Coded data generation or conversion – Digital code to digital code converters – Substituting specified bit combinations for other prescribed...
Reexamination Certificate
1999-08-06
2001-10-30
Ton, David (Department: 2133)
Coded data generation or conversion
Digital code to digital code converters
Substituting specified bit combinations for other prescribed...
C341S051000, C382S239000
Reexamination Certificate
active
06310564
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and an apparatus for compressively coding digital data, and a method and an apparatus for decoding coded data which is read from a recording medium in which the coded data is recorded or is transmitted through a transmission line, to provide reproduced data.
BACKGROUND OF THE INVENTION
In a current AV system, a signal is often handled as a digital signal. The digital signal is more resistive to noises around it than an analog signal, and is reproduced satisfactorily when transmitted or recorded in a recording medium such as a CD (compact disc) or a DVD (digital video disc). When the digital data is stored in the recording medium or transmitted, in order to make it possible to store or transmit enormous amount of data, it is compressed. MPEG (Moving Picture Experts Group) is established as a standardization group for a compression technique. Among MPEG standards, MPEG
1
and MPEG
2
have been standardized as a compression method for a moving picture and MPEG-Audio has been standardized as a compression method for audio.
Hereinafter, conception of the compression technique for audio data according to MPEG-Audio standard or the like, will be described.
FIG. 19
is a block diagram showing a stereo using a compression and decompression technique for the digital audio data.
The stereo shown in
FIG. 19
comprises a compression section
120
for compressing the digital audio data, and a decompression section
125
for restoring digital audio data from compressed data.
The compression section
120
comprises a band division (sub-band creating) unit
121
, a psychoacoustic model
122
, a bit allocation unit
123
, and a bit stream generation unit
124
. Since the digital audio data input to the compression section
120
is in a temporary domain, the band division unit
121
divides the digital audio data into sub-band data to transform it into digital data
151
in a frequency domain and outputs the digital data
151
to the psychoacoustic model
122
and the bit allocation unit
123
. The psychoacoustic model
122
has a model showing a minimum level of audio perceived by human auditory sense, which generally employs a minimum audible limit on stillness or a masking characteristic relating to a background noise and the like. The psychoacoustic model
122
determines the number of bits to-be-allocated
152
of the digital data
151
with quantization precision of a masking level set considering a critical band according to the psychoacoustic model, and outputs the number of bits to-be-allocated
152
to the bit allocation unit
123
. The bit allocation unit
123
performs bit allocation starting from an MSB (most significant bit) of the digital data
151
according to the number of bits to-be-allocated
152
, thereby compressively coding the digital data
151
. The resulting compressed data
103
and the number of allocated bits
152
are output from the bit allocation unit
123
to the bit stream generation unit
124
. The bit stream generation unit
124
multiplexes the compressed data
103
and the number of allocated bits
152
to generate a bit stream, and records the bit stream in a storage medium.
When the stereo is used in a digital music broadcast apparatus, the bit stream can be sent to a channel such as the Internet.
The decompression section
125
comprises a bit stream decomposition unit
126
, a bit decompression unit
127
, and a band composition unit
128
. The bit stream decomposition unit
126
decomposes the bit stream input to the decompression section
125
through the storage medium (channel) into compressed data
203
and a number of allocated bits
162
, which are output to the bit decompression unit
127
. The bit decompression unit
127
calculates a bit length of bits omitted by masking, from the number of bits of the digital data
151
and the number of allocated bits
162
, generates data of the bit length, and couples the data to lower bits of the compressed data
203
. The bit decompression unit
127
couples the bits omitted by the compression unit
120
to the lower bits of the compressed data
203
, resulting in bit-decompressed digital data
161
of an original length. Since the bit-decompressed digital data
161
is data in the frequency domain, the band composition unit
128
transforms this data into data in the temporary domain, to restore original digital audio data.
Subsequently, compression and decompression operations of the digital audio data by the stereo will be described. Here it is assumed that “b”′ is added before binary numbers in numeric representation.
In the compression section
120
, the band division unit
121
divides the digital audio data in the temporary domain into sub-band data to transform it into the digital data
151
in the frequency domain. As an example of the digital data
151
output from the band division unit
121
, “b′ 10100100” of which the number of bits “8” is used. The number of bits “8” is predetermined as a fixed value. When the psychoacoustic model
122
decides that data transmission should be performed at a masking level of the number of bits “5”, the bit allocation unit
123
allocates 5 bits starting from the MSB of the digital data
151
“b′ 10100100”, to create the compressed data
103
“b′ 10100”. The bit allocation unit
123
outputs the “b′ 10100” as the compressed data
103
and “5” as the number of allocated bits
152
to the bit stream generation unit
124
. The bit stream generation unit
124
multiplexes the compressed data
103
“b′ 10100” and the number of allocated bits
152
“5”, and records the bit stream in the storage medium such as the DVD.
Thus, in the compression section
120
, the bit allocation unit
123
performs bit allocation of the digital data
151
to reduce the bits of the digital data
151
by unallocated bits, whereby the digital data
151
is compressed.
In the decompression section
125
, the bit stream decomposition unit
126
decomposes the bit stream sent from the storage medium into the compressed data
203
“b′ 101001”, and the number of allocated bits
162
“5”. In order to restore the digital data
151
, the bit decompression unit
127
subtracts the number of allocated bits
162
“5” from the number of bits “8” of the digital data
151
, to generate data “b′ 000” of the number of bits “3” and couples the bits “b′ 000”, to a lower side of the compressed data
203
“b′ 10100”. The resulting bit-decompressed data
161
“b 10100000” is output from the bit decompression unit
127
. Although the bit-decompressed data
161
“b′ 10100000” is different from the digital data
151
“b′ 10100100”, difference between them is within an error which is hard for human ears to hear, and it is therefore assumed that these two signals are the same in practice. The digital data
161
output from the bit decompression unit
127
is transformed from data in the frequency domain into data in the temporary domain by the band composition unit
128
, to restore the digital audio data.
Thus, in the decompression section
125
, the bit decompression unit
127
couples omitted bits to the lower bits of the compressed data
203
, whereby the bit-decompressed digital data
161
of the original length is obtained.
Further, since storage capacities of the storage media or communication capacities of channels are limited, there is a need for an improved compression technique as data capacities increase. In particular, as for the compression technique for the digital audio data, there is a need for a technique which further increases the compression ratio without degrading the audio quality.
However, using the conventional compression technique, like the stereo, it is impossible to encode the digital data
151
with bits fewer than the bits of the number of bits to-be-allocated
152
which is determined by the psychoacoustic model
122
.
SUMMARY OF THE INVENTION
The present invention is directed to solving the above-mentioned problem, and an object of the present
Matsushita Electric - Industrial Co., Ltd.
Ton David
Wenderoth , Lind & Ponack, L.L.P.
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