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Claims  |
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What is claimed is:
1. An apparatus for compressing a digital input signal, the apparatus
comprising:
band division filter means for dividing the digital input signal in
frequency into plural signals, each of the plural signals being in a
respective one of plural frequency ranges, the plural signals including a
frequency range signal in one of the plural frequency ranges;
block length decision means, operating in response to an index, for
determining a division of the frequency range signal into blocks to
provide a block length decision signal indicating a block length for each
of the blocks;
block floating processing means, operating in response to the block length
decision signal from the block length decision means and in response to
the index, for applying block floating processing to the blocks of the
frequency range signal, each of the blocks having the block length
indicated by the block length decision signal, the block floating
processing circuit providing a block of a block floating processed
frequency range signal from each of the blocks of the frequency range
signal;
orthogonal transform means for orthogonally transforming the block of the
block floating processed frequency range signal to produce plural spectral
coefficients; and
adaptive bit allocation means for dividing the plural spectral coefficients
from the orthogonal transform means into bands and for adaptively
allocating a number of quantizing bits to quantize the spectral
coefficients in each of the bands in response to an allowable noise level
in each of the bands.
2. The apparatus of claim 1, wherein:
the frequency range signal includes plural words, each of the words having
an absolute value; and
the apparatus additionally includes generating means for generating the
index by calculating a logical sum of the absolute values of the words.
3. The apparatus of claims 1 or 2, wherein the orthogonal transform means
includes a Discrete Cosine Transform circuit.
4. The apparatus of claim 1, wherein:
(a) the apparatus additionally comprises:
means for subdividing the frequency range signal into sub blocks, and
index generating means for generating an index for each of the sub blocks
obtained by subdividing the frequency range signal;
(b) the block length decision means determines the division of the
frequency range signal into blocks by comparing the indices of consecutive
ones of the sub blocks of the frequency range signal to determine a number
of the consecutive ones of the sub blocks to constitute each of the
blocks; and
(c) the block floating processing means applies block floating processing
to the blocks of the frequency range signal having the block length
indicated by the block length decision signal using, for each one of the
blocks, a block floating coefficient calculated from the indices of the
sub blocks constituting the one of the blocks.
5. The apparatus of claim 4, wherein:
the frequency range signal includes plural words, each of the words having
an absolute value; and
the index calculating means calculates the index for each one of the sub
blocks by determining a maximum absolute value for the one of the sub
blocks.
6. The apparatus of claim 4, wherein
the block length decision means includes:
(1) a comparing means for comparing the indices of the consecutive ones of
the sub blocks to provide a comparison result, and
(2) a block defining means, responsive to the comparison result provided by
the comparing means, for determining a division of the frequency range
signal into blocks constituted a selected one of:
(i) one of the adjacent sub blocks;
(ii) two of the adjacent sub blocks; and
(iii) four of the adjacent sub blocks.
7. The apparatus of claim 6, wherein:
the means for subdividing the frequency range signal into sub blocks is
additionally for subdividing the frequency range signal into frames, the
frames including a frame constituted of a first sub block and a second sub
block;
the comparing means is for comparing the indices of the first sub block and
the second sub block to provide the comparison result; and
the block defining means determines a division of the frequency range
signal wherein the frame of the frequency range signal constituted of the
first sub block and the second sub block is divided into two equal blocks
when the comparison result provided by the comparing means indicates that
the index of the second sub block is twenty or more times the index of the
first sub block, and otherwise determines a division of the frequency
range signal wherein the frame is divided into a single block.
