JP3060577B2 - Digital 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 high efficiency digital signal encoding apparatus for encoding an input digital signal.

[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.

In the above-mentioned high efficiency coding, an audio signal or the like on a time axis is transformed into an axis (frequency axis) orthogonal to the time axis by an orthogonal transformation such as a fast Fourier transform (FFT) every predetermined unit time. Then, the data is divided into a plurality of bands, and the FFT coefficient data of each of the divided bands is encoded by adaptive bit allocation. This encoded data is transmitted.

[0004]

When the FFT coefficient data for each band is encoded by the adaptive bit allocation, for example, the FFT coefficient data on the frequency axis is divided into blocks, and In many cases, bit compression is further performed by performing a so-called block floating process. For this reason, the configuration for later decoding includes FFT coefficient data that is band-divided and subjected to block floating processing for each block, and word length information corresponding to the floating coefficient and the number of allocated bits for each block. Is transmitted.

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

Accordingly, the present invention has been proposed in view of the above-described circumstances, and has as its object to provide a digital signal encoding method capable of performing higher bit compression.

[0007]

SUMMARY OF THE INVENTION A digital signal encoding method according to the present invention has been proposed to achieve the above-mentioned object. An input digital signal is orthogonally transformed and divided into a critical band. Encoding the signal component of each critical band with the number of bits according to the level of the difference between the allowable noise level for each critical band and the energy for each critical band set based on the energy for each band, and A digital signal encoding method in which signal components after orthogonal transformation are divided into blocks, block floating processing is performed for each block, and floating coefficients for each block are transmitted, wherein the block floating processing is performed for a band narrower than the critical band. In the case of using small blocks, the number of bits allocated to one small block among small blocks in each critical band is The Flip word length information is obtained so as to transmit. When the block floating process is performed on a large block having a band wider than the critical band, the word length information of one critical band among the critical bands in the large block and the information on the allowable noise level of each critical band are provided. It is also possible to transmit that. Here, when determining the number of allocated bits, for example,
It is desirable to obtain a so-called masking amount in consideration of human auditory characteristics from the energy of the amplitude information for each critical band, and determine the number of bits allocated to each critical band using an allowable noise level based on the masking amount.

[0008]

According to the present invention, when the block floating process is performed on a small block having a band narrower than the critical band, a plurality of small blocks exist in one critical band. By transmitting only the word length information of one small block without transmitting the word length information of all the small blocks in the critical band, the word length information of other small blocks in this critical band is transmitted. Can be reduced in the number of bits for transmission.

[0009]

Embodiments of the present invention will be described below with reference to the drawings. As shown in FIG. 1, the digital signal encoding method of the present invention is such that an input digital signal is orthogonally transformed by, for example, a fast Fourier transform (FFT) and the bandwidth becomes wider in a higher frequency band in consideration of human auditory characteristics. The number of bits according to the level of the difference between the allowable noise level NL for each critical band and the energy for each critical band, which is divided into so-called critical bands (B) and set based on the energy for each critical band Encodes the signal component (FFT coefficient data) of each critical band as described above,
A digital signal encoding method in which signal components after the orthogonal transform are divided into blocks, block floating processing is performed for each block, and floating coefficients for each block are transmitted, wherein the block floating processing is narrower than the critical band B. When performing with the small blocks b1 to b4 of the band, the small blocks b1 to b4 in each critical band B
Of the small blocks b1 (for example, the word length W of the small block b1)
1) Information is transmitted. FIG.
Is extracted and shown as one band B on the high frequency side where the bandwidth of the critical band is wide.

Further, in the present embodiment, as described above, encoding processing of FFT coefficient data by adaptive bit allocation is performed. That is, the encoding process by the adaptive bit allocation in the present embodiment obtains a so-called masking amount in consideration of human auditory characteristics as described later based on the energy of each critical band, and based on the masking amount. The number of bits to be allocated is determined according to the level of the difference between the set allowable noise level (that is, a substantially constant allowable noise level for each critical band) and the energy of each critical band, and the number of allocated bits for each critical band is determined. It is performed according to.

Further, in this embodiment, a block is formed for each fixed number of a plurality of FFT coefficient data in each of the above critical bands, and a so-called block floating process is performed for each block to perform bit compression. I have. Therefore, as in the example of FIG. 1 described above, a plurality of the small blocks (for example, four small blocks b1 to b4) exist in the high band of the critical band, that is, the band B having a wide bandwidth.

