Low bit-rate pattern encoding and decoding with a reduced number of excitation pulses

• US 4,945,565 A
• Filed: 07/05/1985
• Issued: 07/31/1990
• Est. Priority Date: 07/05/1984
• Status: Expired due to Term

• Current Assignee: NEC Corporation
• Original Assignee: NEC Corporation
• Inventors: Araseki, Takashi, Ozawa, Kazunori
• Primary Examiner(s): NOT, DEFINED
• Assistant Examiner(s): NOT, DEFINED
• Application Number:
  US06/751,818
• Time in Patent Office: 1,852 Days
• Field of Search: 381/36-41, 381/29-35, 381/51-53, 381/49, 364/513.5
• US Class Current: 704/223
• CPC Class Codes: G10L 19/10 the excitation function being a multipulse excitation

Abstract

In an encoder operable in response to a discrete pattern signal divisible into a succession of segments to produce an output code sequence, a pitch parameter and a spectral parameter are extracted in a parameter calculator from each segment and from a spectral interval. In an excitation pulse producing circuit, each spectral interval is divided into a plurality of subframes, namely, pitch periods with reference to the pitch parameter to divide each segment. A minor group of excitation pulses is calculated from the segment at every subframe to form a major group of the excitation pulses in the spectral interval. The excitation pulses of the major group are reduced in number with reference to adjacent ones of the minor groups in each spectral interval and are modified into a succession of modified excitation pulses. The modified excitation pulses are combined with the spectral parameter into the output code sequence. In a decoder, the modified excitation pulses and the spectral parameter are extracted from the output code sequence. The pitch parameter is recovered by the use of the extracted and mofified excitation pulses and is used to produce a reproduction of the discrete pattern signal. Alternatively, the pitch parameter may be sent from the encoder together with the spectral parameter and the modified excitation pulses as the output code sequence and extracted from the output code sequence in the decoder.

Claims

1. A method of encoding a discrete pattern signal into an output code sequence and of decoding said output code sequence into a reproduction of said discrete pattern signal, said discrete pattern signal including pitch pulses and being composed of a succession of segments, said method comprising the steps of:

extracting, from said discrete pattern signal, a pitch parameter representative of a pitch period of said pitch pulses and a spectral parameter specifying short time spectrum envelope characteristics of said discrete pattern signal;

dividing each of said segments into a succession of subframes each of which has a length equal to the pitch period determined by the pitch parameter;

calculating excitation pulses for a first subframe;

calculating excitation pulses for a second subframe following said first subframe;

calculating first and second signal-to-noise ratios for said first and second subframes, respectively;

determining a ratio R of said second signal-to-noise ratio to said first signal-to-noise ratio;

comparing the ratio R to a predetermined threshold value Th;

generating a repeat signal for the second subframe when the ratio R is not greater than the threshold value Th, so as to repeat the excitation pluses of the first subframe for the second subframe, and otherwise generating modified excitation pulses calculated from the first and second subframes, the excitation pulses of the first subframe and the modified excitation pulses being produced as practical excitation pulses;

producing said output code sequence which is obtained by encoding said spectral parameter, said repeat signal, and the practical excitation pulses;

separating said output code sequence into the spectral parameter, the practical excitation pulses, and the repeat signal;

decoding the practical excitation pulses for said at least one subframe in said subframes within each of said segments to produce decoded excitation pulses when the practical excitation pulses are given and to produce reconstructed excitation pulses by the use of said repeat signal and said decoded excitation pulses when said repeat signal is given; and

producing a reconstructed discrete pattern signal for each of said segments by the use of said decoded and said reconstructed excitation pulses and said spectral parameter.

2. A method as claimed in claim 1, wherein said reconstructed discrete pattern signal producing step comprises the steps of:

extracting a reproduction of said pitch parameter from said decoded excitation pulses; and

using said reproduction of the pitch parameter to divide said segment into said subframes and produce said reconstructed discrete pattern signal by repeating said decoded excitation pulses as said reconstructed excitation pulses in the subframes within each of said segments when repetition of said decoded excitation pulses is indicated by said repeat signal.

3. An encoder for encoding a discrete pattern signal into an output code sequence, said discrete pattern signal including pitch pulses and being composed of a succession of segments, said encoder comprising:

extracting means for extracting, from said discrete pattern signal, a pitch parameter representative of a pitch period of said pitch pulses in each of said segments of said discrete pattern signal and a spectral parameter specifying short time spectrum envelope characteristics of said discrete pattern signal;

calculating means for successively calculating excitation pulses for a first subframe and excitation pulses for a second subframe following said first subframe;

calculating means for calculating first and second signal-to-noise ratios for the first and second subframes, respectively;

determining means for determining a ratio R of said second signal-to-noise ratio to said first signal-to-noise ratio;

comparing means for comparing the ratio R to a predetermined threshold value Th;

generating means for generating a repeat signal for the second subframe when the ratio, R, is not greater than the threshold, Th, so as to repeat the excitation pulses of the first subframe for the second frame, and otherwise generating modified excitation pulses calculated from the first and second subframes, the excitation pulses of the first subframe and the modified excitation pulses being produced as practical excitation pulses which are specified by amplitudes and locations;

calculating means for calculating said amplitudes and said locations of the practical excitation pulses; and

signal producing means for combining said amplitudes and said locations of the excitation pulses and said spectral parameter to produce said output code sequence.

4. An encoder as claimed in claim 3, wherein said signal combining means includes means for combining said pitch parameter and said repeat signal with said amplitudes and said locations of the excitation pulses to produce said output code sequence.

5. A decoder for decoding an encoded discrete pattern signal in the form of an output code sequence which includes amplitudes and locations of excitation pulses, a repeat signal, a pitch parameter and a spectral parameter of each segment of said encoded discrete pattern signal, said repeat signal being produced in consideration of signal-to-noise ratios between two adjacent subframes obtained by dividing each segment, said decoder for decoding said output code sequence into a reproduction of said discrete pattern signal, said decoder comprising:

separating means for separating said output code sequence into said spectral parameter, said repeat signal, and the amplitudes and locations of said excitation pulses; and

producing means for producing said reproduction of said encoded discrete pattern signal by the use of said spectral parameter, said pitch parameter, said repeat signal, and the amplitudes and locations of said excitation pulses.

6. A decoder as claimed in claim 5, wherein said producing means comprises:

first local decoding means for decoding the amplitudes and locations of said excitation pulses, said pitch parameter, said repeat signal, and said spectral parameter, and

second local decoding means for decoding said excitation pulses into the reproduction of said encoded discrete pattern signal by dividing each of said segments into subframes each of which has a length equal to a pitch period determined by said pitch parameter, by generating the excitation pulses for the at least one subframe in each of said segments, and by repeating the excitation pulses in other subframes except said at least one subframe within each of said segments as a reconstructed excitation signal when said repeat signal indicates repetition of the excitation pulses.

7. A decoder as claimed in claim 6, wherein said producing means includes means for producing said reproduction of said discrete pattern signal by the use of said spectral parameter and said reconstructed excitation signal.