Speech coding apparatus using multimode coding

Claim 2

A speech coding apparatus as claimed in claim 1, characterized in that: said decision means (4) comprises means (101) for providing a threshold value based on said current error rate and said error correcting abilities of said m error correcting coders and for comparing said evaluated quality of each of said reproduced speech signals with said threshold value to thereby output comparison results; and said decision means selects said one of the m coders on the basis of said comparison results.

Claim 3

A speech coding apparatus as claimed in claim 1 or claim 2, characterized in that said multipiexed transmission signal has a fixed bit rate.

Claim 4

A speech coding apparatus coupled to a transmission channel, comprising m coders (1₁-1 m ), m being an integer greater than 1, for coding an input speech signal for every frame of said input speech signal to thereby generate coded speech signals having mutually different bit rates, characterized in that said speech coding apparatus comprises: m decoders (2₁-2 m ) respectively provided for said m coders and generating reproduced speech signals from said coded speech signals for every frame; (m - 1) error correcting coders (FEC1-FEC4) provided for (m - 1) coders from among said m coders and carrying out, for every frame, an error correction coding for said coded speech signals generated by said (m - 1) coders to thereby generate error correcting codes having mutually different error correcting abilities; evaluating means (4), coupled to said m coders and said m decoders, for evaluating a quality of each of said reproduced speech signals from said input speech signal and said reproduced speech signals and for outputting an evaluated quality of each of said reproduced speech signals, said quality of each of said reproduced speech signals being evaluated in a state having no transmission error; decision means (5), coupled to said evaluating means, for identifying one of said m coders which provides the reproduced speech signal having a smallest distortion on the basis of said evaluated quality of each of said reproduced speech signals, a current error rate of said transmission channel and error correcting abilities of said (m - 1) error correcting coders and for generating a coder identification number representative of said one of the m coders; and output means (6, 7), coupled to said m coders, said (m - 1) error correcting coders and said decision means, for outputting a multiplexed transmission signal including the coded speech signal generated by said one of the m coders identified by said decision means and including the error correcting code generated by a corresponding one of the (m - 1) error correcting coders when said one of the m coders identified by said decision means is one of (m - 1) coders associated with said (m - 1) error correcting coders.

Claim 5

A speech coding apparatus as claimed in claim 4, characterized in that: said decision means (4) comprises means (101) for providing a threshold value based on said current error rate and said error correcting abilities of said (m - 1) error correcting coders and for comparing said evaluated quality of each of said reproduced speech signals with said threshold value to thereby output comparison results; and said decision means selects said one of the m coders on the basis of said comparison results.

Claim 6

A speech coding apparatus as claimed in claim 4 or claim 5, characterized in that said multiplexed transmission signal has a fixed bit rate.

Claim 7

A speech coding apparatus coupled to a transmission channel, comprising: m coders (21-24), m being an integer greater than 1, for coding an input speech signal for every frame of said input speech signal to thereby generate coded speech signals having mutually different bit rates, each of said m coders having a linear predictor; and analyzing means (25) for carrying out a linear predictive coding (LPC) analysis for said input speech signal for every frame and for generating LPC parameters, characterized in that said speech coding apparatus comprises: (m - 1) error correcting coders (FEC1-FEC4) provided for (m - 1) coders from among said m coders and carrying out, for every frame, an error correction coding for said coded speech signals generated by said (m - 1) coders to thereby generate error correcting codes having mutually different error correcting abilities; evaluating means (26, 126), coupled to said analyzing means, for evaluating a quality of a reproduced speech signal reproduced by using current LPC parameters and previous LPC parameters and for outputting an evaluated quality obtained in a state having no transmission error, said evaluated quality showing a spectral distortion; decision means (5), coupled to said evaluating means, for identifying one of said m coders which provides a reproduced speech signal having a smallest distortion on the basis of said evaluated quality, a current error rate of said transmission channel and error correcting abilities of said (m - 1) error correcting coders and for generating a coder identification number representative of said one of the m coders; and output means (71-73, 6), coupled to said m coders, said (m - 1) error correcting coders and said decision means, for outputting a multiplexed transmission signal including the coded speech signal generated by said one of the m coders identified by said decision means and including the error correcting code generated by a corresponding one of the (m - 1) error correcting coders when said one of the m coders identified by said decision means is one of (m - 1) coders associated with said (m - 1) error correcting coders.

