JP3089692B2 - Highly efficient digital data 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 highly efficient digital data encoding method for encoding input digital data.

[0002]

BACKGROUND ART _Audio, in high-efficiency encoding of the signal of voice, etc., audio, adaptive with, the number of bits of each channel is divided into a plurality of channels of the input signal in the time axis or the frequency axis such as voice There is a coding technique based on bit allocation (bit allocation).
For example, the encoding technique based on the above-mentioned bit allocation of an audio signal or the like includes band division encoding (sub-coding) in which an audio signal or the like on a time axis is divided into a plurality of frequency bands and encoded.
Band coding: SBC) or so-called adaptive conversion coding (ATC) in which a signal on the time axis is converted into a signal on the frequency axis (orthogonal conversion), divided into a plurality of frequency bands, and adaptively encoded for each band. Alternatively, the SBC is combined with so-called adaptive prediction coding (APC), a signal on the time axis is divided into bands, and each band signal is converted into a baseband (low band), and then a multi-order linear prediction analysis is performed. There is an encoding technique such as so-called adaptive bit allocation (APC-AB) for predictive encoding.

[0003]

For example, in the above-mentioned adaptive transform coding among the above-mentioned various high-efficiency codings, an audio signal or the like on a time axis is converted into a fast Fourier transform (FFT) or the like for each predetermined time block. Is converted to an axis (frequency axis) orthogonal to the time axis by the orthogonal transformation, and then divided into a plurality of bands, and the FFT coefficient data of each of the divided bands is encoded by adaptive bit allocation. Further, the FFT coefficient data is represented by a real part and an imaginary part of a complex spectrum, and is encoded with the same number of bits.

However, in the high-efficiency coding, it is desired to further increase the compression efficiency and reduce the signal degradation.

Accordingly, the present invention has been proposed in view of the above-described circumstances, and has as its object to provide a high-efficiency encoding method for digital data that enables higher bit compression and causes less signal degradation. Is what you do.

[0006]

SUMMARY OF THE INVENTION A high efficiency coding method for digital data according to the present invention has been proposed to achieve the above-mentioned object. Is converted to the amplitude information and the phase information, at the time of the encoding, for a predetermined band of the frequency analysis output, the number of bits allocated to the phase information, the amplitude information And perform encoding with fewer bits than
The bit number of the phase information and the bit number of the amplitude information are associated with each other to reduce the bit allocation information. Here, it is preferable that the predetermined band in which the number of bits of the phase information is smaller than the amplitude information is a high band in consideration of, for example, human auditory characteristics. As a method of dividing the band when obtaining the predetermined band, a so-called critical band (critical band) considering human auditory characteristics is used.
It is desirable that the division be made. Furthermore, if the number of bits of the phase information with respect to the amplitude information to be reduced is determined in advance for each of the bands, the information on the number of allocated bits to be transmitted together with the coded data of the amplitude information and the phase information can be used as the amplitude information or phase information. , Only one of the information on the number of allocated bits can be used, and the other information on the number of allocated bits does not have to be transmitted. That is, since the other assigned bit number information can be obtained from one assigned bit number information, the number of bits can be reduced by the other assigned bit number information. Further, when determining the number of allocated bits, a so-called masking amount is obtained from the energy of the amplitude information for each band, and the number of allocated bits for each band is determined using an allowable noise level based on the masking amount. Is also possible.

[0007]

According to the present invention, the human auditory characteristics include:
Even if the allocated bit number information for the phase information is reduced from the allocated bit number information for the amplitude information, there is little adverse effect on the perception, and the bit compression rate can be increased by reducing the allocated bit number information for the phase information. Can be.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. As shown in FIG. 1, the digital data high-efficiency encoding apparatus according to the present embodiment comprises a frequency analysis circuit 40 for frequency-analyzing input digital data and converting it into amplitude information Am and phase information Ph. It has an amplitude information encoding circuit 60 for encoding Am and a phase information encoding circuit 70 for encoding the phase information Ph. When the encoding circuits 60 and 70 perform encoding, the frequency analysis circuit For a predetermined band of the 40 outputs, the number of bits allocated to the phase information Ph is smaller than the number of bits allocated to the amplitude information Am to perform encoding.

That is, in FIG. 1, the input terminal 1
Is supplied with, for example, digital audio data. The digital audio data is sent to the frequency analysis circuit 40. In the frequency analysis circuit 40, the audio data is converted into amplitude information Am and phase information Ph by a fast Fourier transform (FFT) described later or the like. The amplitude information Am from the frequency analysis circuit 40 is sent to the amplitude information encoding circuit 60, and the phase information Ph is sent to the amplitude information encoding circuit 60.
70. The amplitude information Am is also sent to the allocated bit number calculation circuit 50. In the allocated bit number calculating circuit 50, the allocated bit number information for encoding the amplitude information Am and the allocated bit number information for encoding the phase information Ph are obtained based on the amplitude information Am as described later. Are sent to the corresponding encoding circuits 60 and 70.