By the way, normally, when the block floating processing is performed as described above, at the time of the subsequent decoding processing, the floating coefficient of the floating processing and the word length determined according to the number of allocated bits are used. Information is needed. That is, for a configuration for later decoding, usually, the information of the floating coefficient for each small block and the number of allocated bits based on the level difference between the level of the floating coefficient and the allowable noise level of the critical band It is necessary to transmit the information of the word length according to. In other words, at the time of subsequent decoding, the most significant bit (MSB) in the block floating processing is determined from the information of the floating coefficient,
The least significant bit (LSB) is determined from the word length information, and the allowable noise level is determined. Further, the signal size is determined from the FFT coefficient data (main data) of each small block.

Here, usually, the information of the floating coefficient is represented by 6 bits, for example, and the information of the word length is represented by 4 bits, respectively. In the case of DFT (Discrete Fourier Transform), a magnitude (amplitude), a phase or a real part and an imaginary part are represented by the four bits. For this reason,
For example, when one critical band is divided by a plurality of floating blocks, the total number of transmission bits of the critical band according to the number of small blocks (that is, the number of divided bands) in the block floating process is as shown in Table 1. become.

[Table 1]

In Table 1, when the critical band is represented by one block (one division), a total of 10 bits of 6 bits for the floating coefficient and 4 bits for the word length are transmitted. When the critical band is represented by two small blocks (divided into two), the floating coefficient is 6
× 2 (= 12 bits) and a word length of 4 × 2 (= 8 bits), a total of 20 bits are transmitted. Similarly, when divided into three, the floating coefficient is 6 × 3
(= 18 bits) and the word length is 4 × 3 (= 12 bits)
In the case of four divisions (example of FIG. 1), a total of 40 bits of 6 × 4 (= 24 bits) for the floating coefficient and 4 × 4 (= 16 bits) for the word length are transmitted. As described above, the number of transmitted bits increases as the number of small blocks in one critical band B increases.

On the other hand, in the example shown in FIG. 1 of the embodiment of the present invention, each word length W in one critical band B is set.
Of the information of 1, w2 to w4, the word length W is transmitted.
Only information of 1 is set, and information of other word lengths w2 to w4 is not transmitted. That is, information to be transmitted is information on the floating coefficients Fc1 to Fc4 in the critical band B and information on the word length W1. In other words,
In the subsequent decoding process, if information of one word length W1 is transmitted, each floating coefficient Fc1 to Fc
4, the information of the remaining word lengths w2 to w4 can be obtained. Specifically, the allowable noise level NL is determined by the floating coefficient Fc1 and the word length W1.
If the allowable noise level NL can be obtained, the remaining word lengths w2 to w4 can be known from the allowable noise level NL and the floating coefficients Fc2 to Fc4. For this reason, it is possible to prevent transmission of the information of the remaining word lengths w2 to w4. Therefore, the bit for transmission of the information of the three word lengths w2 to w4 with respect to the critical band B is determined. The number can be reduced.

Here, as described above, the allowable noise level NL is determined for each critical band in consideration of human auditory characteristics, and in the critical band, the allowable noise level is substantially equal within one critical band. It can be considered constant. Therefore, in each of the small blocks b1 to b4 in the critical band B in FIG. 1, the allowable noise level NL can be considered to be the same level. However, if the entire dynamic range is, for example, 120 dB and the floating coefficient is represented by the 6 bits, the floating coefficient has an accuracy of about 2 dB, and if the word length information is represented by the 4 bits, The word length information has an accuracy of about 6 dB. Therefore, in each of the small blocks b1 to b4 in FIG. 1 described above, the floating coefficients Fc1 to Fc4 and the word length W
The transmission noise level determined from the information of 1, w2 to w4 has a deviation of about 2 dB steps as shown in FIG. At this time, the noise level transmitted in each of the small blocks b1 to b4 is approximately ± 3d as shown in FIG.
B falls within the range of B. In other words, the transmission noise level of each of the small blocks b1 to b4 is set so that the word length changes by one bit if the transmission noise level deviates by more than the range of approximately ± 3 dB.

