JP2007116655A - Moving picture coding apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a moving picture coding apparatus, for performing moving picture coding according to a single-path variable bit rate control system, which reduces image quality deterioration in a sequence hard to code in such a case that a sequence hard to code and a sequence easy to code alternately continue, and is capable of suppressing an image quality difference between the sequence easy to code and the sequence hard to code. <P>SOLUTION: A coding excess/deficiency code amount calculation unit 201 calculates a too-much/too-little code amount by subtracting a reference target code amount for each GOP from a generated code amount for each GOP. If a buffer fullness rate is 0 or more and the excess/deficiency code amount is 0 or more, a code distribution rate calculation unit 203 calculates a code amount distribution rate from the buffer fullness rate in accordance with a first non-linear function and if the buffer fullness rate is 0 or more and the excess/deficiency code amount is negative, the code amount distribution rate is calculated from the buffer fullness rate in accordance with a second non-linear function. If the buffer fullness rate is negative, the code amount distribution rate is defined as 0. In accordance with the code amount distribution rate, a rate of change of a quantization step to be supplied to a quantization unit 103 is set smaller than conventional one. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a moving picture coding apparatus, and more particularly to a moving picture coding apparatus suitable for performing variable bit rate coding control in real time.

  When transmitting or storing a moving image signal, it is often compressed and encoded into a bit stream for efficiency. As a moving image compression encoding method, a method such as MPEG (Moving Picture Experts Group) has been standardized and widely used. Therefore, it is important for efficient use of a transmission system or a storage system to compress a moving image signal by a method such as MPEG and handle it as a bit stream.

  In a system such as MPEG, a constant bit rate (CBR) control system in Test Model 5 (TM5) of MPEG-2 is known as a control system that keeps a generated code amount constant within a certain unit time. . However, generally, since the moving image signal is not stationary, the information amount of the input moving image signal changes with time. For this reason, it is known that, when an input moving image signal is encoded, if a variable bit rate (VBR) control method is used, a higher image quality can be realized with the same code amount than a fixed bit rate control method.

  In the above variable bit rate control method, the amount of code to be allocated is set based on the amount of code generated when the entire sequence is provisionally encoded using a fixed quantization width, and this encoding process is executed. There is a “two pass” variable bit rate control scheme. This “two-pass” variable bit rate control method performs encoding after knowing in advance the encoding characteristics of the entire sequence, so that high image quality can be realized.

  However, in the above-described “2-pass” variable bit rate control method, it is necessary to perform provisional encoding processing and main encoding processing, and the processing amount is about twice that of the main encoding processing alone. Therefore, it is not suitable for a device that encodes a moving image signal transmitted by means such as broadcasting or communication in real time and records it on a recording medium such as an optical disk or a magnetic disk.

  Therefore, there is a “one-pass” variable bit rate control method as a method for realizing and encoding variable bit rate control in real time. This “one-pass” variable bit rate control method monitors the amount of generated code per unit time, compares it with the allocated code amount per unit time, and calculates the allocated code amount of the next coding unit, The quantization width of the next coding unit is changed according to the calculated allocated code amount.

  As a conventional example of the “one-pass” variable bit rate control method, a moving picture coding characterized in that a quantization width is adjusted for each image unit from an excess or deficiency between a generated code quantity and a preset average bit rate. An apparatus is conventionally known (see, for example, Patent Document 1 and Patent Document 2). Specifically, the “one-pass” variable bit rate control method in the above-described conventional video encoding apparatus sets a target bit rate in proportion to the buffer occupancy and approaches the target bit rate. Such a quantization width is set.

  In addition, as a conventional moving picture coding apparatus adopting another one-pass variable bit rate control method, a moving picture code of a method in which a margin that is a margin of a generated code amount is set and a margin is distributed at a constant rate An apparatus is also known (see, for example, Patent Document 3).

JP 2000-261800 A Republished patent WO 00/68542 Japanese Patent Laid-Open No. 2001-25015

  However, in the conventional one-pass variable bit rate control method described in Patent Document 1 or Patent Document 2, the buffer occupancy and the target bit rate are in a proportional relationship. In the conventional moving image encoding device described above, in order to allocate a margin at a certain ratio, when a sequence that is difficult to encode and a sequence that is easy to encode continue alternately, In the latter half part, the conventional video encoding device described in Patent Document 1 or Patent Document 2 increases the buffer occupancy, and the conventional video encoding device described in Patent Document 3 uses the allocated code amount due to the margin reduction. As a result of the decrease, the quantization width increases, and there is a problem that image quality deterioration is conspicuous in order to forcibly suppress the generated code amount.

