Multimode speech encoder and decoder apparatuses

Details Multimode speech encoder and decoder apparatuses Multimode speech encoder and decoder apparatuses

2000-04-18

2001-12-25

Dorvil, Richemond (Department: 2641)

Data processing: speech signal processing, linguistics, language

Speech signal processing

Synthesis

C704S201000, C704S500000, C704S203000, C704S221000, C704S229000, C704S230000

Reexamination Certificate

active

06334105

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a low-bit-rate speech coding apparatus which performs coding on a speech signal to transmit, for example, in a mobile communication system, and more particularly, to a CELP (Code Excited Linear Prediction) type speech coding apparatus which separates the speech signal to vocal tract information and excitation information to represent.
BACKGROUND ART
Used in the fields of digital mobile communications and speech storage are speech coding apparatuses which compress speech information to encode with high efficiency for utilization of radio signals and recording media. Among them, the system based on a CELP (Code Excited Linear Prediction) system is carried into practice widely for the apparatuses operating at medium to low bit rates. The technology of the CELP is described in “Code-excited Linear Prediction (CELP):High-quality Speech at Very Low Bit Rates” by M. R. Schroeder and B. S. Atal, Proc. ICASSP-85, 24.1.1, pp.937-940, 1985.
In the CELP type speech coding system, speech signals are divided into predetermined frame lengths (about 5 ms to 50 ms), linear prediction of the speech signals is performed for each frame, the prediction residual (excitation vector signal) obtained by the linear prediction for each frame is encoded using an adaptive code vector and random code vector comprised of known waveforms. The adaptive code vector and random code vector are selected for use respectively from an adaptive codebook storing previously generated excitation vectors and a random codebook storing the predetermined number of pre-prepared vectors with predetermined shapes. Used as the random code vectors stored in the random codebook are, for example, random noise sequence vectors and vectors generated by arranging a few pulses at different positions.
The CELP coding apparatus performs the LPC synthesis and quantization, pitch search, random codebook search, and gain codebook search using input digital signals, and transmits the quantized LPC (L), pitch period (P), a random codebook index (S) and a gain codebook index (G) to a decoder.
However, the above-mentioned conventional speech coding apparatus needs to cope with voiced speeches, unvoiced speeches and background noises using a single type of random codebook, and therefore it is difficult to encode all the input signals with high quality.
DISCLOSURE OF INVENTION
An object of the present invention is to provide a multimode speech coding apparatus and speech decoding apparatus capable of providing excitation coding with multimode without newly transmitting mode information, in particular, performing judgment of speech region
on-speech region in addition to judgment of voiced region/unvoiced region, and further increasing the improvement of coding/decoding performance performed with the multimode.
In the present invention, the mode determination is performed using static/dynamic characteristics of a quantized parameter representing spectral characteristics, modes of various codebooks for use in coding excitation vectors are switched based on the mode determination indicating the speech region
on-speech region or voiced region/unvoiced region. Further, in the present invention, the modes of various codebooks for use in decoding are switched using the mode information used in the coding in decoding.


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