For this reason, for the subsequent decoding process, the difference between the transmission noise level of the small block b1 given the word length W1 and the level closest to the allowable noise level NL is taken together. Try to transmit. That is,
As information indicating the difference from the level closest to the allowable noise level NL, information indicating which level within the range of approximately ± 3 dB in FIG. 2 the transmission noise level of each small block comes, for example, 2 Transmission is performed using a bit determination bit. For example, when the two determination bits are “00”, it indicates that the two bits are shifted to the + side (+1),
"01" indicates that there is no deviation (0), and "10"
At the time of (1), it is indicated that (-1) is shifted to the-side. Note that "11" indicates that it is not used or does not change.

Thus, the floating coefficient Fc
1 and the word length W1, the transmission noise level of the small block b1 is obtained.
By adding the difference from the level closest to L (the level difference represented by the determination bit), the level closest to the allowable noise level NL can be obtained. At this time, as described above, the noise level transmitted from each of the small blocks b1 to b4 does not deviate more than the level next to the level closest to the allowable noise level NL.
From 2 to Fc4, each word length w2 to w4 can be obtained.

Table 2 shows how the number of bits is reduced in the example of FIG. 1 in comparison with Table 1 above.

[Table 2]

In Table 2, when the critical band B is represented by one small block (one division), the floating coefficient is transmitted by 6 bits and the word length W is transmitted by 4 bits. However, in this case, the above determination bit (2 bits) is not used. Therefore, a total of 10 bits are transmitted in the one division. Similarly, when the critical band B is represented by two small blocks b (divided into two), a total of 18 bits including 6 × 2 = 12 bits for the floating coefficient, 4 bits for the word length W, and 2 bits for the decision bit are transmitted. Will be. Similarly, when the data is divided into three, the floating coefficient is 6 × 3 = 18 bits, the word length W is 4 bits, and the judgment bit is 2 bits, that is, a total of 24 bits, and is divided into four (the example in FIG. 1).
In the case of, the floating coefficient is 6 × 4 = 24 bits, the word length W is 4 bits, and the decision bits are 2 bits, for a total of 30
Bits will be transmitted. Therefore, when the number of transmission bits in the example of Table 1 is set to 100% and compared with the example of Table 2, in the example of Table 2, 100% is obtained in one division and 90% in three divisions. As the number of divisions (the number of small blocks) increases, such as 80% for division and 75% for four divisions, the bit reduction rate increases. Therefore, it can be understood that the method of this embodiment is very effective.

FIG. 1 shows an example in which the floating processing is performed on a small block having a band narrower than the bandwidth of the critical band. In the case where the processing is performed with a large block having a wide band, the number of transmission bits can be reduced as shown in FIG.

That is, in the digital signal encoding method shown in FIG. 3, an input digital signal is orthogonally transformed by, for example, a fast Fourier transform (FFT) and divided into a critical band B.
The signal component of each critical band (FFT coefficient data) is represented by the number of bits corresponding to the level of the difference between the allowable noise level NL of each critical band and the energy of each critical band set based on the energy of each critical band. ), A signal component after the orthogonal transformation is divided into blocks, block floating processing is performed for each block b, and a floating coefficient Fc for each block b is transmitted. When the floating process is performed on a large block having a band wider than the critical band, information on the word length of one of the critical bands B1 to B4 in the large block b (for example, the word length W1 of the critical band B1). And information on allowable noise levels NL1 to NL4 of each of the critical bands B1 to B4. . That is, in FIG.
If the large block b is wider than the critical band, the information of the floating coefficient and all the critical bands B1 in the large block b
To the large block b, without transmitting the word length information of one critical band (for example, the word length W1 of the critical band B1) and the respective critical bands B1 to B4.
By transmitting the information on the allowable noise levels NL1 to NL4 of four, the floating coefficient Fc and the number of bits for transmitting the information of the word length w2 to w4 of the other critical bands B2 to B4 in the large block b Can be reduced. Note that the example of FIG. 3 shows each band on the lower band side where the critical bandwidth is narrow. When the critical bandwidth is narrow (low band of the critical band) as in the example of FIG. A plurality of critical bands (for example, four critical bands B1 to B1)
B4) comes to exist.