Here, in the allocation bit number calculation circuit 50, as will be described later, the amplitude information Am is converted into a so-called critical band in consideration of human auditory characteristics.
, And a so-called masking amount is obtained from the energy of the amplitude information Am for each band, and the coding of the amplitude information Am for each band and the sign of the phase information Ph are determined using an allowable noise level based on the masking amount. The number of bits to be allocated is determined. Further, when determining the number of allocated bits for each band, the number of allocated bits of the phase information Ph is made smaller than the number of allocated bits of the amplitude information Am in consideration of human auditory characteristics. That is, the human hearing is sensitive to the amplitude (power) in the frequency domain, but considerably insensitive to the phase. In this embodiment, the number of bits allocated to the phase information Ph is reduced in this embodiment. I have. In this manner, by reducing the number of bits allocated to the phase information Ph, the phase information encoding circuit 7
The number of bits of the encoded data of the phase information output from 0 is reduced. Further, even if the number of bits allocated to the phase information Ph is reduced, human auditory perception does not perceive as sound quality deterioration. The details of the processing such as the determination of the number of allocated bits will be described later with reference to FIG.

The encoded data of the amplitude information Am from the amplitude information encoding circuit 60 is output through an output terminal 61, and the phase information P from the phase information encoding circuit 70 is output.
The data in which h is encoded is output via the output terminal 71. The assigned bit number information output from the assigned bit number calculation circuit 50 is output via an output terminal 51. Using the allocated bit number information, decoding processing of the coded data of the amplitude information Am and the coded data of the phase information Ph in the decoding device shown in FIG.

Further, the allocated bit number information sent from the allocated bit number calculating circuit 50 to the decoding device is phase information P
Although it is possible to send the assigned bit number information of h and the assigned bit number information of the amplitude information Am as they are, in the present embodiment, the bit number of the assigned bit number information is also compressed and sent. ing. That is, in the allocated bit number calculation circuit 50 of the present embodiment, the number of bits by which the phase information Ph with respect to the amplitude information Am is reduced is determined in advance for each band. For example, the number of bits allocated to the phase information Ph in the high band (for example, 10 kHz or more) of the predetermined band is set to be smaller by, for example, one bit than the number of bits allocated in the amplitude information Am.
And the number of bits allocated to each of the amplitude information Am. With this configuration, the information on the number of allocated bits to be transmitted together with the encoded data of the amplitude information Am and the phase information Ph is only one of the amplitude information Am and the information on the number of allocated bits of the phase information Ph. This eliminates the need to transmit the other assigned bit number information.
That is, if the later decoding device notifies the predetermined bit reduction condition, the decoding device will
The information about the number of allocated bits can be obtained from the information about the number of allocated bits alone. For this reason, it is not necessary to transmit the number of bits corresponding to the other assigned bit number information, and the number of bits for the assigned bit number information can be reduced.
When the information transmitted as the allocated bit number information is the allocated bit number information for the phase information Ph, the number of transmitted bits is smaller than when the allocated bit number information for the amplitude information Am is transmitted. It can be lowered.

Each circuit of FIG. 1 will be described with reference to a more detailed block circuit diagram of the apparatus of this embodiment shown in FIG.

That is, in FIG. 2, the digital audio data on the time axis supplied to the input terminal 1 is:
Fast Fourier transform circuit 11 and amplitude / phase information generating circuit 12
And transmitted to the fast Fourier transform circuit 11 of the frequency analysis circuit 40. The fast Fourier transform circuit 11 converts the audio data on the time axis into data on the frequency axis for each unit time (unit block),
FFT coefficient data including the real component value Re and the imaginary component value Im is obtained. The FFT coefficient data is transmitted to the amplitude / phase information generation circuit 12, and the amplitude / phase information generation circuit 12 obtains amplitude information Am and phase information Ph from the real component value Re and the imaginary component value Im. The information Am and the phase information Ph are output.

The amplitude information Am of the amplitude information Am and the phase information Ph thus obtained is sent to the allocated bit number calculation circuit 50, and the amplitude information encoding circuit is used by using the amplitude information Am. The adaptive allocation bit number information in the phase information encoding circuit 60 and the phase information encoding circuit 70 is obtained.
That is, although human hearing is generally sensitive to the amplitude (power) in the frequency domain, it is rather insensitive to the phase. Therefore, in the present embodiment, the above-mentioned allocated bit number information is obtained using only the amplitude information Am. I have to.

