Coded speech decoding system with low computation

Details Coded speech decoding system with low computation Coded speech decoding system with low computation

1998-08-06

2002-12-10

Tsang, Fan (Department: 2654)

Data processing: speech signal processing, linguistics, language

Audio signal bandwidth compression or expansion

With content reduction encoding

C704S502000

Reexamination Certificate

active

06493674

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to coded speech decoding systems and, more particularly, to a method of decoding coded speech with less computational effort than in the prior art in case when the number of channels of speech signal that a coded speech decoder outputs is less than the number of channels that are encoded in a coded speech signal.
Heretofore, multi channel speech signals have been coded and decoded by, for instance, a system called “Dolby AC-3”. “Dolby AC-3” techniques are detailed in “ATSC Doc. A/52”, Advanced Television Systems Committee, November 1994 (hereinafter referred to as Literature Ref. 1, and incorporated herein in its entirety).
The prior art coded speech decoding system will first be briefly described. In the prior art coded speech decoding system, input speech signal is first converted through an MDCT (modified discrete cosine transform), which is in the mapping transform, to MDCT coefficients as frequency domain. In this mapping transform, either one of two different MDCT functions prepared in advance is used depending on the character of speech signal to be coded. Which one of the MDCT functions is to be used is coded in auxiliary data. The MDCT coefficients thus obtained are coded separately as exponents and mantissas in the case of expressing in a binary number of floating point system. The mantissas are variable run length coded based on the importance of the subjective coding quality of the MDCT coefficients. Specifically, the coding is performed by using a larger number of bits for the mantissa of an MDCT coefficient with greater importance and a smaller number of bits for the mantissa of an MDCT coefficient with less importance. The exponents and mantissas obtained as a result of the coding and also the auxiliary data, are multiplexed to obtain the coded speech (in the form of a coded bit stream).
FIG. 3
is a block diagram showing a prior art coded speech decoding system. The illustrated prior art coded speech decoding system comprises a coded speech input terminal
1
, a coded speech separating unit
2
, an exponent decoding unit
3
, a mantissa decoding unit
4
, an assigned bits calculating unit
5
, an IMDCT (inverse MDCT: mapping) unit
60
and a decoded speech output terminal
7
. In the following description of operation of the prior art coded speech decoding system, a case is taken, in which coded speech, obtained as a result of coding of an n-channel speech signal, is decoded to an m-channel decoded speech signal. This process of converting a number n of coded audio channels to a smaller number m of decoded channels without loss of information is known in the art as downmixing (see Ref. 1, p. 82). It is used, for example to convert coded five-channel “surround” sound (n=5) to two-channel stereo (m=2), and the following description will be presented in terms of that application.
The coded speech signal obtained through the coding of the 5 channel speech signal is inputted to the coded speech signal input terminal
1
. The coded speech signal inputted to the input terminal
1
is outputted to the coded speech signal separating unit
2
.
The coded speech signal separating unit
2
separates the coded speech bit stream into exponent data, mantissa data and auxiliary data, and outputs these data to the exponent decoding unit
3
, the mantissa decoding unit
4
and the IMDCT unit
4
, respectively.
The exponent decoding unit
3
decodes the exponent data to generate 256 MDCT exponent coefficient per channel for each of the 5 channels. The generated exponent MDCT coefficient for the 5 channels are outputted to the assigned bits calculating unit
5
and the IMDCT unit
60
. Hereinunder, the MDCT exponent coefficient of CH-th (CH=1, 2, . . . , 5) channel is referred to as EXP(CH,
