JPH04373286A - Picture encoder - Google Patents

Abstract

PURPOSE:To improve encoding efficiency by making a processed unit to be a fixed length one so as to suppress generation of a surplus bit which does not contributes to transferring of information. CONSTITUTION:A block forming circuit 12 divides a picture data into nXn blocks. A DCT circuit 13 calculates a conversion coefficient by processing a picture data of each block with DCT. A quantizer Qm has quantization widths which differ from each other and quantizes the conversion coefficient of a processing unit consisting of blocks of specified numbers. A data amount inferencing circuit 14c inferring the amount of data of a processed unit by quantizing a picture data, and controls a selector 14b for the quantizer Qm of the minimum quantization width and of a data amount less than a target to be selected, and in addition to that, a surplus data amount is added to a specified data amount, while the result taken as a target data amount of the next processed unit. A data amount detecting circuit 14d detected the surplus data which is the difference of actual data and the target data and discards the excess data exceeds the prescribed amount of data.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image encoding apparatus, and more particularly to an image encoding apparatus that encodes image data with high efficiency using discrete cosine transform.

0002

2. Description of the Related Art When transmitting image data or recording it on a recording medium such as a magnetic tape, various encoding methods are used to compress image information. For example, so-called predictive coding, transform coding, vector quantization, etc. are known.

[0003] By the way, the above-mentioned transform encoding utilizes the correlation that image signals have, transforms sample values (hereinafter referred to as image data) into mutually orthogonal axes, and decorrelates the correlation between image data. The so-called basis vectors are orthogonal to each other, the sum of the average signal power before conversion is equal to the sum of the average power of the so-called transformation coefficients obtained by orthogonal transformation, and the degree of power concentration in low frequency components is reduced. Excellent orthogonal transformations have been adopted, such as the so-called Hadamard transform, Haar transform, Kärnen-Louvé (K-L) transform, and discrete cosine transform (hereinafter referred to as DCT).
Discrete Sine Transform (hereinafter referred to as DST), Discrete Sine Transform (hereinafter referred to as DST)
rm), slant transformation, etc. are known.

[0004] Here, the above-mentioned DCT will be briefly explained. DCT divides an image into image blocks each consisting of n pixels (n×n) in both the horizontal and vertical directions in a spatial arrangement, and orthogonally transforms the image data within the image block using a cosine function. This DCT has come to be widely used for transmitting and recording image data due to the existence of a high-speed calculation algorithm and the realization of a one-chip so-called LSI that enables real-time conversion of image data. In addition, DCT has almost the same characteristics as the above-mentioned K-L transform, which is an optimal transform in terms of the degree of power concentration in low frequency components, which directly affects efficiency, in terms of encoding efficiency. Therefore, by encoding only the components in which power is concentrated in the transform coefficients obtained by DCT, it is possible to significantly reduce the amount of information as a whole.

Specifically, transform coefficients obtained by DCT of image data are expressed as, for example, Cij (i=0 to n-1, j=0
˜n−1), the transform coefficient C00 corresponds to a DC component representing the average brightness value within the image block, and its power is typically considerably larger than the other components. Therefore, when this DC component is coarsely quantized, so-called block distortion, which is noise peculiar to orthogonal transform coding, which is visually felt as a large deterioration in image quality, occurs. For example, the visual characteristic that the visual spatial frequency decreases at high frequencies is used for the conversion coefficients Cij (excluding C00) of other components other than the DC component (hereinafter referred to as AC components). The higher the frequency component, the lower the number of bits allocated to it for quantization.

In the transmission and recording of image data, after the transform coefficients obtained by DCT of the image data are quantized as described above, so-called Huffman coding and run-length coding are used for further compression. Variable length coding such as run length coding is applied, and a synchronization signal, parity, etc. are added to the resulting coded data for transmission or recording.

Furthermore, for example, in a digital video tape recorder (hereinafter simply referred to as a VTR) that records a video signal as a digital signal on a magnetic tape, the amount of data per frame or field is constant (fixed length) when editing, variable speed playback, etc. are taken into account. ), and considering the circuit scale, it is also desirable that the processing unit, which collects encoded data for a predetermined number of image blocks, has a fixed length. Therefore, in a VTR, multiple quantizers with different quantization widths are prepared, and one quantizer is used for all image blocks within the processing unit. The quantizer whose data amount is less than a predetermined value and whose quantization width is the smallest is selected to perform quantization. This is because if quantization is performed by switching and selecting a quantizer for each image block within a processing unit, the information of the quantizer used must be transmitted for each image block, which requires a large amount of data (overhead). ) increases, so this is to avoid this.