8. The apparatus of claim 6, wherein:
the means for subdividing the frequency range signal into sub blocks
subdivides the frequency range signal into half sub blocks and into
quarter sub blocks, and is additionally for subdividing the frequency
range signal into frames, the frames including a frame constituted of a
first half sub block and a second half sub block, the frame is also
constituted of four quarter sub blocks, the four quarter sub blocks
including pairs of consecutive ones of the quarter sub blocks, each of the
pairs consisting of a first quarter sub block and a second quarter sub
block;
the comparing means is for comparing the indices of the first half sub
block and the second half sub block to provide a first comparison result,
and is additionally for comparing the indices of the first quarter sub
block and the second quarter sub block in each of the pairs of consecutive
ones of the quarter sub blocks to provide a second comparison result; and
the block defining means determines a division of the frequency range
signal wherein the frame of the frequency range signal is divided into:
four equal blocks when the second comparison result from the comparing
means indicates that the index of the second quarter sub block of any one
of the pairs of the consecutive ones of the quarter sub blocks is twenty
or more times greater than the index of the first quarter sub block of the
one of the pairs of the consecutive ones of the quarter sub blocks,
two equal blocks when the first comparison result from the comparing means
indicates that the index of the second half sub block is ten or more
times, but less than twenty times, the index of the first half sub block,
and
a single block when the first comparison result from the comparing means
indicates that the index of the second half sub block less than ten times
the index of the first half sub block.
9. An apparatus for compressing a digital input signal, the apparatus
comprising:
band division filter means for dividing the digital input signal into
plural signals, each of the plural signals being in a respective one of
plural frequency ranges, the plural signals including a frequency range
signal in one of the plural frequency ranges;
block floating processing means for applying block floating processing to
blocks of the frequency range signal to provide a block floating processed
frequency range signal;
orthogonal transform means for orthogonally transforming blocks of the
block floating processed frequency range signal to provide plural spectral
coefficients; and
adaptive bit allocation means for dividing the spectral coefficients from
the orthogonal transform means into bands and for adaptively allocating a
number of quantizing bits for quantizing the spectral coefficients in each
of the bands in response to an allowable noise level in each of the bands,
the adaptive bit allocation means including:
allowable noise level calculation means for calculating the allowable noise
level for each of the bands,
comparison means for comparing, in each of the bands, the allowable noise
level with a minimum audible level and, for each of the bands in which the
minimum audible level is higher than the allowable noise level, for
setting a flag, and
means for selecting, in each of the bands in which the flag is set, the
minimum audible level as the allowable noise level.
10. The apparatus of claim 9, wherein:
the adaptive bit allocation means quantizes the spectral coefficients using
an actual number of quantizing bits;
the apparatus additionally includes:
means for providing an output signal including a target number of bits, and
means for determining an error between the actual number of bits and the
target number of bits; and
the allowable noise level calculation means calculates the allowable noise
level from an energy in each of the bands, and includes means for
adjusting the allowable noise level in response to the error between the
actual number of bits and the target number of bits.
11. The apparatus of claim 10, wherein the adaptive bit allocation means
additionally includes means for adjusting the number of quantizing bits
allocated to each of the bands by changing the allowable noise level.
12. The apparatus of claim 9, wherein:
the apparatus additionally comprises a block length decision means,
operating in response to an index, for determining a division of the
frequency range signal into the blocks;
the block floating processing means applies block floating processing to
the blocks of the frequency range signal using the index as a block
floating coefficient; and
the orthogonal transform means transforms the block floating processed
frequency range signal divided into blocks determined by the block length
decision means.
13. The apparatus of claims 9, 10, 11, or 12, wherein the orthogonal
transform means includes a Discrete Cosine Transform (DCT) circuit.
14. The apparatus of claim 9, wherein:
the adaptive bit allocation means is additionally for dividing the spectral
coefficients in one of the bands from the orthogonal transform means into
plural sub bands, the plural sub bands including a lowest-frequency sub
band; and
the comparison means is for comparing, in the one of the bands, the
allowable noise level for the one of the bands with the minimum audible
level for the lowest-frequency sub band, and for setting the flag for the
one of the bands when the minimum audible level for the lowest-frequency
sub band is higher than the allowable noise level.
15. An apparatus for compressing a digital input signal, the apparatus
comprising:
index generating means for generating an index in response to the digital
input signal;
block length decision means for determining a division of the digital input
signal into blocks in response to the index;
block floating processing means for applying block floating processing to
the blocks of the digital input signal in response to the index to provide
block floating processed blocks of the digital input signal;
orthogonal transform means for orthogonally transforming each of the block
floating processed blocks of the digital input signal to produce plural
spectral coefficients; and
adaptive bit allocation means for dividing the plural spectral coefficients
from the orthogonal transform means into bands, and for adaptively
allocating a number of quantizing bits to quantize the spectral
coefficients in each of the bands.