  In addition, in the conventional moving image encoding device described in each of the above-mentioned patent documents, when the buffer occupancy decreases, that is, in the case of a sequence that is easy to encode, an attempt is made to reduce the quantization width. You will use the amount. Therefore, in the conventional moving image encoding apparatus described in each of the above patent documents, the image quality between the sequences is such that the sequence that is difficult to encode has a large image quality deterioration, and the sequence that is easy to encode extremely improves the image quality. There is a problem that a difference occurs.

  The present invention has been made in view of the above points. In the case where a sequence that is difficult to encode and a sequence that is easy to encode continue alternately, image quality degradation in the sequence that is difficult to encode is reduced, and It is an object of the present invention to provide a moving picture coding apparatus that performs moving picture coding by a one-pass variable bit rate control method that can suppress a difference in image quality between a simple sequence and a difficult sequence.

In order to achieve the above object, the present invention generates a prediction signal in a predetermined encoding unit by a prediction unit from a moving image signal to be encoded and a reference image signal, and the prediction signal and the moving image signal The difference signal is orthogonally transformed by an orthogonal transformation unit to generate an orthogonal transformation coefficient, and a reference image signal is generated by a reference image signal generation unit from a signal obtained by quantizing the orthogonal transformation coefficient based on an external quantization step, and A coding unit that performs variable-length coding by the variable-length coding unit and outputs a coded signal; a generated code amount for each predetermined image group coding unit in the variable-length coding unit; and a predetermined reference target code amount A quantization step for supplying to the encoding unit such that the generated code amount for each predetermined image group encoding unit in the variable length encoding unit becomes the reference target code amount according to the excess / deficiency code amount that is the difference between Variable control A more composed video encoding apparatus and Goka control unit,
The encoding control unit calculates an excess / deficiency code amount by subtracting a reference target code amount for each image group coding unit from a generated code amount for each image group coding unit in the variable length coding unit. And the buffer filling rate, which is the ratio between the buffer filling amount obtained by integrating the excess and deficient code amount and the allowable buffer size of the buffer that stores the code for each image group coding unit in the variable length coding means A buffer filling rate calculating means, and when the buffer filling rate is 0 or more and the excess / deficiency code amount is 0 or more, the code amount allocation rate is calculated from the buffer filling rate according to the first nonlinear function, and the buffer filling rate Is 0 or more, and when the excess / deficiency code amount is negative, the code amount distribution rate is calculated from the buffer filling rate according to the second nonlinear function. When the buffer filling rate is negative, the code amount distribution rate is set to 0. Code amount distribution rate calculation A target code amount calculating means for calculating a target code amount for the current image group coding unit from the code amount distribution rate and excess / deficient code amount calculated by the code amount distribution rate calculating means, and a target code amount calculation Quantization step calculation means for calculating a quantization step from the ratio between the target code amount of the current image group coding unit calculated by the means and the target code amount of the previous image group coding unit. The first nonlinear function is a function that increases the code amount allocation rate nonlinearly as the buffer filling rate increases, and the second nonlinear function is a function that decreases the code amount allocation rate nonlinearly as the buffer filling rate increases. It is a function to be made.

  In this invention, an excess / deficiency code amount calculated by subtracting a reference target code amount for each image group coding unit from a generated code amount for each image group coding unit, and a buffer sufficient amount obtained by integrating the excess / deficiency code amount And the buffer filling rate calculated from the allowable buffer size of the buffer for storing the code for each image group coding unit, when the buffer filling rate is 0 or more and the excess / deficiency code amount is 0 or more, The code amount allocation rate is calculated from the buffer filling rate according to the first nonlinear function. When the buffer filling rate is 0 or more and the excess / deficiency code amount is negative, the code amount from the buffer filling rate according to the second nonlinear function is calculated. The allocation rate is calculated, the target code amount of the current image group coding unit is calculated from the code amount allocation rate and the excess / deficiency code amount, and the quantum amount is calculated from the target code amount of the previous image group coding unit. To calculate the conversion step Since the, after calculating the bit allocation rate in accordance with monotonically increasing function or a monotonically decreasing function, as compared with the prior art in which to calculate the target code amount and the quantization step, can reduce the change rate of the quantization step.

  That is, according to the present invention, when the buffer filling rate is 0 or more and the excess / deficiency code amount is 0 or more, the code is encoded according to the first nonlinear function that increases the code amount allocation rate nonlinearly as the buffer filling rate increases. By calculating the amount allocation rate, the rate of change of the quantization step is reduced compared to the conventional method, and when the buffer filling rate is 0 or more and the excess / deficiency code amount is negative, the code is increased as the buffer filling rate increases. By calculating the code amount distribution rate according to the second non-linear function that decreases the amount distribution rate non-linearly, the change rate of the quantization step is reduced as compared with the conventional case.