[0008]

[Problems to be Solved by the Invention] As mentioned above, if one frame or one field of image data is subdivided into processing units each consisting of a predetermined number of image blocks, and the processing units are made to have a fixed length, each processing unit Surplus bits are generated that do not contribute to information transmission, and these surplus bits are accumulated, resulting in a considerable amount of bits not being used for information transmission in one frame or one field. In other words, encoding efficiency will decrease.

The present invention has been made in view of the above circumstances, and it quantizes each block of a processing unit consisting of a predetermined number of blocks with the same quantization width, and also quantizes the data of the processing unit. When the amount of data is less than the target data amount and is quantized (finely) using the minimum quantization width, the processing unit is set to a fixed length, and the surplus bits (bits not used for information transmission) generated in each processing unit are It is an object of the present invention to provide an image encoding device that can suppress the generation of surplus bits in one frame or one entire field due to accumulation and has high encoding efficiency.

0010

[Means for Solving the Problems] In order to solve the above problems, the present invention provides image data in a spatial arrangement of n×n
a discrete cosine transform means for orthogonally transforming the image data of each block from the blocking means using a cosine function to calculate transformation coefficients; A quantization means that quantizes the transform coefficients from the transform means to form quantized data, and outputs the quantized data, and a difference between the data amount of the quantized data of a processing unit consisting of a predetermined number of blocks and the target data amount. Detect the surplus data amount, add the surplus data amount to a predetermined data amount to set the target data amount for the next processing unit, and quantize the transform coefficient with the minimum quantization width that is less than the target data amount. and control means for controlling the quantization means so as to discard excess data exceeding the predetermined data amount.

[0011]

[Operation] In the image encoding device according to the present invention, image data is divided into blocks each having n×n blocks in a spatial arrangement, and the image data of each block is orthogonally transformed using a cosine function to obtain transformation coefficients. , quantize these transform coefficients to form quantized data, and when outputting the quantized data, the difference between the amount of quantized data in a processing unit consisting of a predetermined number of blocks and the target amount of data is calculated. Detect a certain surplus data amount, add this surplus data amount to a predetermined data amount to set the target data amount for the next processing unit, and quantize the transformation coefficient with the minimum quantization width that is less than the target data amount. At the same time, excess data exceeding the predetermined data amount is discarded.

0012

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of an image encoding apparatus according to the present invention will be described with reference to the drawings. FIG. 1 shows the circuit configuration of an image encoding device to which the present invention is applied, and FIG.
3 shows the circuit configuration of the recording system of a digital video tape recorder (hereinafter simply referred to as VTR) to which this image encoding device is applied, and FIG. 3 shows the circuit configuration of the reproduction system of the VTR.

First, this VTR will be explained. As shown in FIG. 2, this VTR converts an analog video signal into a digital signal, performs data processing such as so-called conversion encoding on the resulting image data, and compresses the data. A recording system for recording on the magnetic tape 1 and a magnetic head 31 from the magnetic tape 1 as shown in FIG.
and a reproduction system that binarizes the reproduced signal reproduced by the system, performs data processing such as decoding, converts it into an analog signal, and reproduces the analog video signal.