16. The apparatus of claim 15, wherein:
the digital input signal comprises plural words, each of the plural words
having an absolute value; and
the index generating means generates the index by calculating a logical sum
of the absolute values of the words.
17. The apparatus of claims 15 or 16, wherein the orthogonal transform
means includes a Discrete Cosine Transform circuit.
18. The apparatus of claim 15, wherein:
(a) the index generating means generates an index for each of plural sub
blocks obtained by dividing the digital input signal;
(b) the block length decision means includes comparing means for comparing
the indices of consecutive ones of the sub blocks of the digital input
signal to determine a number of the consecutive ones of the sub blocks of
the digital input signal to constitute each one of the blocks; and
(c) the block floating processing means applies block floating processing
to the blocks of the digital input signal determined by the block length
decision means using, for each one of the blocks, a block floating
coefficient calculated from the indices of the sub blocks constituting the
one of the blocks.
19. The apparatus of claim 18, wherein:
the digital input signal includes plural words, each of the words having an
absolute value; and
the index calculating means calculates the index for each one of the sub
blocks by determining a maximum of the absolute values of the words in the
one of the sub blocks.
20. The apparatus of claim 18, wherein:
(a) the index generating means includes:
(1) means for dividing the digital input signal into the sub blocks, and
(2) index calculating means for calculating an index for each of the sub
blocks; and
(b) the block length decision means additionally includes block defining
means, responsive to the comparing means, for determining a division of
the digital input signal into blocks constituted of a selected one of:
(i) one of the sub blocks,
(ii) two consecutive ones of the sub blocks, and
(iii) four consecutive ones of the sub blocks.
21. The apparatus of claim 20, wherein:
the means for dividing the digital input signal into the sub blocks
includes:
means for dividing the digital input signal into frames, and
means for dividing each of the frames into a first sub block and a second
sub block;
the comparing means is for comparing the indices of the first sub block and
the second sub block to provide a comparison result; and
the block defining means determines a division of the digital input signal
into blocks in which the frames of the digital input signal for which the
comparison result indicates that the index of the second sub block is
twenty or more times the index of the first sub block are divided into two
equal blocks, and otherwise determines a division of the digital input
signal into blocks in which the frames are each divided into a single
block.
22. The apparatus of claim 20, wherein:
(a) the means for dividing the digital input signal into sub blocks
includes:
(1) means for dividing the digital input signal into flames, and
(2) means for dividing each of the flames into a first half sub block, a
second half sub block, and into four quarter sub blocks, the four quarter
sub blocks including pairs of consecutive ones of the quarter sub blocks,
each of the pairs consisting of a first quarter sub block and a second
quarter sub block;
(b) the comparing means is for comparing the indices of the first half sub
block and the second half sub block to provide a first comparison result,
and for comparing the indices of pairs of the first quarter sub block and
the second quarter sub block in each of the pairs of consecutive ones of
the quarter sub blocks to provide a second comparison result; and
(c) the block defining means determines a division of the digital input
signal into blocks in which:
(1) ones of the flames for which the second comparison result indicates
that the index of the second quarter sub block of any one of the pairs of
consecutive ones of the quarter sub blocks is twenty or more times greater
than the index of the first of the one of the pairs of the consecutive
ones of the quarter sub blocks are divided into four equal blocks,
(2) ones of the flames for which the first comparison result indicates that
the index of the second half sub block is ten or more times, but less than
twenty times, the index of the first half sub block is divided into two
equal blocks, and
(3) ones of the flames for which the first comparison result indicates that
the index of the second half sub block is less than ten times the index of
the first half sub block are divided into a single block.
23. The apparatus of claim 15, wherein the adaptive bit allocation means is
for dividing the spectral coefficients into bands corresponding to
critical bands.
24. The apparatus of claim 15, wherein the adaptive bit allocation means is
for dividing the spectral coefficients towards higher frequencies into
bands corresponding to a fraction of a critical band.
