Motion image data compression coding apparatus

Abstract: A motion image data compression coding apparatus and an image data compression coding method capable of smoothing compressed data among frames are disclosed. A data amount of compressed data is obtained by a hybrid coding of a combination of an orthogonal transform such as a discrete cosine transform, a quantization and a variable length coding of motion image data of one frame. A quantization step size is expressed by a product of a scaling factor f and quantization matrix factors q. The scaling factor f is determined from a code amount produced up to the preceding frame, a target code amount and an AC level of the transform factors of the present frame. Prior to the quantization and the variable length coding, a code amount actually resulted by the same coding is detected, and by considering the actual code amount, the scaling factor f for the last quantization is corrected.

Description: The present invention relates to a motion image data compression coding apparatus for compressing a data amount by successively carrying out an orthogonal transform, a quantization and a variable length coding of motion image data of one frame. Now, a recording and reproducing system is being developed, and in this system, a frame structure of motion image data for games, educations or the like are coded for compressing the data amount, and the coded data are recorded onto or reproduced out of a large capacity recording medium such as a CD-ROM or the like. Conventionally, as a high performance one of various coding systems for data compression, a hybrid coding system is known, and in this system, equal division of one frame of image data into blocks, an orthogonal transform such as discrete cosine transform (DCT) or the like, a quantization and a variable length coding (VLC) are applied. The detail of this hybrid coding system is disclosed in JP-A-87-313850 and JP-A-90-184242. In this conventional hybrid coding system, with a change of the data amount and the compression efficient depending upon the image conditions, the data amount (produced code amount) after compression is also changed every frame. Hence, on reproducing, a relatively large capacity of buffer circuit and a complex control system for the buffering are required. In the coding, as described above, in order to reduce the buffer memory size and simplify the synchronization control in the reproducing part, it is desirable to make smooth one frame of compressed data as flat as possible, and in order to realize this, a variety of methods have been proposed, for example, one frame of entire code amount is calculated, and to the direction to keep this, the step size of the quantization is dynamically varied. However, even in such conventional various data smoothing methods including the dynamic change of the quantization step size, it is still insufficient in respect of the data smoothing, and a more effective smoothing method is demanded. It is therefore an object of the present invention to provide a motion image data compression coding apparatus in view of the problems of the prior art, which is capable of smoothing a produced code amount among frames and compressed data among the frames. This object is solved with the features of the claims. The objects, features and advantages of the present invention will become more apparent from the consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram of a motion image data compression coding apparatus according to an illustrative example; Figs. 2A and 2B show quantization matrix factors for luminace and chrominance signals, respectively, to be used for quantization in the apparatus shown in Fig. 1; Fig. 3 is a graphical representation of characteristics to be used for determining a scaling factor from an AC level of one frame and a target produced code amount in the apparatus shown in Fig. 1; Fig. 4 is a block diagram of a motion image data compression coding apparatus according to the embodiment of the present invention; Fig. 5 is shows a prediction table for determining an initial value of a scaling factor f from an AC level of the head frame and a target produced code amount in the apparatus shown in Fig. 4; Fig. 6 is a graphical representation showing a result of a compressed produced coding in the apparatus shown in Fig. 4; Fig. 7 is a graphical representation showing a variation result of an interframe difference G of an AC component corresponding to the result shown in Fig. 6; Referring now to the drawings, wherein like reference characters designate like or corresponding parts throughout the views and thus the repeated description thereof can be omitted for brevity, there is shown in Fig. 1 a motion image data compression coding apparatus of the illustrative example. As shown in Fig. 1, the motion image data compression coding apparatus comprises a DCT (discrete cosine transform) part 1 coupled to an input terminal IN of motion image data to be compressed, a frame buffer memory 2, a quantization (Q) part 3 of a variable step size, a variable length coding (VLC) part 4 connected to an output terminal OUT, a produced code amount calculator 5, an AC level detector 6, a target produced code amount calculator 7, a step size set part 8, a preceding quantization