The above recording system, as shown in FIG.
An analog/digital converter (hereinafter referred to as an A/D converter) 11 samples a video signal and converts it into a digital signal to form image data, and the image data from the A/D converter 11 is The image is divided into image blocks Gh (h=0 to H, where H depends on the number of pixels in one frame or one field and the number of pixels n2 in one image block), and a predetermined number of image blocks. A blocking circuit 1 that forms a processing unit that is, for example, one unit of data processing or transmission, consisting of Gh.
2, and the image data from the blocking circuit 12 is subjected to orthogonal transformation (hereinafter referred to as DCT) using a cosine function.
Cosine Transform) and transform coefficients Cij (i=0 to n-1,
A discrete cosine variable circuit (hereinafter referred to as DC
T circuit) 13, a quantization circuit 14 that quantizes the transform coefficient Cij from the DCT circuit 13 for each processing unit to form quantized data, and the quantized data from the quantization circuit 14, for example, The coded data VLCij (i=0 to n-1, j=0 to n-1) is encoded using a variable length code.
), a parity addition circuit 17 that adds parity for each processing unit, for example, for error detection or error correction, to the encoded data VLCij from the encoding circuit 15, and the parity addition circuit. Synchronization signal insertion in which an identification bit (hereinafter referred to as ID) for identifying the synchronization signal and the number h of the image block Gh, etc. is added to the coded data VLCij to which parity has been added from No. 17 for each processing unit to form transmission data. A circuit 18, a parallel/serial (hereinafter referred to as P/S) converter 19 that converts transmission data sent as parallel data from the synchronization signal insertion circuit 18 into serial data, and a For example, so-called scramble or NRZI
A recording signal is generated by performing modulation processing, and the magnetic head 2
1 and a channel encoder (hereinafter referred to as ENC) 20.

[0015] After converting the video signal supplied as an analog signal through the terminal 2 into image data, this recording system divides, for example, one frame or one field worth of image data into image blocks Gh. DCT the image data of the image block Gh to calculate the transform coefficient Cij, quantize the transform coefficient Cij for each processing unit to form quantized data, and encode the quantized data using a variable length code. Data VLCij is formed. In addition, this recording system adds a synchronization signal etc. to encoded data VLCij for each processing unit to form transmission data, and then performs modulation suitable for recording on this transmission data, such as scrambling or NRZI modulation processing, and then performs magnetic Recording is performed on the magnetic tape 1 by the head 21.

[0016] Thus, the image encoding device according to the present invention,
That is, the main part of the VTR configured as described above is composed of the blocking circuit 12 to the quantizing circuit 14, and specifically, the configuration is as follows.

The blocking circuit 12 includes, for example, a memory 12a having a storage capacity for one frame or one field and storing image data, as shown in FIG.
Then, the image data from the memory 12a is divided into image blocks Gh in which one block is n×n in spatial arrangement, and a processing unit consisting of a predetermined number of image blocks Gh obtained by dividing one frame or one field into a plurality of blocks. It is composed of a block generator 12b that reads data every time.

The blocking circuit 12 stores the image data supplied via the terminal 4 in the memory 12a for each frame or field, and also arranges the image data stored in the memory 12a in a spatial arrangement. For example, the image block Gh is divided into 8×8 image blocks Gh, and a predetermined number of image blocks Gh
The image data is read out in units of processing, and the read image data is supplied to the DCT circuit 13.

The DCT circuit 13 is, for example, a so-called DSP.
(Digital Signal Processor
), etc., and the image data supplied from the blocking circuit 12 for each processing unit is orthogonally transformed using the cosine function as described above to calculate the transformation coefficient Cij, and this transformation coefficient Cij is sent to the quantization circuit. 14.

As also shown in FIG. 1, the quantization circuit 14 has a buffer memory 14a that stores the conversion coefficients Cij from the DCT circuit 13, and has different quantization widths from each other. A quantizer Qm (m=1 to M) that quantizes each of the output transform coefficients Cij to form quantized data of mutually different amounts of data for the same processing unit; A selector 14b that selects one of the outputs, and data that quantizes the transform coefficient Cij from the DCT circuit 13 for each processing unit to estimate the data amount of the processing unit, and controls the selector 14b based on this data amount. The amount estimation circuit 14c detects the surplus data amount which is the difference between the data amount of the quantized data of the processing unit selected by the selector 14b and the target data amount, and also detects the excess data exceeding the predetermined data amount. and a data amount detection circuit 14d to be discarded.

The quantization circuit 14 converts the transform coefficients of each processing unit from the DCT circuit 13 so that the data amount of the quantized data of the processing unit is less than the target data amount and the quantization distortion is minimized. Cij is quantized with a minimum quantization width that is less than or equal to the target data amount targeted for each processing unit to form quantized data, and when this quantized data exceeds a predetermined data amount, the excess The quantized data obtained in this way is transferred to the encoding circuit 1.
5.

