Digital speech coder having improved vector excitation source

A means for generating a set of 2.sup.M codebook vectors for a vector quantizer, said codebook vector generating means comprising:

• means for converting a set of selector codewords into a plurality of interim data signals;
• means for inputting a set of M basis vectors;
• multiplying said set of basis vectors by said plurality of interim data signals to produce a plurality of interim vectors;
• and
• means for summing said plurality of interim vectors to produce said set of codebook vectors.

A method of generating an excitation vector codebook for a speech synthesizer, said codebook having at least 2.sup.M excitation vectors u.sub.i (n), each having N elements, where 1.ltoreq.n.ltoreq.N, and where 0.ltoreq.i.ltoreq.2.sup.M -1, from a memory containing M basis vectors v.sub.m (n), each having N elements, where 1.ltoreq.n.ltoreq.N and where 1.ltoreq.m.ltoreq.M, using a set of 2.sup.M digital codewords I.sub.i, each having M bits, where 0.ltoreq.i.ltoreq.2.sup.M -1, comprising the steps of:

• (a) identifying a signal .theta..sub.im for each bit of each codeword I.sub.i, such that .theta..sub.im has a first value if bit m of codeword I.sub.i is of a first state, and such that .theta..sub.im has a second value if bit m of codeword I.sub.i is of a second state;
• and
• (b) calculating said codebook of 2.sup.M excitation vectors u.sub.i (n) according to the equation:
• ##EQU29## where 1.ltoreq.n.ltoreq.N.

A method of reconstructing a signal from a set of basis vectors and from a particular excitation codeword, said signal reconstructing method comprising the steps of:

• (a) defining a plurality of interim data signals based upon said particular codeword;
• (b) multiplying said set of basis vectors by said plurality of interim data signals to produce a plurality of interim vectors;
• (c) summing said plurality of interim vectors to produce a single excitation vector;
• and
• (d) signal processing said excitation vector to produce said reconstructed signal.

A method of selecting a codeword for a code-excited signal coder, said selected codeword corresponding to a particular excitation vector having characteristics favorable to those of a given input signal, said particular excitation vector being one of a set of Y excitation vectors, said codeword selecting method comprising the steps of:

• (a) identifying a test codeword;
• (b) defining a plurality of interim data signals based upon said test codeword;
• (c) inputting a set of X basis vectors, where X<Y;
• (d) generating a test excitation vector from said set of basis vectors and said plurality of interim data signals;
• (e) signal processing said test excitation vector to produce a reconstructed signal;
• (f) calculating an error signal representative of the difference between said reconstructed signal and said input signal;
• and
• (g) repeating steps (a) through (f) identifying different test codewords, and selecting one test codeword that produces an error signal which meets predetermined error criteria.

A method of selecting a single excitation codeword for a code-excited signal coder, said single codeword corresponding to a particular excitation vector having characteristics most favorable to those of a portion of a given input signal, said single codeword being one of a set of codewords corresponding to a set of Y possible excitation vectors, said codeword selecting method comprising the steps of:

• (a) generating an input vector corresponding to said input signal portion;
• (b) inputting a set of X basis vectors, where X<Y;
• (c) generating a plurality of processed vectors from said basis vectors;
• (d) producing comparison signals based upon said processed vectors and said input vector;
• (e) calculating parameters for each of said set of codewords based upon said comparison signals;
• and
• (f) evaluating said calculated parameters for each codeword, and selecting one particular codeword having parameters which meet predetermined criteria, without generating said set of Y possible excitation vectors.

A codebook search controller for a code-excited signal coder, said codebook search controller capable of selecting a particular codeword from a set of codewords, said particular codeword corresponding to a desired code vector, said desired code vector being one of at least 2.sup.M possible code vectors, said particular codeword selected according to similarity characteristics between a given input signal and a reconstructed signal derived from said desired code vector, said codebook search controller comprising:

• means for generating a set of processed vectors from a set of M basis vectors;
• means for generating an input vector corresponding to said input signal;
• means for producing comparison signals based upon said processed vectors and said input vector;
• means for calculating parameters for each codeword corresponding to each of said 2.sup.M possible code vectors, said parameters based upon said comparison signals;
• and
• means for selecting a particular codeword having calculated parameters which meet predetermined criteria, without generating said 2.sup.M possible code vectors.

In a code-excited signal coder, a method of selecting a particular excitation codeword I from a set of Y excitation codewords, said particular excitation codeword representative of a desired excitation vector u.sub.I (n) capable of coding a portion of a given input signal, said input signal portion divided into a plurality of N signal samples, said selecting method comprising the steps of:

• (a) generating an input vector y(n) from said input signal portion, where 1.ltoreq.n.ltoreq.N;
• (b) compensating said input vector y(n) for previous filter states, thereby providing compensated vector p(n);
• (c) inputting a set of M basis vectors v.sub.m (n), where 1.ltoreq.m.ltoreq.M<Y;
• (d) filtering said basis vectors to produce zero-state response vectors q.sub.m (n) for each of said M basis vectors;
• (e) generating correlation signals from said zero-state response vectors q.sub.m (n) and said compensated vector p(n);
• (f) identifying a test codeword i from said set of Y excitation codewords;
• (g) calculating parameters for said test codeword i based upon said correlation signals;
• and
• (h) repeating only steps (f) and (g) identifying different test codewords i from said set of Y excitation codewords, and selecting the particular excitation codeword I having calculated parameters which meet predetermined criteria.

A method of generating an excitation signal for a code-excited speech coder, said generating method comprising the steps of:

• (a) signal processing an input signal to produce an input vector;
• (b) providing a set of basis vectors from a memory;
• (c) signal processing said basis vectors to produce a plurality of processed vectors;
• (d) comparing said processed vectors with said input vector to produce comparison signals;
• (e) providing a set of address words;
• (f) calculating parameters for each address word using said comparison signals;
• (g) selecting a particular address word having calculated parameters which meet predetermined error criteria;
• (h) converting said particular address word into a plurality of interim data words;
• and
• (i) generating said excitation signal from said set of basis vectors and said plurality of interim data words.

A speech coder comprising:

• input means for providing an input vector corresponding to a segment of input speech;
• means for providing a set of codewords corresponding to a set of Y possible excitation vectors;
• a first signal path including:
• means for filtering excitation vectors;
• a second signal path including:
• means for providing X basis vectors, where X<Y;
• means for filtering said basis vectors;
• means for comparing said filtered basis vectors to said input vector, thereby providing comparison signals;
• controller means for evaluating said set of codewords and said comparison signals, and for providing a particular codeword representative of a single excitation vector which, when passed through said first signal path, most closely resembles said input vector;
• and
• generator means for generating said single excitation vector by performing a linear transformation on said basis vectors as defined by said particular codeword,
• whereby the evaluation of said set of Y possible excitation vectors is simulated without passing each of said Y possible excitation vectors through said first signal path.