Audio signal encoding apparatus

Details Audio signal encoding apparatus Audio signal encoding apparatus

2002-01-09

2003-06-10

JeanPierre, Peguy (Department: 2819)

Coded data generation or conversion

Digital code to digital code converters

C341S065000

Reexamination Certificate

active

06577252

ABSTRACT:

This application is based on Application No. 2001-052113, filed in Japan on Feb. 27, 2001, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an audio signal encoding apparatus for encoding a wide-band audio signal and multiplexing and transmitting an encoded bit string generated by the encoding processing to a transmission line. More specifically, the present invention relates to a technique of preventing deterioration in objective characteristics such as an S/N ratio (signal-to-noise ratio), etc., in cases where the component in the form of a frequency component such as a sine wave of a signal to be processed exists in a narrow band.
2. Description of the Related Art
As a typical example of conventional audio signal encoding apparatuses, reference is made to one illustrated in the ISO/IEC 13818-7 standard (hereinafter, referred to as an MPEG-2 AAC method). Here, note that the MPEG-2 AAC method is defined in detail in that standard.
FIG. 15
illustrates a block diagram of the MPEG-2 AAC method as such a conventional audio signal encoding apparatus. In this figure, the conventional audio signal encoding apparatus includes a psychoacoustic model section
1
, an MDCT (Modified Discrete Cosine Transform) processing section
2
, an iterative loop processing section
3
, and a multiplexer section
4
. The psychoacoustic model section
1
includes an FFT (Fast Fourier Transform) operation section
11
, a block type determination section
12
and an SMR (Signal Mask Ratio) operation section
13
. The iterative loop processing section
3
includes an allowable error amount calculation section
31
, a bit amount/error amount control section
32
, a normalization processing section
33
, a quantization section
34
, and a Huffman encoding section
35
.
Next, the operation of this audio signal encoding apparatus will be described below.
An input signal input to the psychoacoustic model section
1
is subjected to FFT operation processing in the FFT operation section
11
to generate an FFT frequency spectrum.
Now, the processing block type will be briefly described prior to an explanation of the block type determination section
12
. When a signal on a time base is converted into a signal on a frequency base, there are two kinds of processing block types, one being a long type in which a signal to be analyzed is expanded in time for improved frequency resolution, the other being a short type in which a signal to be analyzed is shortened in time for improved time resolution. The former type is used in the case where there exists only a stationary signal, whereas the latter is used when there is a rapid signal change. In the MPEG-2 AAC method, by properly using these two kinds of processing block types according to the characteristics of a signal to be analyzed, it is possible to prevent the generation of unpleasant noise called a pre-echo, which would otherwise result from an insufficient time resolution.
The block type determination section
12
calculates a masking threshold from an FFT frequency spectrum from the FFT operation section
11
, determines the block type of the input signal based on the masking threshold thus obtained, and passes the result of determination to the MDCT processing section
2
and the multiplexer section
4
as a processing block type.
Then, the SMR operation section
13
calculates an SMR based on the FFT frequency spectrum from the FFT operation section
11
and the masking threshold in the block type determination section
12
, and sends the SMR thus generated to the allowable error amount calculation section
31
in the iterative loop processing section
3
.
The MDCT processing section
2
performs conversion processing, i.e., frequency orthogonal transformation processing, from the time base to the frequency base based on the processing block type received from the block type determination section
12
. As a result, the MDCT frequency spectrum thus generated is passed to the allowable error amount calculation section
31
and the normalization processing section
33
in the iterative loop processing section
3
.
The allowable error amount calculation section
31
in the iterative loop processing section
3
performs multiplication between the MDCT frequency spectrum and the reciprocal (1/SMR) of the SMR to provide an allowable amount of error. The amount of error as mentioned here represents an indication of a difference between the MDCT frequency spectrum from the MDCT processing section
2
and the dequantized value generated through quantization/dequantization, that is, a quantizing error. If this quantizing error is within an allowable range, noise can not be perceived by the human ear.
The amount of error calculated in the allowable error amount calculation section
31
is passed to the bit amount/error amount control section
32
where this amount of error is used as an index of determination as to whether the MDCT frequency spectrum generated through quantization/dequantization satisfies the allowable amount of error.
The normalization processing section
33
normalizes the MDCT frequency spectrum passed from the MDCT processing section
2
by using a scale factor selected in the bit amount/error amount control section
32
.
The quantization section
34
quantizes the MDCT frequency spectrum normalized by the normalization processing section
33
, and passes the result of quantization to the Huffman encoding section
35
. In addition, the quantization section
34
performs dequantization so as to calculate an amount of error in the quantization, and the value thus obtained by the dequantization is passed to the bit amount/error amount control section
32
.
The quantized MDCT frequency spectrum is subjected to Huffman encoding in the Huffman encoding section
35
, so that an amount of bits actually needed are supplied to the bit amount/error amount control section
32
, and a Huffman code book number and a Huffman code are passed to the multiplexer section
4
.
The bit amount/error amount control section
32
calculates a difference between the MDCT frequency spectrum from the MDCT processing section and the dequantized MDCT frequency spectrum obtained from the quantization section
34
, that is, an amount of error due to quantization, which is then compared with the amount of error calculated by the allowable error amount calculation section
31
. As a result, when it is determined that the amount of error due to the quantization is greater than the amount of error calculated by the allowable error amount calculation section
31
, the value of the scale factor is reduced and then passed to the normalization processing section
33
.
On the other hand, when it is determined that the amount of error due to the quantization is smaller than the amount of error calculated by the allowable error amount calculation section
31
, a comparison is made between an amount of used bits obtained from the Huffman encoding section
35
and an allowable amount of bits calculated from the bit rate specified upon encoding. As a result, when it is determined that the amount of the used bits is greater than the allowable amount of bits, the value of the scale factor is increased and then passed to the normalization processing section
33
. On the other hand, when it is determined that the amount of used bits is smaller than the allowable amount of bits, the processing in the iterative loop processing section
3
is ended, and the process is shifted to multiplex processing.
As described above, the processing in the iterative loop processing section
3
, which is constituted by the allowable error amount calculation section
31
, the bit amount/error amount control section
32
, the normalization processing section
33
, the quantization section
34
and the Huffman encoding section
35
, is reiterated until when the quantized MDCT frequency spectrum actually becomes lower than the allowable amount of error, and when the amount of bits required for quantization actually becom

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