Coded data generation or conversion – Digital code to digital code converters
Reexamination Certificate
1999-09-13
2001-09-25
JeanPierre, Peguy (Department: 2819)
Coded data generation or conversion
Digital code to digital code converters
C704S229000
Reexamination Certificate
active
06295009
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an audio signal encoding method and apparatus and an audio signal decoding method and apparatus whereby reduced amounts of encoding and decoding delay can be achieved.
In recent years there has been considerable research and development concerning digital audio signal encoding methods, and the MPEG-1 method of audio encoding (specified as the international standard ISO/IEC 11172-3) has become widely utilized, since it enables high-quality audio reproduction to be achieved even when the encoded data are generated at a low bit rate.
FIGS. 13 and 14
illustrate the basic features of an audio encoding/decoding system which conforms to the MPEG-1 standard.
FIG. 13
is a block diagram of the basic MPEG-1 audio encoder, while
FIG. 14
is a block diagram of the corresponding decoder. There are three different models for practical encoding/decoding systems under the MPEG-1 audio standard, having successively increasing levels of complexity, which are respectively referred to as Layer 1, Layer 2, and Layer 3.
FIGS. 15
,
16
and
17
respectively illustrate the frame formats of MPEG-1 audio Layer 1 encoding, Layer 2 encoding and Layer 3 encoding. The degree of coding efficiency increases as the layer numbers go higher, i.e., Layer 3 encoding enables data to be encoded and transmitted at a lower bit rate, without loss of reproduction quality, than does Layer 2 encoding, and Layer 2 encoding is similarly superior to Layer 1 encoding. However the amounts of encoding and decoding delay times are increased in accordance with increases in the layer number.
In
FIG. 13
, the MPEG-1 audio encoder apparatus is made up of a mapping section
112
, a psychoacoustic model section
113
, a quantization and coding section
114
and a frame packing section
115
. The mapping section
112
of this encoder is a sub-band filter, which decomposes each of respective sets of successive PCM digital audio data sample into a plurality of sets of frequency-domain sub-band samples, with these sets of sub-band samples corresponding to respective ones of a fixed plurality of sub-bands. With MPEG-1 audio Layer 2 encoding, each set of 32 input digital audio samples is mapped onto a corresponding set of 32 sub-band samples, and the contents of twelve of these sets of 32 input audio samples (i.e., a total of 384 successive audio data samples) are transferred in the form of quantized and encoded sub-band samples by each frame of an encoded bit stream, as described in Annex C of ISO/IEC 11172-3. Thinning-out of data samples occurs with this transform from the time domain to the frequency domain, since for each frame, there will be some sub-bands for which the samples are of insufficient magnitude to be quantized and encoded.
In encoding each frame, the psychoacoustic model section
113
derives respective mask values for each of the sub-bands, with each mask value expressing an audio signal level which must be exceeded by any signal component, such as quantization noise, in order for that signal component to become audible to a person hearing the final reproduced audio signal. In the case of MPEG-1 audio Layer 1 encoding, the quantization and coding section
114
utilizes the mask values for the respective sub-bands and the signal-to-noise ratios of the sub-band samples of a sub-band, to derive corresponding mask-to-noise ratios for each of the sub-bands, and to accordingly generate bit allocation information which specifies the respective numbers of bits to be used to quantize each of the sub-band samples of a sub-band (with zero bits being allocated in the case of each sub-band for which the samples are of insufficient magnitude for encoding).
The bit allocation information is derived such that the values of mask-to-noise ratio for each of the sub-bands, after quantization, are made substantially balanced, i.e., by assigning a relatively large number of quantization bits to a sub-band having a relatively small scale factor and assigning smaller numbers of quantization bits to the sub-bands having relatively large values of scale factor. With MPEG-1 audio Layer 1 encoding, this is achieved by a simple iterative algorithm for distributing the bits that are available within a frame for quantizing the samples, which is described in Annex C of ISO/IEC 11172-3.
The frame packing section
115
receives the output data generated for each frame by the quantization and coding section
114
, and also any ancillary data which may be required to be included in the frame, generates the frame header and error check data, and assembles these as one frame, in the requisite bitstream format.
The specific manner of operation of the quantization and coding section
114
, and the frame format that is generated by the frame packing section
115
, are determined in accordance with whether the Layer 1, Layer 2, or Layer 3 model is utilized.
The MPEG-1 decoder
121
shown in
FIG. 14
is formed of a frame unpacking section
122
, a reconstruction section
123
and an inverse mapping section
124
. The operation of the decoder
121
is as follows. As the series of bits constituting one frame are successively supplied to the frame unpacking section
122
, the respective data portions of the frame, described above, are separated by the frame unpacking section
122
, with the ancillary data being output from the decoder and the remaining data of the frame being supplied to the reconstruction section
123
. The reconstruction section
123
dequantizates the sub-band samples of the respective sub-bands, and supplies the resultant samples to the inverse mapping section
124
. The inverse mapping section
124
executes an inverse mapping operation to that of the mapping section
112
of the encoder, i.e. to convert the dequantized sub-band samples conveyed by the frame to a corresponding set of PCM digital audio data samples. Assuming that 384 audio data samples are encoded for one frame, as described above, the inverse mapping section
124
will correspondingly convert the sub-band samples conveyed by each frame to 384 PCM audio data samples, i.e., the sample rate of the output data from the inverse mapping section
124
of the decoder
121
is identical to the sample rate of the audio data which are input to the encoder
111
. This is either 32 kHz, 44.1 kHz, or 48 kHz.
As stated hereinabove, the higher the layer number, of the Layer 1, Layer 2 and Layer 3 MPEG-1 bitstream formats, the greater is the coding efficiency. Hence, high-quality audio reproduction can be achieved from the decoded PCM audio data even with a low bit rate for the MPEG-1 encoded data, if the Layer 2, or especially the Layer 3 format is utilized.
FIG. 15
illustrates the MPEG-1 bitstream format in the case of Layer 1. As shown, each frame is formed of a header
131
, followed by an error check portion
132
, an audio data portion
133
, and an ancillary data portion
134
. The audio data portion
133
is made up of a bit allocation information portion containing respective bit allocation information for each of the sub-bands, a scale factor portion containing respective scale factors for each of the sub-bands, and a data sample portion containing the quantized encoded sub-band samples.
FIG. 16
illustrates the MPEG-1 bitstream format in the case of Layer 2. As shown, this differs from the bitstream format of Layer 1 described above only in that the audio data portion further includes scale factor selection information.
FIG. 17
illustrates the MPEG-1 bitstream format in the case of Layer 3. As shown, this differs from the bitstream format of Layer 1 described above in that the audio data portion
153
is formed of an “additional information” portion, and a “main information” portion. In this case the sub-band samples have been subjected to Huffman encoding, and the main data is made up of bits which express the scale factors, the Huffman encoded data, and the ancillary data. In the actual bitstream which is generated by the encoding, with the Layer 3 MPEG-1 audio encoding, the “main information” portion of a frame is
Jean-Pierre Peguy
Matsushita Electric - Industrial Co., Ltd.
Woo Louis
LandOfFree
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