  In order to achieve the above object, the present invention provides an image target code amount for calculating the image target code amount included in the current image group coding unit calculated from the target code amount. The code output from the calculation means, the fixed bit rate quantization step calculation means for calculating the fixed bit rate quantization step in a predetermined coding unit according to the image target code amount, and the code output from the variable length coding means are predetermined. Buffer capacity monitoring means for monitoring whether or not the accumulated code amount of the buffer is less than a predetermined capacity related to the image target code amount, and the buffer capacity monitoring means If it is determined that the fixed bit rate quantization step calculation means calculates, the fixed bit rate quantization step calculated by the fixed bit rate quantization step calculating means is output to the encoding unit. When it is determined that the accumulated code amount of the buffer exceeds a predetermined capacity, the quantization step switching output for outputting the variable bit rate quantization step calculated by the quantization step calculation means to the encoding unit And a means further comprising a means.

  In this invention, when performing quantization parameter control in units of image group encoding in the variable bit rate quantization step, when it is determined that the accumulated code amount of the buffer is below a predetermined capacity, Since the fixed bit rate quantization step is output to the encoding unit and operated at the fixed bit rate, the variable bit rate can be controlled while maintaining the specified buffer capacity.

  According to the present invention, when the buffer filling rate is 0 or more and the excess / deficiency code amount is 0 or more, the code is encoded according to the first non-linear function that increases the code amount distribution rate non-linearly as the buffer filling rate increases. By calculating the amount distribution rate, the change rate of the quantization step is reduced as compared with the conventional case. Therefore, when a difficult sequence and a simple sequence continue alternately, the buffer fullness rate is 0 or more. In addition, the excess / deficiency code amount is 0 or more, and in this case, it is possible to reduce the rate of change of the quantization step in a difficult sequence, thereby reducing the image quality degradation in the latter half of the difficult sequence. It is possible to suppress.

  According to the present invention, when the buffer filling rate is 0 or more and the excess / deficiency code amount is negative, according to the second nonlinear function that nonlinearly decreases the code amount distribution rate as the buffer filling rate increases. By calculating the code amount distribution rate, the change rate of the quantization step is reduced compared to the conventional case, so that the buffer fullness rate such that a sequence that is easy to encode (simple) continues is 0 or more, and Even when the excess / deficiency code amount is negative, the change rate of the quantization step can be made smaller than before, so that the quantization step can be made smaller than necessary and no extra code amount is used. The phenomenon that the image quality is extremely improved in an easy sequence can be suppressed, and the image quality can be maintained between sequences. In the present invention, a one-pass variable bit rate control system having the above effects can be realized.

  Furthermore, according to the present invention, when the quantization parameter control is performed in units of image group coding in the variable bit rate quantization step, it is determined that the accumulated code amount of the buffer is less than a predetermined capacity. In this case, the variable bit rate is controlled while maintaining the specified buffer capacity by outputting the fixed bit rate quantization step to the encoding unit and operating it at the fixed bit rate. It is possible to generate a bit stream that conforms to the standard with higher image quality than in the past.

  Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of a moving picture encoding apparatus according to the present invention. However, in the present embodiment, it is assumed that the image group coding unit is GOP (Group Of Pictures), which is a set of several images, and the image type coding unit is one picture. Also, in this specification, “image group” is a set of coding units for each image type, and “image type” is an intra-screen prediction image (I-picture), an inter-screen forward prediction image. (P-picture) is an inter-screen bidirectional prediction image (B-picture).

  As shown in FIG. 1, the moving image encoding apparatus according to the present embodiment includes a moving image encoding unit 100 that encodes an input moving image signal, and encoding that performs variable bit rate control of the encoded moving image signal. And a control unit 200. The moving image encoding unit 100 includes a prediction unit 101, an orthogonal transform unit 102, a quantization unit 103, a variable length encoding unit 104, an inverse quantization unit 105, an inverse orthogonal transform unit 106, an image memory unit 107, and a subtractor 108. Composed. The encoding control unit 200 includes an encoding excess / deficiency code amount calculation unit 201, a buffer fullness rate calculation unit 202, a code distribution rate calculation unit 203, a target bit rate calculation unit 204, a target code amount calculation unit 205 by image type, an image type, It is composed of another quantization step calculation unit 206.