25. An apparatus for compressing a digital input signal, the apparatus
comprising:
block length decision means for determining a division of the digital input
signal into blocks in response to an index;
block floating means for applying block floating processing to each of the
blocks of the digital input signal using the index as a block floating
coefficient; and
means for deriving spectral coefficients from the block floating processed
blocks of the digital input signal; and
adaptive bit allocation means for dividing the spectral coefficients by
frequency into bands and for adaptively allocating a number of quantizing
bits for quantizing the spectral coefficients in each of the bands in
response to an allowable noise level for each of the bands, the adaptive
bit allocation means including:
allowable noise level calculation means for calculating an allowable noise
level for each of the bands,
comparing means for comparing, in each of the bands, the allowable noise
level with a minimum audible level, and
selecting means for selecting the minimum audible level as the allowable
noise level in each of the bands for which the comparing means determines
that the minimum audible level is higher than the allowable noise level.
26. The apparatus of claim 25, wherein the means for deriving spectral
coefficients from the digital input signal includes an orthogonal
transform circuit.
27. The apparatus of claim 26, wherein the orthogonal transform circuit is
a Discrete Cosine Transform (DCT) circuit.
28. The apparatus of claims 25 or 26, wherein:
the bands into which the adaptive bit allocation means divides the spectral
coefficients include a band corresponding to a critical band;
the adaptive bit allocation means is additionally for dividing the spectral
coefficients in the band corresponding to a critical band into sub bands,
the sub bands including a lowest-frequency sub band;
the comparing means is for comparing, in the band corresponding to a
critical band, the allowable noise level for the band corresponding to a
critical band with the minimum audible level for the lowest-frequency sub
band; and
the selecting means is for selecting, as the allowable noise level for the
sub bands in the band corresponding to a critical band, the respective
minimum audible levels for the sub bands when the comparing means
indicates that the minimum audible level for the lowest-frequency sub band
is higher than the allowable noise level.
29. The apparatus of claims 25 or 26, wherein:
the number of quantizing bits adaptively allocated by the adaptive bit
allocation means is an actual number of bits;
the apparatus additionally includes:
means for providing an output signal including a target number of bits, and
means for determining an error between the actual number of bits and the
target number of bits; and
the allowable noise level calculation means includes means for adjusting
the allowable noise level in ones of the bands in response to the error
between the actual number of bits and the target number of bits.
30. The apparatus of claim 29, wherein the adaptive bit allocation means
adjusts the number of quantizing bits allocated to the bands in response
to changes in the allowable noise level in the ones of the bands caused by
the means for adjusting the allowable noise level.
31. A method for compressing a digital input signal, the method comprising
the steps of:
generating an index in response to the digital input signal;
determining a division of the digital input signal into blocks in response
to the index;
applying block floating processing to the blocks of the digital input
signal in response to the index to provide block floating processed blocks
of the digital input signal;
orthogonally transforming each of the block floating processed blocks of
the digital input signal to produce plural spectral coefficients; and
dividing the plural spectral coefficients into bands, and adaptively
allocating a number of quantizing bits to quantize the spectral
coefficients in each of the bands.
32. The method of claim 31, wherein:
the digital input signal comprises plural words, each of the plural words
having an absolute value; and
in the step of generating an index, the index is generated by calculating a
logical sum of the absolute values of the words.
33. The method of claims 31 or 32, wherein, in the step of orthogonally
transforming each of the block floating processed blocks of the digital
input signal, each of the block floating processed blocks of the digital
input signal is orthogonally transformed using a discrete cosine
transform.
34. The method of claim 31, wherein:
(a) in the step of generating an index, an index is generated for each of
plural sub blocks obtained by dividing the digital input signal;
(b) the step of determining a division of the digital input signal into
blocks includes a step of comparing the indices of consecutive sub blocks
of the digital input signal to determine a number of the consecutive sub
blocks of the digital input signal to constitute each one of the blocks;
and
(c) in the step of applying block floating processing to the blocks of the
digital input signal, block floating processing is applied to the blocks
of the digital input signal using, for each one of the blocks, a block
floating coefficient calculated from the indices of the sub blocks
constituting the one of the blocks.
35. The method of claim 34, wherein:
the digital input signal includes plural words, each word having an
absolute value; and
in the step of generating an index, the index for each one of the sub
blocks is calculated by determining a maximum of the absolute values of
the words in the one of the sub blocks.