Specifically, the data amount estimation circuit 14c:
The transform coefficient Cij from the DCT circuit 13 is quantized with a plurality of quantization widths for each processing unit, and the data amount of the quantized data of the processing unit is estimated. Detect the quantization width of Then, a quantizer selection signal for selecting the quantizer Qm corresponding to the detected quantization width, for example, the number m of the quantizer Qm, is sent to the parity filter shown in FIG. Supplied to additional circuit 17. Further, the data amount estimation circuit 14c calculates the target data amount for the next processing unit by adding the surplus data amount from the data amount detection circuit 14d to a predetermined data amount.

On the other hand, the quantizer Qm divides the area 80 of the transform coefficient Cij of the image block Gh into three areas 81, 82, and 83, as shown in FIG. Quantization is performed in the three regions 81, 82, and 83 with a predetermined quantization width q, and for example, the quantizer Q2
performs quantization with a quantization width q in regions 81 and 82, and performs quantization with a quantization width 2q in a region 83. For example, quantizer Q3 performs quantization with a quantization width q in the region 81. At the same time, quantization is performed in the regions 82 and 83 with a quantization width of 2q, and for example, the quantizer Q4 is
Quantization is performed in three areas 81, 82, and 83 with a quantization width of 2q, and the transformation coefficient Cij is read out for each processing unit from the buffer memory 14a.
quantized data of mutually different amounts of data are formed for the same processing unit, and these quantized data are supplied to the selector 14b.

The selector 14b is the data amount estimation circuit 14.
Based on the quantizer selection signal from c, each quantizer Qm
, and supplies the selected quantized data to the data amount detection circuit 14d.

The data amount detection circuit 14d has a selector 14
The surplus data amount, which is the difference between the data amount of the quantized data selected in b and the target data amount, is detected, and this surplus data amount is supplied to the data amount estimating circuit 14c. Furthermore, when the selected quantized data exceeds a predetermined data amount, the data amount detection circuit 14d discards high-frequency components so that the data amount of the quantized data in the processing unit becomes equal to or less than the predetermined data amount. . For example, as shown in FIG. 5, quantized data is adopted in a zigzag manner from DC to high frequencies in all image blocks 90 included in the processing unit in which the amount of data has exceeded, and the total amount of adopted data is a predetermined amount. When the amount of data reaches , the subsequent high frequency components are discarded.

As a result, the data amount detection circuit 14d outputs quantized data that is obtained by quantizing the data amount of the processing unit within the predetermined data amount and using the minimum quantization width within the target data. be done. At this time, by adding the surplus data amount of the previous processing unit to the predetermined data amount and setting it as the target data amount of the next processing unit, the data amount of the processing unit can be kept below the predetermined data amount (fixed ), it is possible to reduce the occurrence of surplus bits in one frame or entire field due to the accumulation of surplus bits (bits not used for information transmission) in each processing unit, and it is possible to improve encoding efficiency. can.

For example, if one frame has a fixed length, the number of image blocks in one frame is represented by A, the number of image blocks in a processing unit is represented by B, and the transmission rate of quantized data is 20M.
bps, the predetermined amount of data C allocated to each processing unit is determined by the following formula (1). C=20×106/30×(A/B)...(1)

[
Therefore, for example, the kth processing unit from the beginning of the frame is expressed as Pk, the target data amount of this processing unit Pk is expressed as Tk, and the number of surplus bits (the difference between this target data amount Tk and the actual data amount) is If the surplus data amount) is expressed as Rk, the target data amount T for processing unit Pk+1
k+1 can be calculated using the following formula (2). Tk+1 =C+Rk...(2)

That is, the surplus bits generated in the processing unit Pk are carried over to the next processing unit Pk+1, and the next processing unit Pk+1 is added with this surplus data amount Rk as the target data amount Tk+1, and the actual data When the amount of data exceeds a predetermined amount of data, the excess data (high frequency components) is discarded, thereby making the processing unit a fixed length and minimizing the accumulation of surplus bits (ΣRk) generated in each processing unit. It is possible to perform highly efficient quantization.

The encoding circuit 15 uses, for example, a so-called Huffman code for variable length encoding.
) and run length code (Run Length co
The encoding circuit 15 encodes the quantized data selected by the selector 14b using a Huffman code and a run-length code to form encoded data VLCij, and this encoded data VLCij
is connected to the parity addition circuit 1 shown in FIG. 2 through the terminal 5.
7.