  Next, the operation of the moving picture coding apparatus in FIG. 1 will be described. First, the operation of the moving image encoding unit 100 will be described. The moving image encoding unit 100 receives the moving image signal and the quantization step from the encoding control unit 200 as input, and outputs the encoded bit stream as a moving image signal with the same configuration as the encoding device in MPEG or the like. Is encoded. The moving image signal input to the moving image encoding unit 100 uses a reference image signal, which will be described later in the predicting unit 101, and the predicted image signal by intra-screen prediction or inter-screen prediction with past and future decoded images. On the other hand, it is supplied to the orthogonal transform unit 102 through the subtractor 108.

  When generating an intra-screen prediction image, the orthogonal transform unit 102 performs orthogonal transform on the moving image signal input through the subtracter 108 to generate an inter-screen forward prediction image or an inter-screen bidirectional prediction image. When subtracting 108, the subtracter 108 performs orthogonal transform on the difference signal between the input moving image signal (original image) and the predicted image generated by the prediction unit 101, and converts the difference signal to a frequency coefficient (orthogonal transform coefficient). The quantization unit 103 quantizes the frequency coefficient input from the orthogonal transform unit 102 according to a quantization step from the encoding control unit 200 described later, and converts the frequency coefficient into a quantization coefficient.

  The variable length coding unit 104 performs entropy coding or the like on the quantized coefficient input from the quantization unit 103 to convert it into a code string, generates a coded bit stream of a moving image signal, and outputs the coded bit stream to the outside. Output. Further, the above quantization coefficient is inversely quantized by the inverse quantization unit 105, then inversely orthogonally transformed by the inverse orthogonal transform unit 106, and then added to the predicted image signal from the prediction unit 101 by the adder 109. Thus, a decoded image signal is obtained. The decoded image signal is accumulated in the image memory unit 107 and used as a past / future reference image signal in the prediction unit 101.

  Next, the operation of the encoding control unit 200 will be described. The encoding control unit 200 performs variable bit rate control in which an encoding generated code amount is input and a quantization step is output. The encoding excess / deficiency code amount calculation unit 201 generates a generated code based on a preset target bit rate and the amount of encoded generation code input from the variable length encoding unit 104 of the moving image encoding unit 100. The difference code amount with respect to the target code amount is obtained, and the difference code amount is output to the buffer fullness rate calculation unit 202. The buffer fullness rate calculation unit 202 obtains the fullness of the encoding buffer in the variable length encoding unit 104 from the above differential code amount, and calculates the buffer fullness with respect to the size of the encoding buffer.

  The code allocation rate calculation unit 203 calculates a variable code amount of the target bit rate for the next image group coding unit of the code amount overflowing from the target bit rate, based on the buffer full rate calculated by the buffer full rate calculation unit 202. . The target bit rate calculation unit 204 calculates the target bit rate of the next image group coding unit based on the variable code amount calculated by the code distribution rate calculation unit 203 and the target bit rate.

  The target code amount calculation unit 205 for each image type obtains the target code amount for each image type from the target bit rate of the next image group encoding unit calculated by the target bit rate calculation unit 204, and the next image group A target code amount of a coding unit for each image type in the coding unit is calculated. The quantization step calculating unit 206 for each image type includes a target code amount for each image type in the encoded image group encoding unit and a target for the next encoding unit for each image type calculated by the target code amount calculating unit 205 for each image type. The next image type quantization step is calculated from the code amount. The calculated quantization step for each next image type is input to the quantization unit 103 of the moving image encoding unit 100, and quantization is performed by the quantization step.

  Next, the operation of the encoding control unit 200 will be described in more detail. However, hereinafter, the image group coding unit is referred to as GOP. An image is simply called a picture. The unit of code amount and buffer is [bits], and the unit of bit rate is [bps]. In the encoding control unit 200, first, an operation according to the flowchart of FIG.

  That is, the encoding excess / deficiency code amount calculation unit 201 calculates the GOP reference target code amount (T_int) by the following equation based on a predetermined average bit rate (reference target bit rate: TB_int) (FIG. 2). Step S10).

T_int = TB_int × N / PR (1)
In equation (1), PR is the picture rate (fixed value), and N is the number of pictures constituting the encoded GOP.

  Subsequently, the encoding excess / deficiency code amount calculation unit 201 uses the calculated reference target code amount (T_int) and the GOP generation code amount (S_gop) from the variable length encoding unit 104 in the moving image encoding unit 100. The GOP excess / deficiency code amount (dS_gop) is obtained (step S11 in FIG. 2). The GOP excess / deficiency code amount (dS_gop) is given by the difference between the GOP generation code amount (S_gop) and the GOP reference target code amount (T_int). Therefore, the excess / deficiency code amount dS_gop of GOP is given by the following equation.

dS_gop = S_gop-T_int (2)
However, in (2), the initial value of dS_gop is 0.