part 9, a preceding variable length coding part 10, a preceding produced code amount calculator 11 and a preceding step size set part 12. In this example, in the DCT part 1, the motion image data of the frame structure supplied from the input terminal IN are divided into a plurality of blocks every one frame unit and a discrete cosine transform is carried out every block to obtain transform efficient groups including AC and DC components. This discrete cosine transform is carried out in a conventional manner, as disclosed in the aforementioned Japanese Patent applications No.87-313850 and No.90-184242. The transform coefficients output from the DCT part 1 are sent to the quantization part 3 through the frame buffer memory 2 and are quantized with a variable step size dynamically changed according to the produced code amount and the like. As to this quantization method, not a vector quantization method as disclosed in Japanese Patent application No. 87-313850 but a usual scalar quantization method as disclosed in Japanese Patent application No.90-184242 can be used. The quantized data output from the quantization part 3 is converted into variable length codes in the variable length coding part 4, and the obtained variable length codes are output to the produced code amount calculator 5 and a recording device such as a CD-ROM via the output terminal OUT. In the produced code amount calculator 5, one frame of code amount produced by a hybrid coding of a combination of the aforementioned discrete cosine transform, the quantization of the variable step size and the variable length coding is summed with the code amount up to the present frame to calculate the produced code amount, and the calculation result is sent to the target produced code amount calculator 7. In the target produced code amount calculator 7, a target produced code amount to be produced for the following one frame is calculated from the calculation result in the produced code amount calculator 5 and an instruction given from an external part and is output to the step size set part 8 and the preceding step size set part 12. In the AC level detector 6, one frame of the discrete cosine transform coefficients k(u,v) read out of the frame buffer memory 2 are input prior to the preceding quantization in the preceding quantization part 9, and a sum P of absolute values of all AC components except DC components is calculated to detect it as an AC level of its frame according to the following formula. P = Σ Σ |k(u,v)| The detected AC level is fed to the step size set part 8 and the preceding step size set part 12. In the quantization part 3, with the variable quantization step size determined by the step size set part 8, the transform coefficient read out of the frame buffer memory 2 is quantized. Now, assuming that the input to the quantization part 3 is A and the quantization step size is Qs, the output A' from the quantization part 3 is represented by the following formula. A' ≡ A / Qs In the quantization of the discrete cosine transform coefficients, as the quantization step size Qs is enlarged, the code amount (bit sum) of the compressed data after the variable length coding is reduced and the data compression rate is raised. This is the reason why the more the level of the quantization step size Qs is increased, the lower the output A' of the quantization part 3 and why the lower the level of the output A' of low appearance frequency in the following variable length coding part 4, the shorter the bit number of the code allotted. As described above, as the quantization step size Qs is enlarged, the data compression rate is increased, and on the contrary, as the quantization noise increases, the image quality of the reproduced image falls. Hence, although it is necessary to determine the quantization step in consideration of the data compression rate and the image quality deterioration allowance, the control and smoothing of the produced code amount are further considered in the compression coding system of this example. Relating to the motion image data, from the physiological characteristics of a viewer, it is known that the higher the space frequency of the AC component, the lower the image quality deterioration by the quantization noise can be allowed. In this example, the quantization step size Qs is determined by attention to the above-described visual characteristics so as to be a large value as the space frequency of the AC component becomes higher. Hence, the quantization step size Qs is disintegrated into a quantization matrix factor q which depends on an order of a space harmonic and a quantization scaling factor f, as represented by the following formula Qs ≡ q · f/k wherein k is constant. First, the quantization matrix factor q depends on only the order (u,v) of the space harmonic and is determined to a fixed value which increases as the order (u,v) of the space harmonic increases, as shown in Figs. 2A and 2B. For example, as shown in Fig. 2A, a quantization matrix factor q for a luminance signal (Y) is determined to a relatively small value, and as shown in Fig. 2B, another quantization matrix factor q for a chrominance signal (R-Y,B-Y) of which image quality deterioration is inconspicuous is determined to a relatively large value. The