Claim 8

A speech coding apparatus as claimed in claim 7, characterized by further comprising memory means (27, 28) for storing said previous LPC parameters previously generated by said analyzing means and for sending said previous LPC parameters to (m - 1) coders from among said m coders associated with said (m - 1) error correcting coders, wherein said evaluating means (26) receives said current LPC parameters from one coder other than said (m - 1) coders and said previous LPC parameters from said memory means.

Claim 9

A speech coding apparatus as claimed in claim 8, characterized in that: said memory means (27) comprises a switch (28) and a memory (27) having an input terminal and an output terminal; said switch connects the input terminal of said memory to either said analyzing means or the output terminal of said memory; and the output terminal of said memory is coupled to said (m - 1) coders associated with said (m - 1) error correcting coders.

Claim 10

A speech coding apparatus as claimed in claim 9, characterized in that: said switch has a control terminal which receives said coder identification number generated by said decision means (5).

Claim 11

A speech coding apparatus as claimed in any of claims 7 to 10, characterized in that: said evaluating means (26) evaluates the quality of the reproduced speech signal by calculating an equation defined as follows: in the following expression. SN LAR = 10logΣ{[(y A ​ (i) )²]/[(y A ​ (i) - y B ​ (i) )²]} where Sn LAR : said evaluated quality, y A (i) : said current LPC parameters and y B (i) : said previous LPC parameters.

Claim 12

A speech coding apparatus as claimed in any of claim 7 to 11, characterized in that: said decision means (5) comprises means (101) for providing a threshold value based on said current error rate and said error correcting abilities of said (m -1) error correcting coders and for comparing said evaluated quality of said reproduced speech signal with said threshold value to thereby output comparison results; and said decision means selects said one of the m coders on the basis of said comparison results.

Claim 13

A speech coding apparatus as claimed in claim 12, characterized in that said decision means (5) is adapted to generate said coder identification number which identifies one coder other than said (m - 1) coders associated with said (m - 1) error correcting coders when said evaluated quality SN LAR is greater than said threshold value.

Claim 14

A speech coding apparatus as claimed in claim 13, characterized in that when said decision means (5) identifies one of said (m - 1) coders in place of said one coder other than said (m - 1) coders, said decision means is adapted to generate said coder identification number which identifies one of said (m - 1) coders associated with said (m - 1) error correcting coders which carries out the error correcting coding for said previous LPC parameters.

Claim 15

A speech coding apparatus as claimed in any of claims 7 to 14, characterized in that the error correcting code generated by one of said (m - 1) error correcting coders has error correcting bits for protecting said previous LPC parameters.

Claim 16

A speech coding apparatus as claimed in any of claims 7 to 15, characterized in that said current LPC parameters related to one of said m coders to which no said error correcting codes are provided are contained in said multiplexed transmission signal without said error correction coding.

Claim 17

A speech coding apparatus as claimed in any of claims 7 to 16, characterized in that said output means comprises: m multiplexer units (71-73) respectively provided for said m coders, each of said m multplexer units generating said multiplexed transmission signal; and selector means (6), coupled to said m multiplexer units and said decision means, for selecting one of said m multiplexer units in accordance with said coder identification number generated by said decision means and for outputting said multiplexed transmission signal supplied from said one of the m multiplexers to said transmission channel.

Claim 18

A speech coding apparatus as claimed in claim 17, characterized by further comprising an error correcting coder (FEC0) for carrying out an error correcting coding for said coder identification number so that said coder identification number is protected by said error correcting coding.

Claim 19

A speech coding apparatus as claimed in any of claims 7 to 18, characterized in that: each of said m coders is of a code excited linear predictive (CELP) coding type coder; said evaluating means (126) comprises waveform distortion evaluating means (202) for evaluating a waveform distortion of each of m reproduced speech signals supplied from said m coders on the basis of said input speech signal and said m reproduced speech signals; said decision means (5) comprises means for identifying said one of the m coders on the basis of said spectral distortion and said waveform distortion.