  As can be seen from the equation (2), if the GOP excess / deficiency code amount dS_gop is positive, it means that encoding is difficult and the generated code amount exceeds the target code amount, and if it is negative, encoding is not possible. This means that the generated code amount is less than the target code amount.

  Next, the buffer fullness rate calculation unit 202 operates according to the flowchart of FIG. First, the buffer fullness calculation unit 202 calculates the buffer fullness (BktFlns) in the variable length coding unit 104 from the GOP excess / deficiency code amount (dS_gop) (step S20 in FIG. 3). Since this buffer fullness (BktFlns) is the sum of the GOP excess / deficiency code amount, it is given by the following equation.

BktFlns = min (BktBufSize, max (-BktBufSize, BktFlns-dS_gop)) (3)
However, in the formula (3), min (a, b) is a if a ≦ b, b is returned if b <a, and max (a, b) is a if a ≧ b and if a <b. Returns b. The initial value of BktFlns is 0, and BktBufSize indicates the allowable buffer size of the buffer. In addition, the following inequality is established.

-BktBufSize≤BktFns≤BktBufSize
Subsequently, the buffer filling rate calculation unit 202 obtains a buffer filling rate (BktFnsRate) based on the calculated buffer filling amount (BktFlns) (step S21). Here, since the buffer fullness rate (BktFlnsRate) is a ratio of the buffer fullness (BktFlns) to the buffer allowable buffer size (BktBufSize), it is given by the following equation.

BktFnsRate = BktFlns / BktBufSize (4)
However, the initial value of BktFnsRate is 0. Further, -1 ≦ BktFnsRate ≦ 1.

  Next, the code distribution rate calculation unit 203 operates according to the flowchart of FIG. The code allocation rate calculation unit 203 obtains the code rate allocation rate using the function representing the relationship between the buffer full rate (BktFnsRate) and the code rate allocation rate (BitDistRate) from the buffer full rate calculation unit 202 (FIG. 4). Steps S30 to S35). That is, the code allocation rate calculation unit 203 first determines whether or not the buffer fullness rate (BktFlnsRate) is 0 or more (step S30 in FIG. 4), and when it is 0 or more, the GOP excess / deficiency code amount ( It is determined whether or not (dS_gop) is 0 or more (step S31 in FIG. 4).

  When the GOP excess / deficiency code amount (dS_gop) is 0 or more, the code amount accumulated in the buffer in the variable-length encoding unit 104 when the buffer is accumulated in the positive direction from the equation (2), In this case, the code amount distribution rate (BitDistRate) increases as the time increases (step S32 in FIG. 4). That is, in step S32, since a difficult sequence continues and the code amount (GOP generation code amount) accumulated in the buffer increases with time, the code amount distribution rate (BitDistRate) is increased to increase the rate. To do.

  On the other hand, when it is determined in step S31 that the GOP excess / deficiency code amount (dS_gop) is negative, if the code amount hardly accumulates in the positive direction in the buffer, that is, the code amount accumulated in the buffer (GOP occurrence) This is a case where the code amount) does not increase or decreases with time, and the code amount distribution rate (BitDistRate) is increased as the code tends to decrease (step S33 in FIG. 4). That is, when the buffer fullness rate (BktFlnsRate) is positive and the absolute value is small and the GOP excess / deficiency code amount (dS_gop) is negative, the code amount is accumulated in the buffer in the positive direction, This is a case where the code amount decreases. At this time, in order to provide a buffer with a margin for encoding a difficult sequence, in step S33 described above, an attempt is made to reduce the code amount distribution rate (BitDistRate).

  When it is determined in step S30 that the buffer fullness rate (BktFnsRate) is negative, the code amount distribution rate (BitDistRate) is set to 0 (step S34 in FIG. 4). That is, when it is determined in step S30 that the buffer fullness rate (BktFnsRate) is negative, the code amount stored in the buffer in the variable length encoding unit 104 accumulates in the negative direction. When the accumulated code amount decreases with the passage of time, it is considered that a simple sequence continues, and the code amount distribution rate (BitDistRate) is set to 0 in the above step 34, so that the reference target rate is always maintained. Set it.

  Thus, by avoiding allocation in the direction of increasing the rate, a large amount of code can be allocated when switching to the difficult sequence again. The code distribution rate calculation unit 203 outputs the code amount distribution rate (BitDistRate) calculated by the processing in step S32, S33, or S34 described above (step S35 in FIG. 4).

