Patent
1992-09-10
1995-08-22
MacDonald, Allen R.
395 2, 395 271, 395 21, G10L 500
Patent
active
054448166
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a new technique for digitally encoding and decoding in particular but not exclusively speech signals in view of transmitting and synthesizing these speech signals.
2. Brief Description of the Prior Art
Efficient digital speech encoding techniques with good subjective quality/bit rate tradeoffs are increasingly in demand for numerous applications such as voice transmission over satellites, land mobile, digital radio or packed network, for voice storage, voice response and secure telephony.
One of the best prior art methods capable of achieving a good quality/bit rate tradeoff is the so called Code Excited Linear Prediction (CELP) technique. In accordance with this method, the speech signal is sampled and converted into successive blocks of a predetermined number of samples. Each block of samples is synthesized by filtering an appropriate innovation sequence from a codebook, scaled by a gain factor, through two filters having transfer functions varying in time. The first filter is a Long Term Predictor filter (LTP) modeling the pseudoperiodicity of speech, in particular due to pitch, while the second one is a Short Term Predictor filter (STP) modeling the spectral characteristics of the speech signal. The encoding procedure used to determine the parameters necessary to perform this synthesis is an analysis by synthesis technique. At the encoder end, the synthetic output is computer for all candidate innovation sequences from the codebook. The retained codeword is the one corresponding to the synthetic output which is closer to the original speech signal according to a perceptually weighted distortion measure.
The first proposed structured codebooks are called stochastic codebooks. They consist of an actual set of stored sequences of N random samples. More efficient stochastic codebooks propose derivation of a codeword by removing one or more elements from the beginning of the previous codeword and adding one or more new elements at the end thereof. More recently, stochastic codebooks based on linear combinations of a small set of stored basis vectors have greatly reduced the search complexity. Finally, some algebraic structures have also been proposed as excitation codebooks with efficient search procedures. However, the latter are designed for speed and they lack flexibility in constructing codebooks with good subjective quality characteristics.
OBJECTS OF THE INVENTION
The main object of the present invention is to combine an algebraic codebook and a filter with a transfer function varying in time, to produce a dynamic codebook offering both the speed and memory saving advantages of the above discussed structured codebooks while reducing the computation complexity of the Code Excited Linear Prediction (CELP) technique and enhancing the subjective quality of speech.
SUMMARY OF THE INVENTION
More specifically, in accordance with the present invention, there is provided a method of producing an excitation signal that can be used in synthesizing a sound signal, comprising the steps of generating a codeword signal in response to an index signal associated to this codeword signal, such signal generating step using an algebraic code to generate the codeword signal, and filtering the so generated codeword signal to produce the excitation signal.
Advantageously, the algebraic code is a sparse algebraic code.
The subject invention also relates to a dynamic codebook for producing an excitation signal that can be used in synthesizing a sound signal, comprising means for generating a codeword signal in response to an index signal associated to this codeword signal, which signal generating means using an algebraic code to generate the codeword signal, and means for filtering the so generated codeword signal to produce the excitation signal.
In accordance with a preferred embodiment of the dynamic codebook, the filtering means comprises a adaptive prefilter having a transfer function varying in time to shape the frequency charac
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Adoul Jean-Pierre
Laflamme Claude
Dorvil Richemond
MacDonald Allen R.
Universite de Sherbrooke
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