The amplitude information Am sent to the allocated bit number calculation circuit 50 is first transmitted to the band division circuit 13. The band dividing circuit 13 divides the input digital data represented by the amplitude information Am into a so-called critical bandwidth (critical band). The critical bandwidth is based on human auditory characteristics (frequency analysis capability). For example, 0 to 22 kHz is divided into 25 bands, and the higher the frequency band, the wider the bandwidth is selected.
That is, human hearing has characteristics like a kind of band-pass filter, and the band divided by each filter is called a critical band.

The amplitude information Am for each band divided into the critical bands by the band dividing circuit 13 is transmitted to the sum detecting circuit 14, respectively. In the sum detection circuit 14, the energy for each band (spectral intensity in each band) is calculated by summing the amplitude information Am in each band (amplitude information Am).
By taking the peak or the average or the total energy). The output of the sum detection circuit 14, that is, the spectrum of the sum of each band is generally called a bark spectrum, and the bark spectrum SB of each band is, for example, as shown in FIG.
It becomes as shown in. However, in FIG. 3, for simplicity of illustration, the number of the critical bands is represented by 12 bands (B _{1 to} B ₁₂ ).

Here, in order to consider the influence of the bark spectrum SB on so-called masking, a function of a predetermined weight is convolved with the bark spectrum SB (convolution). Therefore, the output of the sum detection circuit 14, that is, each value of the bark spectrum SB is sent to the filter circuit 15. The filter circuit 15 includes, for example, a plurality of delay elements for sequentially delaying input data and a plurality of multipliers (for example, 25 corresponding to each band) for multiplying an output from these delay elements by a filter coefficient (weighting function). Multipliers) and a sum adder for summing the outputs of the multipliers. This filter circuit 1
In each of the multipliers 5, for example, the multiplier M corresponding to an arbitrary band has the filter coefficient 1, the multiplier M- 1 has the filter coefficient 0.15, and the multiplier M- 2 has the filter coefficient 0.00.
19 is multiplied by a filter coefficient 0.000008 by the multiplier M-3.
6, the filter coefficient 0.4 by the multiplier M + 1 and the multiplier M
By multiplying the output of each delay element by the filter coefficient 0.06 by +2 and the filter coefficient 0.007 by the multiplier M + 3, the convolution process of the bark spectrum SB is performed. Here, M is an arbitrary integer of 1 to 25. By this convolution processing, the sum of the parts indicated by the dotted lines in FIG. 3 is obtained. The masking refers to a phenomenon that a certain signal masks another signal and makes it inaudible due to human auditory characteristics.This masking effect includes a masking effect for an audio signal on a time axis. There is a masking effect on signals on the frequency axis.
That is, even if there is noise in the masked portion due to the masking effect, this noise will not be heard. Therefore, in an actual audio signal,
The noise in this masked portion is considered acceptable noise.

Thereafter, the output of the filter circuit 15 is sent to a subtractor 16. The subtracter 16 calculates a level α corresponding to an allowable noise level described later in the convolved area. The level α corresponding to the permissible noise level (permissible noise level) is, as described later, a level at which the permissible noise level of each critical band is obtained by performing inverse convolution processing. is there. Here, an allowance function (a function expressing a masking level) for obtaining the level α is supplied to the subtractor 16. The level α is controlled by increasing or decreasing the allowable function. The permissible function is supplied from a function generation circuit 29 described later.

That is, the level α corresponding to the allowable noise level can be obtained by the following equation, where i is a number sequentially given from the lower band of the critical bandwidth. α = S− (n−ai) In this equation, n and a are constants and a> 0, and S is the intensity of the convolution-processed Bark spectrum.
ai) is an allowable function. In this embodiment, n = 38, a =
It was set to 1. At this time, there was no sound quality deterioration, and good encoding was performed.

In this way, the level α is obtained, and this data is transmitted to the divider 17. The divider 17 is for inversely convolving the level α in the convolved region. Therefore, by performing this inverse convolution processing,
A masking spectrum can be obtained from the level α. That is, this masking spectrum becomes an allowable noise spectrum. Note that the above inverse convolution process requires a complicated operation, but in the present embodiment, inverse convolution is performed using a simplified divider 17.