dynamic control of the quantization step size Qs is practised by the dynamic control of the quantization scaling factor f in the produced code amount calculator 5. Next, as shown in Fig. 3, the quantization scaling factor f is determined to a value which not only decreases at a certain AC level as the target produced code amount increases but also increases at a certain target produced code amount as the AC level of the discrete cosine transform coefficient increases. This is the reason why with the increase of the AC level, the amount of the data to be quantized increases, and the increase of the bit amount of the compressed data after the variable length coding is repressed to smooth the same. As described above, by determining the quantization step size Qs which increases with the increase of the AC level, even when the motion image pattern becomes fine to increase the AC level, the total bit amount of the compressed image data after the variable length coding can be maintained to almost a predetermined value. In the target produced code amount calculator 7, on the basis of the calculation result of the produced code amount calculator 5 and the instruction fed from the external portion, the target produced code amount to be produced for the next one frame is calculated as follows. From the input terminal IN, a total number F0 of the frames are supplied to the DCT part 1 at a fixed frame rate, for example, 30 frames/second. The required time for the data compression by the hybrid coding, that is, the total required time for the recording or reproducing becomes F0/30 seconds. Further, it is considered that a part of the input frames to be recorded is removed at a predetermined thinning rate to exclude from the compression object in the DCT part 1 and thus a frame rate and a bit rate of the compressed image data output from the output terminal OUT are Fr and Br, respectively. Hence, a total number F1 and a total bit amount Bt of the frames after the data compression by the coding are expressed by the respective following formulas. F1 = Fr(F0/30) Bt = Br(F0/30) At the same time of the coding start for the data, the calculations of the total number F1 of the coded frames and the total bit amount Bt produced by the coding are started, and it is considered that these two reach F2 and Bc, respectively, at the point the compression of a certain frame is finished, a remaining frame number F3 and a remaining produced allowable bit amount Bn of the compression object are obtained by the following formulas. F3 = F1 - F2 Bn = Bt - Bc When an allowable value of the bit amount produced by the compression of the next one frame is defined as a target produced code amount Bm, the target produced code amount Bm is obtained by the following formula. Bm = Bn/F3 The calculation of the target produced code amount Bm is performed on the basis of the total bit amount Bc calculated in the produced code amount calculator 5, the frame rate instructed from the external portion, the bit rate and the aforementioned formulas. As described above, with the quantization step size Qs set up by the step size set part 8, the quantization is executed in the quantization part 3 and then the variable length coding is taken place in the variable length coding part 4. However, due to the incompleteness of the characteristics shown in Fig. 3 and the like, the actual produced code amount is shifted from the target value and the variation of the produced code amounts among the frames can be caused. In this example, in order to avoid the produced code amount variation among the frames, prior to the final coding by the step size set part 8, the quantization part 3 and the variable length coding part 4, a preceding coding by the preceding step size set part 12, the preceding quantization part 9 and the preceding variable length coding part 10 is carried out. In this preceding coding, the step size is determined by the preceding step size set part 12 on the basis of the detected value obtained in the AC level detector 6 and the calculated value resulted by the target produced code amount calculator 7. The code amount actually produced by the preceding coding is calculated in the preceding produced code amount calculator 11, and the calculation result is fed to the step size set part 8. In the step size set part 8, the step size determined by the detected value obtained in the AC level detector 6 and the calculation value obtained in the target produced code amount calculator 7, that is, the step size set up in the preceding quantization is corrected on the basis of the error E between the calculation value in the preceding produced code amount calculator 11 and the target produced code amount, and the corrected step size is set up in the quantization part 3. One example of this correction method will be described as follows. That is, as exemplified in Fig. 3, from an intersect (a white circle mark in Fig. 3) between a detected value of an AC level and a target produced code amount, 5 as a scaling factor f is settled, and then a preceding quantization and a preceding variable length coding are carried out by the preceding quantization part 9 and the preceding variable length coding part 10, respectively, to obtain an error E (as shown by a dark circle mark) of