That is, as shown in FIG. 3, when the block floating process is performed on a large block b wider than the critical bands B1 to B4, the floating coefficient Fc and the word lengths w2 to w4 for the critical bands B2 to B4 are determined. By transmitting the information of the word length W1 of the critical band B1 and the allowable noise levels NL1 to NL4, which are information on the allowable noise level of each critical band, without transmitting the information, it is possible to perform the subsequent decoding process. In this case, if information of one word length W1 is transmitted, the remaining word length w2 is determined based on the information of the allowable noise levels NL1 to NL4 of each critical band.
To w4 can be obtained. Specifically, the floating coefficient Fc can be obtained from the allowable noise level NL1 and the word length W1, and if the floating coefficient Fc can be obtained, the floating coefficient Fc and the allowable noise levels NL2 to NL4 are used. The remaining word lengths w2 to w4 can be known. Therefore, the information of the remaining word lengths w2 to w4 can be prevented from being transmitted, so that the number of bits for the information of the three word lengths w2 to w4 can be reduced for the block b. become.

FIG. 4 shows a specific example of a configuration to which the above-described encoding method of the present embodiment is applied.

That is, in FIG. 4, the digital audio data on the time axis supplied to the input terminal 1 is:
The signal is transmitted to the fast Fourier transform circuit 11. 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), and obtains FFT coefficient data including a real component value Re and an imaginary component value Im. Can be These FFT coefficient data are 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 of the information Am is output.
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 is first sent to the band division circuit 1
3 is transmitted. The band dividing circuit 13 divides the input digital signal 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 critical bands by the band division circuit 13 is transmitted to the sum detection 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. 5, the number of the critical bands is represented by 12 bands (B _{1 to} B ₁₂ ) to simplify the illustration.

Here, in order to consider the influence of the bark spectrum SB on so-called masking, a predetermined weighting function 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. 5 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 synthesis 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. Accordingly, the subtractor 19 performs a subtraction operation between the masking spectrum and the bark spectrum SB, thereby obtaining the signal 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 quantization of the amplitude information Am, and outputs the information of the subtraction circuit 19 (the difference between the energy of each band and the output of the noise level setting means). ), And outputs information on the number of allocated bits according to the level. Accordingly, the quantization circuit 24 quantizes the amplitude information Am based on the allocated bit number information. This quantization circuit 24
Is output from the output terminal 2. Note that the delay circuit 2
1 delays the bark spectrum SB from the sum detection circuit 14 in consideration of the delay amount in each circuit before the synthesis circuit 18, and the delay circuit 23 considers the delay amount in each circuit before the ROM 30. This is provided to delay the amplitude information Am.

At the time of synthesizing by the synthesizing circuit 18 described above, the minimum audible curve generating circuit 22 supplies the signals 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. In addition, 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.

[0037]

According to the digital signal coding method of the present invention, an input digital signal is orthogonally transformed and divided into critical bands, and each critical band is adaptively allocated based on the allowable noise level of each critical band. In addition to encoding the signal components of the band and transmitting the floating coefficient obtained by performing block floating processing on the signal components after the orthogonal transformation, when performing the floating processing on a small block of a band narrower than the critical band, By transmitting the word length information of one of the small blocks in each critical band, the number of bits for the word length information can be reduced, thus enabling higher bit compression.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a case where a floating process according to an embodiment of the present invention is performed in a block narrower than a critical band.

FIG. 2 is a diagram for explaining an allowable noise level transmitted in each block according to the embodiment.

FIG. 3 is a diagram for explaining a case where floating processing is performed in a block wider than a critical band.

FIG. 4 is a block circuit diagram showing a specific configuration for setting an allowable noise level.

FIG. 5 is a diagram showing a bark spectrum.

FIG. 6 is a diagram showing a masking spectrum.

FIG. 7 is a diagram in which a minimum audible curve and a masking spectrum are combined.

[Explanation of symbols]

 B, B1 to B4 ... band b, b1 to b4 ... block W1, w2 to w4 ... word length NL, NL1 to NL4 ... allowable noise level Fc1 to Fc4 ... ..Floating coefficient

Continuation of the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H03M 7/30

Claims (1)

(57) [Claims] 1. An input digital signal is orthogonally transformed and divided into critical bands, and a difference between an allowable noise level for each critical band set based on energy for each critical band and energy for each critical band is determined. The digital component that encodes the signal components of each critical band with the number of bits according to the level, blocks the signal components after the orthogonal transformation, performs block floating processing for each block, and transmits a floating coefficient for each block. In the signal encoding method, when the block floating process is performed on small blocks in a band narrower than the critical band, the number of bits assigned to one of the small blocks in each critical band is determined. A digital signal encoding method comprising transmitting word length information.