The parity addition circuit 17 and synchronization signal insertion circuit 18 shown in FIG. In addition to time division multiplexing, parity and synchronization signals are added to form transmission data. As a result, for example, one processing unit includes, in order from the beginning, a synchronization signal, an ID, a number m of a quantizer Qm adopted in the processing unit, and encoded data VLC of a predetermined number of image blocks Gh.
Transmission data consisting of ij and parity is output.

As described above, this image encoding device temporarily stores the image data supplied through the terminal 4 in the memory 12a, and stores the stored image data in the n in the spatial arrangement.
DCT is performed on the image data of each image block Gh, and the resulting transform coefficients Cij are used for each processing. When quantizing using the quantizer Qm with the minimum quantization width within the unit target data amount, variable-length encoding the quantized data, and outputting the resulting encoded data VLCij via the terminal 5, The target data amount for the next processing unit is the sum of the data amount of surplus bits generated in the previous processing unit (surplus data amount), and if the actual data amount exceeds the predetermined data amount, the excess bit amount will be calculated. By discarding data for 1 frame or 1 field, the processing unit can be set to a fixed length, and the generation of surplus bits in one frame or field due to the accumulation of surplus bits (bits not used for information transmission) in each processing unit can be reduced. It is possible to improve encoding efficiency.

Next, the reproduction system of this VTR will be explained. As shown in FIG. 3 above, this reproduction system includes a channel decoder (hereinafter simply referred to as DEC) that performs signal processing such as NRZI demodulation on the reproduction signal reproduced from the magnetic tape 1 by the magnetic head 31 to reproduce transmitted data. 32
and a serial/parallel (hereinafter referred to as S/P) converter 33 that converts the transmission data sent as serial data from the DEC 32 into parallel data, and the synchronization of the transmission data from the S/P converter 33. In addition, a synchronization signal detection circuit 34 that reproduces the encoded data VLCij
and a time axis correction circuit (hereinafter referred to as TBC: Ti
base corrector) 35, an error correction circuit 36 that performs error correction on the encoded data VLCij from the TBC 35, and sets an error flag EF for the encoded data VLCij for which error correction could not be made; A decoding circuit 37 decodes encoded data VLCij subjected to variable length encoding during recording from the circuit 36 to reproduce quantized data, and a dequantization signal is added to the quantized data from the decoding circuit 37. An inverse quantization circuit 38 that performs processing to reproduce the transform coefficients Cij
and an inverse discrete cosine transform circuit (hereinafter referred to as an IDCT circuit) 39 that orthogonally transforms the transform coefficients Cij from the inverse quantization circuit 38 to reproduce image data; A deblocking circuit 40 forms image data for one frame or one field from image data, and the deblocking circuit 4 generates image data based on the error flag EF from the error correction circuit 36.
an error correction circuit 41 that performs error correction on image data from 0; and a digital/analog converter (hereinafter referred to as a D/A converter) 42 that converts the image data from the error correction circuit 41 into an analog signal and outputs it. It consists of

Next, the operation of the reproduction system configured as described above will be explained. The DEC 32 binarizes the reproduction signal reproduced from the magnetic tape 1 by the magnetic head 31, performs NRZI demodulation, performs descramble processing to reproduce the transmission data, and converts this transmission data into S/
It is supplied to the synchronization signal detection circuit 34 via the P converter 33.

The synchronizing signal detection circuit 34 is connected to the S/P converter 3.
A synchronization signal is detected from the transmission data converted to parallel data in step 3, synchronization is pulled in, and the encoded data VL
Cij and convert this encoded data VLCij to TBC.
35.

The TBC 35 performs time axis correction on the encoded data VLCij, absorbs fluctuations in the time axis that occur during reproduction, and supplies the time axis corrected encoded data VLCij to the error correction circuit 36.

The error correction circuit 36 converts the encoded data VL
Cij error correction is performed using the parity added during recording, and an error flag EF is set for encoded data VLCij that has an error exceeding the error correction capability.
and the error-corrected encoded data VLCij
is supplied to the decoding circuit 37.

[0038] The decoding circuit 37 decodes the encoded data VLCij encoded by the Huffman code and run-length code during recording and reproduces the quantized data.
This quantized data is supplied to an inverse quantization circuit 38.