36. The method of claim 34, wherein:
(a) the step of generating an index includes steps of:
(1) dividing the digital input signal into sub blocks, and
(2) calculating an index for each of the sub blocks; and
(b) in the step of determining a division of the digital input signal into
blocks, a division of the digital input signal into blocks constituted of
a selected one of one sub block, two sub blocks, and four sub blocks is
determined in response to the step of comparing the indices of consecutive
sub blocks.
37. The method of claim 36, wherein:
(a) the step of dividing the digital input signal into sub blocks includes
steps of:
(1) dividing the digital input signal into frames, and
(2) dividing each frame into a first sub block and a second sub block;
(b) in the step of the comparing the indices of consecutive sub blocks, the
indices of the first sub block and the second sub block are compared; and
(c) in the step of determining a division of the digital input signal into
blocks, a division of each frame of the digital input signal into two
equal blocks is determined when the step of comparing determines that the
index of the second sub block is twenty or more times the index of the
first sub block, and a division of the frame of the digital input signal
into a single block is otherwise determined.
38. The method of claim 36, wherein:
(a) the step of dividing the digital input signal into sub blocks includes
steps of:
(1) dividing the digital input signal into frames, and
(2) dividing each frame into a first half sub block, a second half sub
block, and into our quarter sub blocks;
(b) in the step of comparing the indices of consecutive sub blocks, the
indices of the first half sub block and the second half sub block are
compared, and the indices of pairs of consecutive quarter sub blocks are
compared, each pair of consecutive quarter sub blocks including a first
quarter sub block and a second quarter sub block; and
(c) in the step of determining a division of the digital input signal into
blocks, the division of each frame of the digital input signal is
determined as follows:
(1) into four equal blocks when the step of comparing indicates that the
index of the second of any pair of consecutive quarter sub blocks is
twenty or more times greater than the index of the first of any pair of
consecutive quarter sub blocks,
(2) into two equal blocks when the step of comparing indicates that the
index of the second half sub block is ten or more times, but less than
twenty times, the index of the first half sub block, and
(3) into a single block when the step of comparing indicates that the index
of the second half sub block less than ten times the index of the first
half sub block.
39. The method of claim 31, wherein, in the step of dividing the plural
spectral coefficients into bands, the spectral coefficients are divided
into bands corresponding to critical bands.
40. The method of claim 31, wherein, in the step of dividing the plural
spectral coefficients into bands, the spectral coefficients towards higher
frequencies are divided into bands corresponding to a fraction of a
critical band.
41. The method of claim 31, wherein:
the method additionally comprises a step of dividing the digital input
signal into plural signals, each of the plural signals being in a
respective one of plural frequency ranges, the plural signals including a
frequency range signal in one of the plural frequency ranges;
in the step of generating an index in response to the digital input signal,
an index is generated in response to the frequency range signal;
in the step of determining a division of the digital input signal into
blocks, the division of the frequency range signal into blocks is
determined in response to the index;
in the step of applying block floating processing to the digital input
signal, block floating processing is applied to the blocks of the
frequency range signal in response to the index to provide block floating
processed blocks of the frequency range signal; and
in the step of orthogonally transforming each of the block floating
processed blocks of the digital input signal, each of the block floating
processed blocks of the frequency range signal is orthogonally transformed
to produce ones of the plural spectral coefficients.
42. A method for compressing a digital input signal, the method comprising
steps of:
determining a division of the digital input signal into blocks in response
to an index;
applying block floating processing to each of the blocks of the digital
input signal using the index as a block floating coefficient;
deriving spectral coefficients from the block floating processed blocks of
the digital input signal;
dividing the spectral coefficients by frequency into bands; and
adaptively allocating a number of quantizing bits for quantizing the
spectral coefficients in each of the bands in response to an allowable
noise level for each of the bands, the step of adaptively allocating a
number of quantizing bits including steps of:
calculating an allowable noise level for each of the bands,
comparing, in each of the bands, the allowable noise level with a minimum
audible level, and
selecting the minimum audible level as the allowable noise level in each of
the bands for which the step of comparing determines that the minimum
audible level is higher than the allowable noise level.