  When the code amount distribution rate (BitDistRate) calculated in the above steps S32, S33, and S34 is expressed by the function f, it is expressed as follows. Here, for simplification in function notation, BktFnsRate = x and BitDistRate = y.

(I) When BktFnsRate ≧ 0 and dS_gop ≧ 0 (S30 Yes, S31 Yes, S32, S35 in FIG. 4)
y = f (x) (5)
(Ii) When BktFnsRate ≧ 0 and dS_gop <0 (S30 Yes, S31 No, S33, S35 in FIG. 4)
y = f (−x + 1) (6)
(Iii) When BktFnsRate <0 (S30 No, S34, S35 in FIG. 4)
y = 0 (7)
For example, when f is a linear function, it is expressed as follows.

f (x) = x (8)
Therefore, when the code amount distribution rate (BitDistRate = y) with respect to the buffer fullness rate (BktFlnsRate = x) is linearly expressed, it is expressed as shown in FIG. That is, since the straight line of y = x shown by the solid line in FIG. 8 is a straight line obtained by substituting the equation (8) into the equation (5), the relationship represented by the following equation is shown.

BitDistRate = BktFlnsRate (0 ≦ BitDistRate ≦ 1, dS_gop ≧ 0) (9)
In addition, since a straight line y = −x + 1 indicated by a dotted line in FIG.

BitDistRate = −BktFlnsRate + 1 (0 ≦ BitDistRate ≦ 1, dS_gop <0) (10)
On the other hand, when the above f is an exponential function that is a nonlinear function, for example, it is expressed as follows.

f (x) = α · (β ^x −1) (β = (α + 1) / α) (11)
When there is not much code in the buffer, to distribute less in the direction of decreasing rate (DS_GOP> 0) and to distribute more in the direction of increasing rate (DS_GOP <0), use f as an exponent. It is desirable to determine appropriate α and β as functions. For example, if α = 1, β = 2 from equation (11), so the exponential function f is expressed as follows.

f (x) = 2 ^x −1 (12)
Therefore, when the code amount distribution rate (BitDistRate = y) with respect to the buffer fullness rate (BktFnsRate = x) is set to a non-linear relationship represented by the equation (12), it is expressed as shown in FIG. That is, since the curve of y = ^2x- 1 shown with a continuous line in FIG. 9 is a curve obtained from Formula (5) and Formula (12), the relationship represented by the following formula is shown.

BitDistRate = 2 ^BktFlnsRate −1 (0 ≦ BitDistRate ≦ 1, dS_gop ≧ 0) (13)
Moreover, since the curve of y = 2 ^{(-x + 1)} -1 shown with a dotted line in FIG. 9 is a curve obtained from (6) Formula and (12) Formula, the relationship represented by following Formula is shown. .

BitDistRate = 2 ^{(-BktFlnsRate + 1)} −1 (0 ≦ BitDistRate ≦ 1, dS_gop <0) (14)
In the present embodiment, the function f of the code amount distribution rate (BitDistRate) based on the excess / deficiency code amount (dS_gop) is calculated from the buffer filling rate (BktFlnsRate) and the reference target bit rate (TB_int) as described above. By setting the exponential function, the target bit rate (TB_gop) is calculated and variable bit rate control is performed.

  Next, the operation of the target bit rate calculation unit 204 in FIG. 1 will be described with reference to the flowchart in FIG. The target bit rate calculation unit 204 calculates the target code amount (T_gop) of the current GOP from the code amount distribution rate (BitDistRate) and the GOP excess / deficiency code amount (dS_gop) (steps S40 to S42 in FIG. 5).

  That is, the target bit rate calculation unit 204 adds the code amount distribution rate (BitDistRate) to the excess / deficiency code amount (dS_gop) of the GOP to add to the target code amount (T_gop) of the next GOP. Since the amount is (dDistBit), first, the distribution code amount (dDistBit) is calculated by the following equation (step S40 in FIG. 5).

dDistBit = dS_gop × BitDistRate (15)
However, the initial value of dDistBit is 0.

  Therefore, the distribution rate (dDistRate) is expressed as follows with respect to the number of pictures N of the current (to be encoded) GOP.

dDistRate = dDistBit × PR / N (16)
Here, when the buffer fullness (BktFlns) is positive and the previous GOP code amount exceeds the target code amount (T_gop), the distribution rate (dDistRate) is 0 or less. That is, when the previous GOP code amount (S_gop) exceeds the target code amount (T_gop) (S_gop> T_gop), dS_gop is negative from the equation (2). Since the code amount distribution rate (BitDistRate) is always 0 or more from the equation (13), the distribution code amount (dDistBit) is 0 or less from the equation (15). Therefore, the distribution rate (dDistRate) is 0 or less from the equation (16). Similarly, when the buffer fullness (BktFlns) is positive and the previous GOP code amount is lower than the target code amount (T_gop), the distribution rate (dDistRate) is positive.