The inverse quantization circuit 38 converts the encoded data VLC
Based on the number m of the quantizer Qm of each processing unit reproduced together with ij, the quantizer Qm of each processing unit used during recording is recognized, and the quantization corresponding to these quantizers Qm is performed. The quantized data of each processing unit is inversely quantized by the width to reproduce the transform coefficient Cij, and this transform coefficient Ci
j is supplied to the IDCT circuit 39.

The IDCT circuit 39 calculates the transformation coefficients Cij using a transposed matrix corresponding to the transformation matrix used during recording.
is orthogonally transformed to reproduce the image data for each image block Gh, and this image data is supplied to the deblocking circuit 40.

The deblocking circuit 40 converts the image block G
One frame or one field of image data is formed from the image data reproduced every h and is supplied to the error correction circuit 41.

The error correction circuit 41 performs an interpolation process using, for example, error-free image data adjacent to the image data for which the error cannot be corrected in the error correction circuit 36, so that the error cannot be corrected for the image data. The error is corrected, and the image data with this error corrected is supplied to the D/A converter 42.

The D/A converter 42 converts the error-corrected image data into an analog signal, and outputs the analog video signal from the terminal 3 as, for example, a luminance signal Y and color difference signals U and V.

As described above, when recording, a processing unit consisting of a predetermined number of image blocks Gh is set to a fixed length, and the transform coefficients Cij of each image block Gh of the processing unit are quantized with the same quantization width. By carrying over and quantizing the surplus bits generated in a processing unit to the next processing unit, recording this quantized data, and recording the number m of the quantizer Qm used in each processing unit. During playback, the processing unit can be set to a fixed length, and editing, variable speed playback, etc. can be easily performed. Furthermore, even if the data amount of one frame is the same (fixed length), the encoding efficiency of the image encoding device according to the present invention is high, so the overall image quality of the reproduced image is improved compared to conventional devices. be able to.

[0045]

[Effects of the Invention] As is clear from the above explanation, in the present invention, image data is divided into blocks of n×n blocks in a spatial arrangement, and the image data of each block is orthogonally divided using a cosine function. When converting to calculate transform coefficients, quantizing the transform coefficients to form quantized data, and outputting this quantized data, the data amount and target of quantized data for a processing unit consisting of a predetermined number of blocks are determined. Detect the surplus data volume which is the difference in data volume, add the surplus data volume to the predetermined data volume to set the target data volume for the next processing unit, and convert with the minimum quantization width that is less than or equal to this target data volume. By quantizing the coefficients and discarding excess data that exceeds a predetermined amount of data, the processing unit can be made into a fixed length, and the surplus bits (bits not used for information transmission) generated in each processing unit can be reduced. It is possible to suppress the generation of surplus bits in one frame or one entire field due to accumulation, and it is possible to improve encoding efficiency compared to conventional devices.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a circuit configuration of a first embodiment of an image encoding device to which the present invention is applied.

FIG. 2 is a block diagram showing a circuit configuration of a recording system of a digital video tape recorder to which the above image encoding device is applied.

FIG. 3 is a block diagram showing a circuit configuration of a reproduction system of a digital video tape recorder to which the above image encoding device is applied.

FIG. 4 is a diagram showing a region of transform coefficients for explaining the quantization width of a quantizer constituting the image encoding device.

FIG. 5 is a diagram showing areas of quantized data for explaining a high-frequency component discard operation in a quantization circuit constituting the image encoding device.

[Explanation of symbols] 12... Blocking circuit 13...DCT circuit 14...Quantization circuit 14c...Data amount estimation circuit 14d...Data amount detection circuit Qm...quantizer

Claims (1)

[Claims] Claim 1: Image data in a spatial arrangement of n×n
a discrete cosine transform means for orthogonally transforming the image data of each block from the blocking means using a cosine function to calculate transformation coefficients; A quantization means that quantizes the transform coefficients from the transform means to form quantized data, and outputs the quantized data, and a difference between the data amount of the quantized data of a processing unit consisting of a predetermined number of blocks and the target data amount. Detect the surplus data amount, add the surplus data amount to a predetermined data amount to set the target data amount for the next processing unit, and quantize the transform coefficient with the minimum quantization width that is less than the target data amount. and control means for controlling the quantization means so as to discard excess data exceeding the predetermined data amount.