43. The method of claim 42, wherein the step of deriving spectral
coefficients from the digital input signal includes a step of orthogonally
transforming the digital input signal.
44. The method of claim 43, wherein, in the step of orthogonally
transforming the digital input signal, the digital input signal is
orthogonally transformed using a discrete cosine transform.
45. The method of claims 42 or 43, wherein the method is for compressing
the digital input signal to provide a compressed signal including a target
number of bits, and wherein:
in the step of adaptively allocating a number of quantizing bits, the
number of bits adaptively allocated is an actual number of bits;
the method additionally includes a step of determining an error between the
actual number of bits and the target number of bits; and
the step of adaptively allocating a number of quantizing bits includes a
step of adjusting the allowable noise level in ones of the bands in
response to the error between the actual number of bits and the target
number of bits.
46. The method of claim 45, wherein, in the step of adaptively allocating a
number of quantizing bits, the number of quantizing bits allocated to the
ones of the bands and the sub bands is adjusted in response to changes in
the allowable noise level caused by the step of adjusting the allowable
noise level.
47. The method of claims 42 or 43, wherein:
in the step dividing the spectral coefficients into bands, the spectral
coefficients are divided into bands including a band corresponding to a
critical band, and the spectral coefficients in the band corresponding to
a critical band are divided into sub bands, the sub bands including a
lowest-frequency sub band;
in the step of comparing, the allowable noise level for the band
corresponding to a critical band is compared with the minimum audible
level for the lowest-frequency sub band; and
in the step of selecting, the minimum audible level for the
lowest-frequency sub band is selected as the allowable noise level for the
band corresponding to a critical band when the step of comparing indicates
that the minimum audible level for the lowest-frequency sub band is higher
than the allowable noise level.
48. The method of claim 42, wherein:
the method additionally comprises a step of dividing the digital input
signal into plural signals, each of the plural signals being in a
respective one of plural frequency ranges, the plural signals including a
frequency range signal in one of the plural frequency ranges;
the step of determining a division of the digital input signal into blocks
includes a step of generating the index in response to the frequency range
signal;
in the step of determining a division of the digital input signal into
blocks, a division of the frequency range signal into blocks is determined
in response to the index;
in the step of applying block floating processing to each of the blocks of
the digital input signal, block floating is applied to the blocks of the
frequency range signal in response to the index to generate a block of a
block floating processed frequency range signal from each of the blocks of
the frequency range signal; and
the step of deriving spectral coefficients from the block floating
processed blocks of the digital input signal includes a step of
orthogonally transforming the block floating processed block of the
frequency range signal to produce ones of the spectral coefficients.
49. An apparatus for expanding a compressed digital signal including plural
quantized spectral coefficients and auxiliary information, the apparatus
comprising:
adaptive bit allocation decoding means, operating in response to the
auxiliary information, for inversely quantizing the quantized spectral
coefficients to provide plural spectral coefficients;
block floating means for applying inverse block floating to the spectral
coefficients to provide inverse block floating processed spectral
coefficients;
inverse orthogonal transform means for inversely orthogonally transforming
the inverse block floating processed spectral coefficients to provide
plural frequency range signals; and
inverse filter means for synthesizing the frequency range signals to
provide an output signal.
50. The apparatus of claim 49, wherein the inverse orthogonal transform
means includes an inverse discrete cosine transform circuit.
51. The apparatus of claim 49, wherein:
(a) the apparatus is for expanding a compressed digital signal wherein:
(1) the spectral coefficients are quantized in critical bands,
(2) the critical bands include a divided band, the divided band being a
higher-frequency one of the critical bands that is divided into plural sub
bands, the sub bands including a lowest-frequency sub band, and
(3) the auxiliary information includes an allowable noise level for each of
the critical bands, the allowable noise level for the divided band being
the allowable noise level for the lowest-frequency sub band; and
(b) the apparatus additionally comprises means for determining an allowable
noise level for each sub band of the divided band in response to the
allowable noise level for the divided band.
