Note: Descriptions are shown in the official language in which they were submitted.
WO 91/01545 PCT/US90/02469
2060310
DIGITAL SPEECH CODER WITH VECTOR EXCITATION
SOURCE HAVING IMPROVED SPEECH QUALITY
Technical Field
This invention relates generally to speech coders,
and more particularly to digital speech coders that use
vector excitation sources.
Background of the Invention
Speech coders are known in the art. Some speech
coders convert analog voice samples into digitized
representations, and subsequently represent the spectral
speech information through use of linear predictive
coding. Other speech coders improve upon ordinary
linear predictive coding techniques by providing an
excitation signal that is related to the original voice
signal. I have described, in previously issued U.S. Patent
No. 4,817,157, a digital speech coder having an improved
vector excitation source wherein a codebook of
excitation vectors is accessed to select an excitation
signal that best fits the available information, and hence
provides a recovered speech signal that closely
represents the original.
WO 91/01545 PCT/US90/02469
In general, the resultant decoded speech signal
will more closely represent the original unencoded
speech signal if there is a significant number of
candidate excitation vectors available for consideration
as the excitation source. Increasing performance in this
way, however, generally results in enlargement of the
codebpok size, and this will usually increase processing
complexity and data rates.
A need therefore exists for a digital speech coder
that uses a vector excitation signal, wherein for a given
size codebook, the quality of the decoded speech signal
is substantially maximized with minimal increase in
complexity and substantially no increase in data rate.
Summary of the Invention
These needs and others are substantially met
through provision of the digital speech coder with vector
excitation source having improved speech quality
disclosed herein. Pursuant to this invention, when
encoding a signal sample, such as a speech sample, the
coder first determines a pitch period parameter for the
speech sample. Relying in part upon this pitch period
parameter, a particular coded excitation signal can be
determined independent of the pitch filter coefficient,
following which the pitch filter coefficient parameter
can be optimized for that particular speech sample.
This methodology allows candidate excitation signals to
be considered without requiring a commensurate
increase in processing complexity or data rates.
In one embodiment, the coded excitation signal is
determined substantially independent from any pitch
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information. In particular, candidate excitation signals
as provided by a codebook are processed to substantially
remove components that are representable, at least in
part, by a reference component that is related, at least
in part, to the intermediate pitch vector. More
particularly, the vector component related to the
intermediate pitch vector is removed from the candidate
excitation signal (a process known as orthogonalizing).
The orthogonalized candidate excitation signals are then
compared with the unencoded speech sample to identify
the candidate excitation signal that best represents this
particular speech sample. The pitch information,
including a pitch filter coefficient parameter, can be
optimized later to best suit the selected excitation
signal to thereby yield an overall optimized coded
representation of the speech signal.
In another embodiment, a second codebook of
candidate excitation signals, wherein two excitation
signals are used to represent the speech sample, is
provided. The first excitation signal can be selected as
described above, and the second excitation signal can be
selected in a similar manner, wherein candidate second
excitation signals are first orthogonalized with respect
to both the intermediate pitch vector and the previously
selected first excitation signal.
Brief Description of the Drawin,
Fig. 1 comprises a block diagrammatic depiction of
the invention; and
Fig. 2 cow rises a simple vector diagram
representing one aspect of the invention.
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6~~~,0
Best Mode For Carr~g Out The Invention:
This invention can be embodied in a speech coder
5 that makes use of an appropriate digital signal processor
such as a Motorola DSP 56000 family device. The
computational functions of such a DSP embodiment are
represented in Fig. 1 as a block diagram equivalent
circuit.
10 A pitch period parameter (101 ) (determined in
accordance with prior art technique) is provided to a
pitch filter state (102) that comprises part of a pitch
filter. The resultant signal (103) comprises an
intermediate pitch vector that is provided to both a first
15 multiplier (104) and two orthogonalizing processes (106
and 107) as described below in more detail. This first
multiplier (104) functions to multiply the resultant
signal by a pitch filter coefficient (108) to yield a pitch
filter output (109). Selection of the pitch filter
20 coefficient (108) will be described below in more detail.
A first codebook (111 ) includes a set of basis
vectors that can be linearly combined to form a plurality
of resultant excitation signals. Depending upon the size
of the memory utilized, and other factors appropriate to
the application, the number of possible resultant
excitation signals can be, for example, between 64 and
2,048, with more of course being possible when
appropriate to a particular application. The problem,
when encoding a particular speech sample, is to select
whichever of these excitation sources best represents
the corresponding component of the original speech
information.
WO 91/01545 2 0 6 0 3 1 0
PCT/US90/02469
Pursuant to this invention, once a particular
resultant signal (103) has been determined, the
excitation signals formulated by the first codebook
(111 ) will be presented in seriatim fashion as candidate
5 excitation sources. Each candidate excitation source
will first be orthogonalized (106) with respect to the
resultant signal. For example, referring momentarily to
Fig. 2, if vector A were considered to represent the
resultant signal and vector B were to represent a
particular candidate excitation source, orthogonalization
of the candidate excitation source signal would result in
the vector denoted by reference character B'. (It should
be understood that in practice, the vector dimension
space is a function of the number of samples comprising
the vectors, which may be upwards of 40 samples or
more. It should also be noted that the candidate
excitation vectors may be readily orthogonalized by
orthogonalizing the basis vectors, wherein linear
combinations of the orthogonadized basis vectors with
one another will result in orthogonalized excitation
vectors.)
Once orthogonalized, the resulting candidate
excitation source can be compared (112) with the
unencoded signal (113) (or an appropriate representative
signal based thereon) to determine the relative
similarity or disparity between the two. The process is
then repeated for each of the excitation sources of the
first codebook (111 ). A determination can then be made
as to which candidate excitation source most closely
aligns with the unencoded signal (113).
In this particular embodiment, a gain factor (114)
can also be used to modify each candidate excitation
WO 91/01545 PCT/US90/02469
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2~ source signal, as well understood in the art. In addition,
if desired, the excitation source selection and gain
compensation can both be accomplished in a
substantially simultaneous manner, as also well
understood in the art.
Once an appropriate excitation source from the
first codebook (111 ) has been selected through this
process, the orthogonalizing process (106) can
thereafter be dispensed with and the exact excitation
source signal selected (116) through an appropriate
control mechanism (117). Thereafter, presuming a
single codebook coder, the pitch information can be
gated (117) and summed (118) together with the
selected excitation source with the pitch filter
coefficient (108) and excitation gain (114) optimized
such that the combined excitation most closely aligns
with the encoded signal (113). Once optimized, the pitch
period parameter, pitch filter coefficient, and particular
excitation source and gain are known, and appropriate
representations thereof may be utilized thereafter as
representative of the original speech sample.
If desired, and as depicted in Fig. 1, an additional
codebook (121 ) can be utilized, which second codebook
(121 ) again includes a plurality of basis vector derived
candidate excitation sources. The use of such multiple
codebooks is understood in the art. Pursuant to this
invention, however, once the first excitation source
from the first codebook (111 ) has been selected as
described above, the candidate excitation sources from
the second codebook (121 ) are orthogonalized (107) with
respect to both the resultant signal (103) and the
selected excitation source signal from the first
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ccdebook (111 ). The selection process can then continue
as described above, with the orthogonalized candidate
excitation source signals from the second codebook
(121 ) being compared against a representative unencoded
signal (113) to identify the closest fit. Once this
excitation source has been selected, the pitch filter
coefficient (108) and excitation gains (114 and 120) can
then be optimized as described above.
What is claimed is:
