JP2575265B2 - Digital audio signal encoding method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital audio signal encoding method for compressing and encoding a music signal and an audio signal.

[0002]

2. Description of the Related Art In a conventional audio signal encoding apparatus using band division, the product of the downsampling frequency and the number of quantization bits, that is, the amount of transmission information for each band becomes equal to the clarity index contribution of the band. (For example, Kazuo Nakata; "High-efficiency coding of speech", 1986 (pp. 56-61)).

FIG. 4 is a block diagram for explaining a conventional audio signal encoding / decoding device. In FIG. 4, 13 is an encoding device, and 14 is a decoding device. In the encoding device 13, 15 is a band division unit for dividing an audio signal into a plurality of sub-bands, and 16 is a quantization and encoding unit for quantizing and encoding the samples of the sub-band signal for each fixed number of samples. A multiplexing unit for transmitting or recording the encoded sub-band signal;
Is a quantization bit number allocating unit that determines the number of quantization bits when quantizing the samples of the subband signal for each fixed number of samples. At the time of encoding, the method of band division is divided so that the clarity index contribution in the band is equal, and the product of the down-sampling frequency and the number of allocated bits, that is, the transmission information amount for each band is equal to the contribution. The number of bits was assigned to be equal.

[0004] The decoding device 14 includes a distributor 19 for dividing the multiplexed transmitted or recorded sub-band signal, and an inverse quantization for de-quantizing and decoding the sample of the sub-band signal. And a decoding unit 20, and a band synthesizing unit 21 for synthesizing the sub-band signal into an audio signal of all bands, and performs decoding.

[0005]

In a conventional quantization bit control method for an audio signal encoding apparatus using band division, bits are controlled so that the information amount for each band becomes equal to the clarity index contribution. Therefore, it was effective to efficiently encode information. However, even for music and speech that are perceived by human hearing, encoding all information uniformly allocates more bits to information that is important to human perception. However, there is a problem that the limited coding capacity cannot be used efficiently.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and provides a digital audio signal encoding method for efficiently encoding information important for human hearing in a limited encoding capacity. It is an object.

[0007]

In order to achieve the above object, the present invention divides a digital audio signal into a plurality of sub-band signals, and determines the number of quantization bits of a sample of the sub-band signal by a characteristic of a human auditory system. In using the minimum audible threshold, which is the lower limit of the human audible range, to reduce and encode based on the sub-band signal, if the level of the sub-band signal is smaller than the minimum audible threshold in the sub-band, Even if the sub-band signal is set to zero and the level of the sub-band signal is smaller than the minimum audible threshold value in the sub-band band, if the level of this sub-band signal is the maximum value of all the sub-band signals, , Quantization bits are allocated to this subband.

[0008]

When the input signal is encoded by using the above-described encoding method for digital audio signals, information important to human hearing can be efficiently encoded. Therefore, each sub-band is encoded at the same bit rate as in the prior art. It works so that good sound quality can be obtained as compared with the case where the sound quality is improved.

[0009]

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an embodiment of a quantization bit number allocating unit to which a digital audio signal encoding method according to the present invention can be applied.

FIG. 1 is a block diagram showing a quantization bit number allocating unit. In FIG. 1, 1 is a level calculation unit, 2
Is a logarithmic value calculator, 3 is an index calculator, 4 is a quantization bit number calculator, 5 is a weighting table for logarithmic values, and 6 is a weighting table for subbands.

Next, the operation of the quantization bit number assignment unit will be described with reference to FIG. First, when a certain number of samples of each subband signal divided into a plurality of subband signals in the band division unit is input to the level calculation unit 1, the level of each subband is calculated. As an example of the level calculation, among a fixed number of samples of each subband signal,
By obtaining the peak value, the level of the corresponding subband can be obtained. As a result, even when the energy of the sub-band signal within a predetermined time is small, if a component having a large peak and important in sound quality is included, the level of the sub-band signal is large, and the level is reduced from energy or the like. There is a possibility that a larger number of quantization bits will be allocated than in the specified case.

Further, a logarithmic value calculator 2 calculates a logarithmic value of the level of each subband. An effective embodiment in the logarithmic value calculation unit 2 will be described with reference to the drawings. FIG. 2 is a diagram showing the minimum audible threshold, and 7 is a curve of the minimum audible threshold when the horizontal axis represents frequency and the vertical axis represents sound pressure. Fletcher et al. Report that people with good hearing can hear the range below this curve. Therefore, when the logarithmic value of the level calculated for each subband signal is smaller than the minimum audible threshold value in the subband band, the corresponding subband signal is set to zero. As a result, limited coded bits can be effectively allocated to higher-level subbands.

Another effective embodiment of the logarithmic value calculator 2 will be described with reference to the drawings. FIG.
Is a diagram showing the minimum audible threshold value and the peak value of each subband, and 8 is a curve of the minimum audible threshold value when the horizontal axis represents frequency and the vertical axis represents sound pressure. 9, 10, 11, and 12 are peak values of each subband. In this case, since the peak values of all subbands are below the minimum audible threshold,
It is possible to set the number of quantization bits for all subbands to zero. However, this does not mean that the given coded bits are being used effectively. Therefore, for example, when the peak values of all the subbands are in a range below the minimum audible threshold value, quantization bits are given to three subbands including the subband having the maximum peak value. As a result, the given coded bits are effectively used, and good characteristics are exhibited even when a minute signal that cannot be detected by the human ear is measured by the measuring instrument.

Next, the index calculating section 3 calculates an index for calculating the number of quantization bits of each subband from the logarithmic value of the level of each subband obtained by the logarithmic value calculating section 2. At this time, a weighting table 5 for logarithmic values in which weighting values determined in advance according to logarithmic values are registered, and a weighting table 6 for subbands in which weighting values similarly defined for each subband are registered in advance The index is calculated by referring to. As an example of the index calculation, a logarithmic value of the level of each subband is multiplied by a weight corresponding to the logarithm and a weight determined for each subband, and can be used as an index of the corresponding subband.

A weighting table 5 for the logarithmic value
The weighting table 6 for subbands and subbands can be derived each time using an equation. For example, the power of the logarithmic value can be used as the weight for the logarithmic value.

Finally, in the quantization bit number calculating section 4,
The number of quantization bits for each subband is calculated. The quantization bit number calculation unit 4 calculates the index for each subband calculated by the index calculation unit 3 and the coding bit rate allowed in the entire band for coding given in advance.
The number of quantization bits for a certain number of samples of each subband signal is calculated.

As described above, according to the present embodiment, by referring to the logarithmic value of the level and the weighting table corresponding to each subband, the number of quantization bits for each subband is determined based on the weight. By obtaining the index and determining the number of quantization bits, it is possible to obtain good sound quality taking into account the level and frequency of the audio signal, which are important factors for human hearing. In addition, since the quantization bit number allocating unit uses the sample values of the respective sub-band signals divided by the band dividing unit, and the weights are registered in advance in the weighting table, effective results can be obtained by simple processing. can get.

[0018]

As described above, according to the digital audio signal encoding method of the present invention, the digital audio signal is divided into a plurality of sub-band signals, and the number of quantization bits of the samples of the sub-band signal is determined by the human auditory system. If the level of the sub-band signal is smaller than the minimum audible threshold in the sub-band to reduce and encode based on the characteristics of the sub-band signal, the sub-band signal is set to zero; Even if the level of the sub-band signal is smaller than the minimum audible threshold in the sub-band, if the level of this sub-band signal is the maximum value of all the sub-band signals, a quantization bit is assigned to this sub-band. Information important to human hearing can be efficiently encoded within the encoding capacity. In addition, since the number of quantization bits is determined at predetermined time intervals according to the characteristics of the input digital audio signal, it is possible to provide an audio signal encoding method suitable for any type of audio signal.

[Brief description of the drawings]

FIG. 1 is a block diagram of a quantization bit number allocating unit to which a digital audio signal encoding method according to an embodiment of the present invention can be applied;

FIG. 2 is a characteristic diagram showing a minimum audible threshold value.

FIG. 3 is a characteristic diagram showing a relationship between a minimum audible threshold value and a subband peak.

FIG. 4 is a block diagram of a conventional audio signal encoding / decoding device.

[Explanation of symbols]

 Reference Signs List 1 level calculating unit 2 logarithmic value calculating unit 3 index calculating unit 4 quantization bit number calculating unit 5 weighting table for logarithmic value 6 weighting table for subband

Claims (1)

(57) [Claims] 1. A digital audio signal is divided into a plurality of sub-band signals, and a quantization bit number of a sample of the sub-band signal is encoded based on characteristics of a human auditory system .
A digital audio signal encoding method,
If the level of the sub-band signal is
If it is less than the listening threshold and all subband signals
If the maximum value of the sub-band signal is quantized,
A method for encoding a digital audio signal, characterized by allocating a number of quantization bits .