52. A method for expanding a compressed digital signal to provide a digital
output signal, the compressed digital signal including:
(a) plural quantized spectral coefficients divided by frequency into bands,
the bands including a divided band in which the spectral coefficients
therein are further divided by frequency into sub bands, the quantized
spectral coefficients in each of the bands and each of the sub bands being
quantized using an adaptively-allocated number of quantizing bits,
(b) an allowable noise level for each band, and
(c) a flag signal for the divided band,
the method comprising the steps of:
setting the allowable noise level of the divided band as the allowable
noise level for the divided band when the flag signal for the divided band
is in a first state, and setting the allowable noise level of the divided
band as the allowable noise level for one of the sub bands constituting
the divided band when the flag signal for the divided band is in a second
state;
determining, when the flag signal for the divided band is second state,
from the allowable noise level of the divided band, an allowable noise
level for each of the other ones of the sub bands constituting the divided
band;
using the allowable noise level for each of the bands and for each of the
sub bands constituting the divided band to inversely quantize the
respective quantized spectral coefficients in each of the bands and in
each of the sub bands constituting the divided band to provide spectral
coefficients; and
deriving the digital output signal from the spectral coefficients.
53. The method of claim 52, wherein:
when the flag signal for the divided band is in the second state, the
allowable noise level for the divided band is the allowable noise level
for the lowest-frequency one of the sub bands constituting the divided
band; and
in the step of determining an allowable noise level for each of other ones
of the sub bands constituting the divided band, the allowable noise level
for each of the other ones of the sub bands higher in frequency than the
lowest-frequency one of the sub band is calculated.
54. The method of claim 52, wherein:
the method additionally includes a step of providing a read-only memory
wherein allowable noise levels are stored; and
the step of determining an allowable noise level for each of other ones of
the sub bands constituting the divided band includes a step of reading an
allowable noise level for each of the other ones of the sub bands from the
read-only memory in response to the allowable noise level for the divided
band.
55. The method of claim 52, wherein the step of deriving the digital output
signal from the spectral coefficients includes steps of:
dividing the spectral coefficients by frequency into plural frequency
ranges;
inversely orthogonally transforming the spectral coefficients in each of
the frequency ranges to provide frequency range signals; and
synthesizing the frequency range signals to provide the digital output
signal.
56. The method of claim 52, wherein, in the compressed digital signal, the
plural quantized spectral coefficients are divided by frequency into bands
corresponding to critical bands.
57. An apparatus for compressing a digital input signal, the apparatus
comprising:
means for deriving spectral coefficients from the digital input signal;
frequency dividing means for dividing the spectral coefficients by
frequency into bands, the bands including a band corresponding to a
critical band, and additionally for subdividing the spectral coefficients
in the band by frequency into sub bands, the sub bands including a
lowest-frequency sub band;
allowable noise level calculation means for calculating a allowable noise
level for each of the bands;
supplying means for supplying a minimum audible level for each of the bands
except the band, and for each of the sub bands in the band;
comparing means for determining, in each of the bands except the band, when
the minimum audible level supplied by the supplying means is greater than
the allowable noise level calculated by the allowable noise level
calculation means, and for determining, in the band, when the minimum
audible level supplied by the supplying means for only the
lowest-frequency sub band is greater than the allowable noise level
calculated by the allowable noise level calculation means for the band;
substituting means for substituting, in each one of the bands in which the
comparing means determines that the minimum audible level is greater than
the allowable noise level, the minimum audible level supplied by the
supplying means for the allowable noise level calculated by the allowable
noise level calculating means as the allowable noise level for the one of
the bands, and for substituting, in each one of the sub bands in the band
when the comparing means determines that the minimum audible level for the
lowest-frequency sub band is greater than the allowable noise level for
the band, the minimum audible level supplied for the one of the sub bands
by the supplying means for the allowable noise level calculated for the
band by the allowable noise level calculation means as the allowable noise
level for the one of the sub bands; and
adaptive bit allocation means for adaptively allocating a number of
quantizing bits among the bands and the sub bands for quantizing the
spectral coefficients therein, the bit allocation means allocating
quantizing bits among the bands and the sub bands in response to the
allowable noise level for each of the bands and sub bands.
58. The apparatus of claim 57, wherein the means for deriving spectral
coefficients from the digital input signal includes an orthogonal
transform circuit.
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