which the calculation value in the preceding produced code amount calculator 11 exceeds the target value. Such an error occurrence is deemed to be attributed to the weakness of the relationship between the AC level and the scaling factor f, and the detected value of the AC level is adjusted to a value (adjusting value) imparting an intersect (a triangular mark) between a characteristic curve of scaling factor 5 and the calculated value. Accordingly, scaling factor 6 nearest to an intersect (a rectangular mark) between the adjusting value and the target produced code amount is determined on the final quantization. In this example, as described above, the two systems of the quantization part and the variable length coding part for the final and preceding codings, are provided, and the final coding for the present frame and the preceding coding for the following frame are simultaneously processed to enable shortening of the processing time. However, in case that a hardware scale reduction is preferential to the processing time reduction, the quantization part, the variable length coding part and the like can be used in common for the final and preceding codings by a time-sharing operation in multiple. In this example, as described above, the produced code amounts among the frames can be smoothed and hence cost reduction of a reproducing part side can be possible. In Fig. 4, there is shown the embodiment of the motion image data compression coding apparatus according to the present invention, having the same construction as the example shown in Fig. 1, except that a code amount controller 20 is provided in place of the members 5 to 12 of the example. In this embodiment, the code amount controller 20 dynamically controls a variable quantization step size to be set up in a quantization part 3 on the basis of AC components, a target code amount, a produced code amount and so forth so as to keep one frame of code amount produced by a hybrid coding of a combination of a DCT, a quantization and a variable length coding in the vicinity of a target produced code amount to be settled to the same value for all frames. In the code amount controller 20, a dynamic control of a quantization step size Qs can be realized by a dynamic control of a quantization scaling factor f. The code amount controller 20 comprises an AC level detector 21, an interframe difference detector 22a, a one frame delay 22b, a produced code amount calculator 23a, a produced code amount error detector 23b, an integration produced code amount error detector 24, an initial value set part 25, an adjusting factor calculator 26, a scaling factor adjustor 27a and a quantization step size set part 27b. The AC level detector 21 reads one frame of transform factors out of the frame buffer memory 2 and calculates the sum of the absolute values of all AC components to detect it as the AC component of its frame. The interframe difference detector 22a subtracts the present frame of the AC component G(n) directly sent from the AC level detector 21 from the preceding frame of the AC component G(n-1) fed from the AC level detector 21 through the one frame delay 22b to calculate an interframe difference G of the AC component and outputs the obtained interframe difference G to the adjusting factor calculator 26. The produced code amount calculator 23a calculates the actually produced code amount (bit sum of the data after the compression coding) B of the frames and outputs the calculation result B to the produced code amount error detector 23b. A target code amount M predetermined in advance as its target value is sent to the produced code amount error detector 23b and the initial value set part 25. The produced code amount error detector 23b subtracts the target code amount M from the actual produced code amount B fed from the produced code amount calculator 23a to obtain a subtracted value (B-M) and outputs the subtracted value (B-M) as a produced code amount error D to the integration produced code amount error detector 24 and the adjusting factor calculator 26. The integration produced code amount error detector 24 integrates the produced code amount error D from the coding start head frame to the preceding frame to obtain an integrated produced code amount error Z to be sent to the adjusting factor calculator 26. The adjusting factor calculator 26 receives the interframe difference G, the produced code amount error D and the integrated produced code amount error Z for the interframe difference detector 22a, the produced code amount error detector 23b and the integration produced code amount error detector 24, respectively, and calculates an adjusting factor α for adjusting the scaling factor predetermined for the preceding frame to obtain a scaling factor for the present frame as follows α ≡ D/k1 + Z/k2 + G/k3 wherein k1, k2 and k3 are constants. The adjusting factor calculator 26 outputs the obtained adjusting factor α to the scaling factor adjustor 27a. On the other hand, the initial value set part 25 receives the AC level of the transform factor detected for the head frame in the AC level detector 21 and the target code amount M and determines the initial value of the scaling factor f from a table shown in Fig. 5 containing a combination of the received two values to