, have also
been reported [12]. However, we particularly focus here on hybrid coders,
which combine AbS coding and harmonic coding. The advantages and
disadvantages of harmonic coding and CELP, and the potential benefits of
combining the two methods have been discussed by Trancoso et al . [13].
Improving the speech quality of the LPC vocoder by using a form of multi-
pulse excitation [14] as a third excitation model at the transitions has also
been reported [15].

9.3.1 PrototypeWaveformInterpolationCoder
Kleijn introduced prototype waveform interpolation (PWI) in order to
improve the quality of voiced speech [7]. The PWI technique extracts pro-
totype pitch cycle waveforms from the voiced speech at regular intervals of
20-30ms. Speech is reconstructed by interpolating the pitch cycles between
the update points. The PWI technique can be applied either directly to the
speech signal or to the LPC residual. Since the PWI technique is not suit-
able for encoding unvoiced speech segments, unvoiced speech is synthesized
using CELP. Even though the motivation behind using two coding techniques
is different in the PWI coder (i.e. waveform coding is not used for transitions),
it combines harmonic coding and AbS coding. The speech classification of
the PWI coder is relatively easier, since it only needs to classify speech into
either voiced or unvoiced.
At the onset of a voiced section, the previously estimated prototype wave-
form is not present at the decoder for the interpolation process. Kleijn suggests
three methods to solve this problem:

• Extract the prototype waveform from the reconstructed CELP waveform

of the previous frame.

• Set to a single pulse waveform (filtered through LPC) with its amplitude

determined from the transmitted information.

• Use a replica of the prototype transmitted at the end of the current synthesis

frame.

The starting phase of the pitch cycles at the onsets can be determined
at the decoder from the CELP encoded signal. At the offsets, the linear
phase deviation between the harmonically synthesized and original speech
is measured and the original speech buffer is displaced, such that the AbS
coder begins exactly where the harmonic coder ended.

9.3.2 CombinedHarmonicandWaveformCodingatLowBit-Rates
This coder, proposed by Shlomot et al ., consists of three modes: harmonic,
transition, and unvoiced [16, 17]. All the modes are based on the source filter