Next, the masking spectrum is transmitted to a subtractor 19 via a synthesizing circuit 18. Here, the subtractor 19 outputs the output of the sum detection circuit 14, that is, the bark spectrum S from the sum detection circuit 14 described above.
B is supplied via a delay circuit 21. Therefore, the subtractor 19 performs a subtraction operation between the masking spectrum and the bark spectrum SB to obtain the subtraction result shown in FIG.
As shown in (1), the bark spectrum SB is masked below the level indicated by the level of the masking spectrum MS.

The output of the subtracter 19 is sent to the ROM 30 via the allowable noise level correction circuit 20. The ROM 30 stores a plurality of pieces of assigned bit number information used for encoding the amplitude information Am and the phase information Ph, and outputs the output of the subtraction circuit 19 (the energy of each band and the output of the noise level setting means). (The level of the difference from the number of bits) is output.
As described above, the number of allocated bits in this case is smaller in the phase information Ph than in the amplitude information Am. The output of the ROM 30 is supplied to the amplitude information encoding circuit 60 and the phase information encoding circuit 70. In the amplitude information encoding circuit 60, the amplitude information A supplied through the delay circuit 23 is determined by the number of bits allocated from the ROM 30.
m, and the phase information encoding circuit 70 encodes the phase information Ph supplied via the delay circuit 24. In other words, in other words, these encoding circuits 6
0, 70, the number of bits allocated according to the level of the difference between the energy of each band of the critical bandwidth and the allowable noise level, and the number of bits in which the phase information Ph is smaller than the amplitude information Am. The components of the band will be encoded. Note that the delay circuit 21 delays the bark spectrum SB from the sum detection circuit 14 in consideration of the delay amount of each circuit before the synthesis circuit 18, and the delay circuit 23 or 24 is a circuit before the ROM 30. Is provided to delay the amplitude information Am or the phase information Ph in consideration of the delay amount.

In addition, at the time of synthesizing by the synthesizing circuit 18 described above, the signal supplied from the minimum audible curve generating circuit 22 shown in FIG.
And data indicating a so-called minimum audible curve RC which is a human auditory characteristic as shown in FIG.
And S can be synthesized. At this minimum audible curve, if the absolute noise level is below this minimum audible curve, the noise will not be heard. Further, even if the coding is the same, the minimum audible curve differs depending on, for example, a reproduction volume at the time of reproduction. However, in a realistic digital system, for example, there is not much difference in how to enter music into the 16-bit dynamic range. For example, if quantization noise in the most audible frequency band around 4 kHz is not heard, It is considered that quantization noise below the level of the minimum audible curve is not audible in other frequency bands. Therefore, assuming that the system is used so that noise around 4 kHz of the word length of the system cannot be heard, and an allowable noise level is obtained by synthesizing the minimum audible curve RC and the masking spectrum MS together, In this case, the permissible noise level can be set up to the shaded portion in the figure. In this embodiment, the 4 kHz level of the minimum audible curve is adjusted to the lowest level corresponding to, for example, 20 bits. Also, FIG.
The signal spectrum SS is also shown.

Here, the allowable noise level correction circuit 2
In the case of 0, the allowable noise level from the subtractor 19 is corrected based on information of a so-called equal loudness curve sent from the correction value determination circuit 28. That is, the correction value determination circuit 28 outputs correction value data for correcting the allowable noise level from the subtractor 19 based on information data of a so-called equal loudness curve. By being transmitted to the noise level correction circuit 20, the allowable noise level from the subtracter 19 is corrected in consideration of the equal loudness curve. The above-mentioned equal loudness curve relates to human auditory characteristics. For example, the equal loudness curve is obtained by obtaining sound pressures of sounds at each frequency that sounds as loud as a pure sound of 1 kHz and connecting them with a curve. Also called a curve. Further, the equal loudness curve draws substantially the same curve as the minimum audible curve RC shown in FIG. In the equal loudness curve, for example, 1 k
1 kHz even if the sound pressure falls 8-10 dB below the Hz
Sounds the same size as z, 1k around 50kHz
Unless it is higher than the sound pressure at Hz by about 15 dB, the sound cannot be heard at the same level. For this reason, it can be seen that noise exceeding the level of the minimum audible curve (allowable noise level) preferably has a frequency characteristic given by a curve corresponding to the equal loudness curve. From this, it can be seen that correcting the allowable noise level in consideration of the equal loudness curve is suitable for human auditory characteristics.

In this embodiment, a configuration may be adopted in which the above-described minimum audible curve synthesizing process is not performed. That is, in this case, the minimum audible curve generating circuit 22 and the synthesizing circuit 18 become unnecessary, and the output from the subtractor 16 is inversely convolved by the divider 17 and immediately transmitted to the subtractor 19. Will be done.