output the initial value of the scaling factor f to the scaling factor adjustor 27a. The scaling factor adjustor 27a multiplies the scaling factor f for the preceding frame by (1+ α) to obtain a scaling factor f' for the present frame in the following formula f' ≡ (1+α)f and outputs the obtained scaling factor f' to the quantization step size set part 27b. The quantization step size set part 27b prepares the quantization step size Qs from the scaling factor f' sent from the scaling factor adjustor 27a and the quantization matrix factors q as exemplified in Figs. 2A and 2B according to formula (3) and outputs the obtained quantization step size Qs to the quantization part 3. Then, in the quantization part 3, in respect of the head frame to be compression-coded, the quantization is carried out with the quantization step size Qs set up as described above in the scaling factor adjustor 27a. For the following frames to the head frame, the adjusting factor calculator 26 calculates another adjusting factor α again from the interframe difference G, the produced code amount error D and the integrated produced code amount error Z according to formula (9). The scaling factor adjustor 27a obtains another scaling factor from another adjusting factor α and the and the quantization step size set part 27b produces another quantization step size Qs to carry out the quantization in the quantization part 3 in the same manner as described above. Fig. 6 illustrates the result of the compression coding in the motion image data compression coding apparatus shown in Fig. 4. In Fig. 6, the horizontal and vertical axes exhibit the frame number and the calculation value of the actually produced code amount (K bit) for each frame, respectively. In this embodiment, the Figs. 2A and 2B and Fig. 5 are used for the quantization, and the target produced code amount is determined to 100 kbits. The constants k1, k2 and k3 in formula (9) are settled to 10000, 4000 and 5000, respectively. Further, as the motion data, the original video signals which are thinned out to 1/3 of the total frames are used, and as to the AC level, the sum of the absolute values of all AC components is used. Fig. 7 shows the variation of the interframe difference G of the AC component with reference to the frame number. Since the original video signals are thinned out to 1/3 of all frames, the adjacent frames to be subjected to the compression coding are aligned at an interval of three times as many as the original frame interval, a fairly large interframe difference G is caused. In spite of the changes of the code amount and the compression efficiency suggested by the occurrence of such a large interframe difference G of the AC component, a good smoothing can be realized, as shown in Fig. 6.

Claims: A motion image data compression coding apparatus for compressing a data amount by successively carrying out an orthogonal transform, a quantization and a variable length coding of motion image data of one frame structure, comprising; AC level detector means (21) for detecting an AC level reflecting an amplitude size of AC components of transform coefficients produced by the orthogonal transform every one frame; interframe difference detector means (22a, 22b) for detecting an interframe difference G of the AC level between a preceding frame and a present frame, detected by the AC level detector means; code amount error detector means (23a, 23b) for detecting a code amount error difference D between a code amount of compressed coded data produced every frame by the variable length coding and a target code amount as a target value of the code amount; addition code amount error detector means (24) for adding up the code amount error differences D from a head frame through the present frame to detect an addition code amount error Z; initial value set means (25) for setting an initial value of a scaling factor f relating to a quantization step size from a combination of the AC level of the detected transform coefficients of the head frame and the target code amount; scaling factor adjusting means (26, 27a) for adjusting the scaling factor f set for the preceding frame on the basis of the code amount error D, the addition code amount error Z and the interframe difference G; and calculator means (27b) for calculating the quantization step size in the quantizing from a product of the scaling factor f adjusted by the scaling factor adjusting means and quantization matrix factors q set for the AC components of the transform coefficients. The apparatus as claimed in claim 1, wherein the scaling factor adjusting means includes an adjusting factor calculator means for calculating an adjusting factor α for adjusting a scaling factor set for the present frame by multiplying the scaling factor f for the preceding frame by (1+α) according to the following formula α = D/k1 + Z/k2 + G/k3 wherein k1, k2 and k3 are constants. The apparatus as claimed in claim 1 or 2, wherein the AC level detector means detects a sum of absolute values of levels of the AC components of the transform coefficients as the AC level.

Inventors: KUNIHIRO HIDETO, KUNIHIRO, HIDETO, Kunihiro, Hideto, c/o NEC Home Elec., Ltd.
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1992
Main classification: H04N7
IPC codes: H04N7