  Subsequently, the target bit rate calculation unit 204 calculates the target bit rate (TB_gop) of the current GOP (step S41 in FIG. 5). Here, the target bit rate (TB_gop) of the current GOP is obtained by adding the distribution bit rate (dDistRate) to the target bit rate of the previous GOP, and is expressed by the following equation.

TB_gop = TB_gop + dDistRate (17)
Then, the target bit rate calculation unit 204 calculates the target code amount (T_gop) of the current GOP using the following equation using the target bit rate of the current GOP calculated in step S41 (step S42 in FIG. 5).

T_gop = TB_gop × N / PR (18)
However, in the above formula, the initial value of T_gop is T_int.

  Next, the operation of the image type target code amount calculation unit 205 in FIG. 1 will be described with reference to the flowchart shown in FIG. The target code amount calculation unit 205 for each image type includes the target code amount (T_gop) of the current GOP calculated by the equation (18) in the target bit rate calculation unit 204 and the picture complexity from the variable length encoding unit 104. Using (X_pic), a target code amount (T_pic) is calculated for each image type (step S50 in FIG. 6).

  Here, when the picture complexity (X_pic) is a product of a generated code amount of a picture and an average value of quantization parameters in the picture, for example, when conforming to Test Model 5 in MPEG-2, The target code amount (T_pic) is obtained for each image type using the equations (19) to (21).

  The “quantization parameter” is an encoding parameter corresponding to the quantization step value. The quantization parameter and the quantization step are, for example, a linear relationship in MPEG-1 and a non-linear relationship in MPEG-2, MPEG-4 AVC, and the like. However, (I), (P), and (B) in the following equations (19) to (21) indicate variables of I picture, P picture, and B picture. Kp and Kb are ratios of the quantization parameter of the P picture and the B picture with respect to the quantization parameter of the I picture, for example, Kp = 1.0 and Kb = 1.4. N_pic is the number of encoded pictures for each image type.

T_pic (I) = T_gop / Z (I) (19)
Z (I) = N_pic (I) + {(N_pic (P) × X_pic (P)) / (X_pic (I) × Kp)}
+ {(N_pic (P) × X_pic (P)) / (X_pic (I) × Kp)}
T_pic (P) = T_gop / Z (P) (20)
Z (P) = {(N_pic (I) × X_pic (I) × Kp) / X_pic (P)} + N_pic (P) + {(N_pic (B) × X_pic (B) × Kp) / (X_pic (P) × Kb)}
T_pic (B) = T_gop / Z (B) (21)
Z (B) = {(N_pic (I) × X_pic (I) × Kb) / X_pic (B)}
+ {(N_pic (P) × X_pic (P) × Kb) / (X_pic (B) × Kp) + N_pic (B)}
Next, the operation of the image type-specific quantization step calculation unit 206 will be described with reference to the flowchart of FIG. The quantization step calculating unit 206 for each image type calculates the target code amount for each image type (T_pic (n): n) calculated by the equation (19) to the equation (21) by the image type target code amount calculation unit 205. Is a quantization step for each image type based on the ratio between the GOP number) and the target code amount for each image type (T_pic (n-1)) of the previous GOP (step S60 in FIG. 7).

  That is, taking MPEG-4 AVC as an example, since the quantization step and the generated code amount are in a proportional relationship, the image type-specific quantization step calculation unit 206 performs the image type-specific target code amount (T_pic (n): The quantization step is set for each image type according to the ratio between n is a GOP number) and the target code amount (T_pic (n-1)) for each image type of the previous GOP. That is, assuming that the quantization step of GOP number n is Qstep_gop (n) and the quantization step of GOP number (n−1) is Qstep_gop (n−1), the quantization step calculation unit 206 for each image type has the GOP number n Image type target code amount T_pic (n) and GOP number (n-1) image type target code amount T_pic (n-1), and the image type quantization step according to the following equation: calculate.

Qstep_gop (n) = Qstep_gop (n-1) × (T_pic (n-1) / T_pic (n)) (22)
The quantization step by image type calculated by the image type quantization step calculation unit 206 for each picture type according to the equation (22) is input to the quantization unit 103 in FIG. 1 and supplied from the orthogonal transform unit 102 here. Quantize the frequency conversion coefficient to be performed. As a result, the quantization coefficient generated by the quantization unit 103 is variable-length encoded by the variable-length encoding unit 104 to generate an encoded bit stream of the moving image signal.

