Speech coding system
View Patent ↗In a speech coding system with an encoder and a decoder cooperating with said encoder, the speech encoder comprises a pre-processor and an ADPCM encoder with a quantizer and step-size adaptation means, while the speech decoder comprises an ADPCM decoder with similar step-size adaptation means as in the ADPCM encoder and with a decoder, and a post-processor. The quantizer is provided with storage means containing values for a correction factor α(c(n)) of the step-size Δ(n), said correction factor being dependent on the quantizer output signal c(n). The step-size adaptation occurs in accordance with the relation: Δ ( n + 1 ) = Δ ( n ) · A · { α ( c ( n ) ) if α ( c ( n ) ) < 1 α ( c ( n ) ) + b if Δ ( n ) · A · c max < Δ max 1 otherwise .
1. Speech coding system with a speech encoder and a speech decoder cooperating with said speech encoder, the speech encoder comprising a pre-processor and an adaptive differential pulse code modulation (ADPCM) encoder with a quantizer and step-size adaptation means and the speech decoder comprising an ADPCM decoder with similar step-size adaptation means as in the ADPCM encoder and with a decoder, and a post-processor, the quantizer being provided with storage means containing values for a correction factor α(c(n)) of the step-size Δ(n), said correction factor being dependent on the quantizer output signal c(n), characterized in that the step-size adaptation occurs in accordance with the relation:
Δ
(
n
+
1
)
=
Δ
(
n
)
·
A
·
{
α
(
c
(
n
)
)
if
α
(
c
(
n
)
)
<
1
α
(
c
(
n
)
)
+
b
if
Δ
(
n
)
·
A
·
c
max
<
Δ
max
1
otherwise
wherein α is a function of c(n), c(n) has a maximum value of c max , and b, A and Δ max are constants.
2. Speech coding system according to claim 1 , characterized in that the storage means in the quantizer contain values of correction factors α(c(n)) in accordance with the tables FIGS. 3A–3F .
3. Speech coding system according to claim 1 , characterized in that the correction factors α(c(n)) are adapted for a coding system, wherein in the pre-processor phase-smearing filter means are applied and in the post-processor inverse phase-smearing filter means are applied within the speech decoder.