As described above, in the apparatus according to the present embodiment, the input digital data is subjected to the frequency analysis to obtain the amplitude information A.
m and phase information Ph, and when encoding the amplitude information Am and the phase information Ph, the number of bits to be assigned to the phase information Ph for the high frequency band of the output of the frequency analysis circuit 40 Since the encoding is performed with the number of bits smaller than the number of bits assigned to the amplitude information Am, the encoded data can be compressed. Further, since the band for reducing the allocated bit number information of the phase information Ph is a high band of a critical band in consideration of human auditory characteristics,
Even if sound quality deterioration in hearing is small, it suffices. Further, for example, for each band, the phase information Ph for the amplitude information Am
If the number of bits in which the number of bits of
The information on the number of allocated bits to be transmitted together with the encoded data of the amplitude information Am and the phase information Ph is represented by the amplitude information Am and the phase information P.
h can be used as only one of the allocated bit number information, and it is not necessary to transmit the other allocated bit number information, and the number of bits for transmitting the allocated bit number information can be further reduced. It becomes possible. Furthermore, the number of bits allocated to each band is determined using an allowable noise level based on the so-called masking amount obtained from the energy of the amplitude information Am for each band. Bit compression with little deterioration of the data.

FIG. 6 is a block circuit diagram showing a schematic configuration of a decoding apparatus corresponding to the encoding apparatus shown in FIG. In this decoding device, the amplitude information Am via the input terminal 161
And the phase information P via the input terminal 171
The coded data of h is decoded based on the information on the number of allocated bits via the input terminal 151. The encoded data of the amplitude information Am via the input terminal 161 is sent to an amplitude information decoding circuit 160 that performs a decoding process corresponding to the encoding process in the amplitude information encoding circuit 60 in FIG. The encoded data of the phase information Ph via the input terminal 171 is transmitted to the phase information encoding circuit 7 shown in FIG.
0 is sent to the phase information decoding circuit 170 that performs a decoding process corresponding to the encoding process at 0. The allocated bit number information via the input terminal 151 is decoded by the allocated bit number information decoding
60, 170. Therefore, in these decoding circuits 160 and 170, decoding processing is performed based on the information obtained by decoding the allocated bit number information. The amplitude information Am and the phase information Ph decoded by the decoding circuits 160 and 170 respectively perform a process (for example, an inverse fast Fourier transform (IFFT) or the like) reverse to the frequency analysis process in the frequency analysis circuit 40. The data is sent to the inverse conversion circuit 140, and the audio data from the circuit 140 is output via the output terminal 101.

[0029]

According to the high-efficiency encoding method for digital data of the present invention, the input digital data is subjected to frequency analysis and converted into amplitude information and phase information. The number of bits allocated to the phase information for the predetermined band is reduced and encoded from the number of bits allocated to the amplitude information, and the bit allocation information is given by associating the number of bits of the phase information with the number of bits of the amplitude information. Is reduced, the bit compression rate can be increased, and the transmission bit rate can be reduced.

[Brief description of the drawings]

FIG. 1 is a block circuit diagram showing a schematic configuration of a digital data high-efficiency encoding apparatus according to an embodiment of the present invention.

FIG. 2 is a block circuit diagram showing a more detailed configuration of the high-efficiency digital data encoding apparatus of the present embodiment.

FIG. 3 is a diagram showing a bark spectrum.

FIG. 4 is a diagram showing a masking spectrum.

FIG. 5 is a diagram in which a minimum audible curve and a masking spectrum are combined.

FIG. 6 is a block circuit diagram illustrating a schematic configuration of a decoding device corresponding to the encoding device of the present embodiment.

[Explanation of symbols]

 40 frequency analysis circuit 50 allocated bit number calculation circuit 60 amplitude information encoding circuit 70 phase information encoding circuit

Continuation of the front page (56) References JP-A-1-205200 (JP, A) JP-A-2-501507 (JP, A) "Hearing and voice", edited by The Institute of Electronics and Communication Engineers, August 1979 Published by Corona Co., Ltd., 10th edition, pp. 72-73, "2.4.5 Problems with phase and timbre", p. 419, "Estimation of information receiving ability of hearing" (58) Fields surveyed (Int. Cl. ⁷ , DB name) H03M 7/30

Claims (1)

(57) [Claims] An input digital data is frequency-analyzed, converted into amplitude information and phase information, and the amplitude information and phase information are encoded. for band, the number of bits allocated to the phase information, performs encoding by reducing than the number of bits allocated to the amplitude information, the number of bits of the phase information
Bit assignment by associating
A highly efficient encoding method for digital data, characterized in that the information is reduced by using the method.