Method and apparatus for speech coding
View Patent ↗A method and apparatus for prediction in a speech-coding system is provided herein. The method of a 1 st order long-term predictor (LTP) filter, using a sub-sample resolution delay, is extended to a multi-tap LTP filter, or, viewed from another vantage point, the conventional integer-sample resolution multi-tap LTP filter is extended to use sub-sample resolution delay. This novel formulation of a multi-tap LTP filter offers a number of advantages over the prior-art LTP filter configurations. Particularly, defining the lag with sub-sample resolution makes it possible to explicitly model the delay values that have a fractional component, within the limits of resolution of the over-sampling factor used by the interpolation filter. The coefficients of such a multi-tap LTP filter are thus largely freed from modeling the effect of delays that have a fractional component. Consequently their main function is to maximize the prediction gain of the LTP filter via modeling the degree of periodicity that is present and by imposing spectral shaping.
1. A method for coding speech by a speech coder, the method comprising the steps of:
generating, by a processor, a plurality of weighted adaptive codebook vectors ( c ′ 0 (n) . . . c ′ K′ (n)) based on a sub-sample resolution delay value, an adaptive codebook, and a weighted synthesis filter;
receiving an input speech signal s(n);
generating a target vector p(n) based on the input speech signal;
generating a plurality of correlation terms (R cc (i,j),R pc (i)) based on the target vector p(n) and the plurality of weighted adaptive codebook vectors;
generating a plurality of symmetric multi-tap long-term predictor filter coefficients (β i 's) based on the plurality of correlation terms, wherein the plurality of symmetric multi-tap long-term predictor filter coefficients comprises coefficients β 0 =αθ and
β
1
=
(
1
-
α
)
θ
2
and wherein α is a shaping coefficient of a shaping filter and θ is an overall long-term predictor gain value; and
constraining values of the shaping coefficient α such that a characteristic of the shaping filter is low-pass.
2. The method in claim 1 wherein the step of generating a target vector p(n) based on the input speech signal s(n) comprises the step of generating a target vector p(n) by perceptually weighting the input speech signal s(n).
3. The method in claim 1 wherein the step of generating a plurality of symmetric multi-tap long-term predictor filter coefficients further comprises solving a set of simultaneous linear equations in response to an error minimization criterion.
4. The method of claim 1 further comprising computing the shaping coefficient α as follows:
α
=
α
2
α
5
-
2
α
4
α
3
α
2
α
4
-
2
α
1
α
5
,
wherein:
α
1
=
R
cc
(
0
,
0
)
+
R
cc
(
1
,
1
)
4
-
R
cc
(
1
,
0
)
α
2
=
R
cc
(
1
,
0
)
-
R
cc
(
1
,
1
)
2
α
3
=
R
cc
(
1
,
1
)
4
α
4
=
R
pc
(
0
)
-
R
pc
(
1
)
2
α
5
=
R
pc
(
1
)
2
.
5. The method of claim 1 where the step of constraining values of the shaping coefficient α such that the characteristic of the filter is low-pass comprises constraining the values of the shaping coefficient to an interval 0.5≦α≦1.0.
6. An apparatus for speech coding comprising:
means for generating a plurality of weighted adaptive codebook vectors ( c ′ 0 (n) . . . c ′ K′ (n)) based on a sub-sample resolution delay value, an adaptive codebook, and a weighted synthesis filter,
means for receiving an input speech signal s(n);
means for generating a target vector p(n) based on the input speech signal s(n);
means for generating a plurality of correlation terms (R cc (i,j),R pc (i)) based on the target vector p(n) and the plurality of weighted adaptive codebook vectors;
means for generating a plurality of symmetric multi-tap long-term predictor filter coefficients (β i 's) based on the plurality of correlation terms, wherein the plurality of symmetric multi-tap long-term predictor filter coefficients comprises coefficients β 0 =αθ and
β
1
=
(
1
-
α
)
θ
2
,
and wherein α is a shaping coefficient of a shaping filter and θ is an overall long-term predictor gain value; and
means for constraining values of the shaping coefficient α such that a characteristic of the shaping filter is low-pass.
7. An apparatus for speech coding comprising:
a plurality of weighted adaptive codebook vectors ( c ′ 0 (n) . . . c ′ K′ (n)) based on a sub-sample resolution delay value, an adaptive codebook, and a weighted synthesis filter;
a perceptual error weighting filter receiving an input speech signal s(n) and outputting a target vector p(n) based on at least s(n);
a correlation generator receiving the weighted adaptive codebook vectors and the target vector p(n), and outputting a plurality of correlation terms (R cc (i,j),R pc (i)) based on the target vector p(n) and the weighted adaptive codebook vectors; and
error minimization circuitry receiving the plurality of correlation terms and outputting a plurality of symmetric multi-tap long-term predictor filter coefficients (β i 's) based on the plurality of correlation terms, wherein the plurality of symmetric multi-tap long-term predictor filter coefficients comprises coefficients β 0 =αθ and
β
1
=
(
1
-
α
)
θ
2
and wherein α is a shaping coefficient of a shaping filter and θ is an overall long-term predictor gain value; and
means for constraining values of the shaping coefficient α such that a characteristic of the shaping filter is low-pass.