0
), EXP(CH,
1
), . . . , EXP(CH,
255
), and N in MDCT exponent coefficient EXP(CH, N) is referred to as frequency exponent.
The assigned bits calculating unit
5
generates assigned bits data for MAXCH channels in a procedure described in Literature Ref. 1, taking human's psychoacoustic characteristics into considerations, with reference to the MDCT exponent coefficient inputted from the exponent decoding unit
3
, and outputs the generated assigned bits data to the mantissa decoding unit
4
.
The mantissa decoding unit
4
generates the MDCT mantissa coefficients, each expressed as a floating point binary number, for the 5 channels.
The generated MDCT mantissa coefficients for the 5 channels are outputted to the IMDCT unit
60
. Hereinunder, CH-th (CH=1, 2, . . . , 5) channel MDCT mantissa coefficients are referred to as MAN(CH, N), is referred to as the N'th frequency mantissa.
The IMDCT unit
60
first derives the MDCT coefficients from the MDCT mantissa coefficients and MDTC exponent coefficients. Then, the unit
60
converts the MDTC coefficients to the MAXCH-channel speech signal through IMDCT using the transform function designated by the auxiliary data and by windowing. Finally, the unit
60
converts the 5-channel speech signal to 2-channel decoded speech signal through weighting multiplification of the 5-channel speech signal by weighting coefficients each predetermined for each channel. The 2-channel decoded speech signal thus generated is outputted from the decoded speech signal output terminal
7
.
FIG. 4
is a block diagram showing an example of the internal structure of the IMDCT unit
60
in the prior art coded speech signal decoding system when the number of the channels is 5.
MDCT exponent coefficient EXP(CH, N) of CH-th (CH=1, 2, . . . , 5) channel for N'th frequency exponent (N=0, 1, . . . , 255) is inputted to the input terminal
100
.
MDCT mantissa coefficient MAN(CH, N) of CH-th (CH=1, 2,. . . , 5) channel for frequency exponent N (N=0, 1, . . . , 255) is inputted to the input terminal
101
.
Auxiliary data including identification of transform function data of CH-th (CH=1, 2, . . . , 5) channel is inputted to the input terminal
102
.
The MDCT exponent coefficient EXP(CH, N) and the MDCT mantissa coefficient MAN(CH, N) are outputted to an MDCT coefficient generator
110
.
The MDCT coefficient generator
110
generates MDCT coefficient MDCT(CH, N) of CH-th (CH=1, 2, . . . , 5) channel for N'th frequency exponent (N=0, 1, . . . 255) by executing computational operation expressed as
MDCT
(CH,
N
)=MAN(CH,
N
)×2{circumflex over ( )}(_EXP(CH,
N
))
where X{circumflex over ( )}Y represents raising X to power Y.
MDCT coefficient MDCT(CH, N) of the CH-th channel (CH=1, 2, . . . , 5) channel for frequency exponent N (N=0, 1, . . . , 255), is outputted to transform function selector
12
-CH of CH-th channel (i.e., transform function selectors
12
-
1
to
12
-
5
as shown in FIG.
4
).
Transform function selection data of the CH-th (CH=1, 2, . . . , 5) channel inputted to the input terminal
102
, is outputted to the pertinent transform function selectors
12
-CH. According to the transform function data of CH-th (CH=1, 2, . . . , 5) channel ,transform function selector
12
-CH selects either a 512- or a 256-point IMDCT
22
-CH or
23
-CH for the CH-th channel as transform function to be used, and outputs CH-channel MDCT coefficient MDCT(CH,
0
), MDCT(CH,
1
), . . . , MDCT(CH,
225
) to the selected MDCT function.
CH-channel 512-point IMDCT
22
-CH, when selected for CH-th (CH=1, 2, . . . , 5) channel by the pertinent CH-channel transform function selector
12
-CH, converts MDCT coefficient MDCT (CH, N) of CH-channel to windowing signal WIN(CH, N) of CH-channel for frequency exponent N (N=0, 1, . . . , 255) through 512-point IMDCT.
The windowing signal WIN(CH, N) of CH-th channel thus obtained is outputted to windowing processor
24
-CH of CH-channel. At this time, 256-point IMDCT
23
-CH of CH-channel is not operated and does not output any signal. 256-point IMDCT
23
-CH of CH-channel, when selected by the pertinent CH-channel transfer function selector
12
-CH, con

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