  Note that the quantization step obtained above may be calculated for each picture in the GOP, or may be calculated for each macroblock which is a coding unit in the picture. As these calculation methods, for example, a method such as Test Model 5 in MPEG-2 may be applied, or another method may be used.

  As described above, in the present embodiment, the code amount distribution rate (BitDistRate) is calculated from the buffer fullness rate (BktFlnsRate) according to an exponential function that is a predetermined nonlinear function, and the code amount distribution rate (BitDistRate) and the GOP The target code amount (T_gop) of the current GOP is calculated from the excess / deficiency code amount (dS_gop), and the quantization step is performed according to the ratio between the target code amount (T_gop) of the current GOP and the target code amount of the previous GOP And the variable bit rate control is performed by quantizing the moving image signal in the quantization step. Therefore, according to the present embodiment, for example, when the buffer fullness rate (BktFlnsRate) is small, the code amount distribution rate (BitDistRate) is made smaller than the conventional one by setting the above exponential function. The rate of change can be made smaller than the conventional proportional relationship.

  That is, the code amount distribution rate based on the excess / deficiency code amount from the reference target bit rate is expressed based on the equation (9) in the conventional proportional relationship, whereas in the present embodiment (13) Since it is set exponentially based on the equation, the change rate of the quantization step can be reduced when the buffer fullness is low.

This will be described with a specific example. Examples buffer fill rate (BktFlnsRate) is small, for example when it is BktFlnsRate = 0.1, the code allocation ratio in this embodiment (BitDistRate) is BitDistRate from (13) = (2 ^0.1) -1 = 0.07. On the other hand, conventionally, BitDistRate = 0.1 from the equation (9). Therefore, the code amount distribution rate (BitDistRate) is smaller in the present embodiment.

  In addition, when the GOP excess / deficiency code amount (dS_gop) is negative, that is, when the GOP generated code amount exceeds the target code amount (T_gop), the distribution rate (dDistRate) is negative from Equations (15) and (16). Thus, from the equation (17), the current GOP target bit rate (TB_gop) is smaller than the previous GOP target bit rate. For example, assuming that dS_gop = −100000 [bits], PR = 30 [Hz], N = 15, and the previous GOP target bit rate (TB_gop) is 300000 [bits], in this embodiment, the code amount allocation rate Since (BitDistRate) is 0.07 as described above, the allocated code amount (dDistBit) is −7000 [bits] (= −100,000 × 0.07) from the equation (15). On the other hand, since the code amount distribution rate (BitDistRate) is 0.1 as described above, the distribution code amount (dDistBit) is −10000 [bits] (= −100,000 × 0.1) from the equation (15). )

  Therefore, the distribution rate (dDistRate) is −14000 [bps] (= −7000 × (30/15)) in the present embodiment from the equation (16), and is −20000 [bps] (= −10000 × (30) conventionally. / 15)). As a result, the target bit rate (TB_gop) of the current GOP is 286000 [bps] (= 300000 + (− 14000)) in the present embodiment from the equation (17), whereas conventionally the target bit rate (TB_gop) is 280000 [bps] ( = 300000 + (− 20000)). Therefore, the target code amount (T_gop) of the current GOP is 143000 [bits] (= 286000 × (15/30)) in the present embodiment according to the equation (18), whereas in the past, 140000 [bits]. (= 280000 × (15/30)).

  Here, the target code amount (T_pic) for each picture type is proportional to the target code amount (T_gop) of the current GOP from the equations (19) to (21), and the change rate of the quantization step from the equation (22). Is represented by the ratio (T_pic (n-1) / T_pic (n)) between the current code type target code amount and the previous GOP picture type target code amount, so that the current GOP target code amount (T_gop) In the present embodiment, which is larger than the conventional value, the current code type target code amount (T_pic (n)) and the previous picture type target code amount (T_pic (n-1)) The difference is smaller. Therefore, the change rate of the quantization step is smaller in the present embodiment than in the prior art.

  As a result, when the difficult sequence and the simple sequence continue alternately, the change rate of the quantization step can be reduced in the difficult sequence according to the present embodiment. It is possible to suppress the image quality deterioration in the latter half part more than before. Even in a sequence that is easy to encode (simple), in the present embodiment, the rate of change of the quantization step can be reduced as compared with the conventional method. It is possible to suppress the phenomenon that the image quality is extremely improved in a sequence that is easy to encode without using a code amount, and it is possible to maintain the image quality between sequences.

  Next, another embodiment of the present invention will be described. FIG. 10 is a block diagram showing the main part of another embodiment of the moving picture coding apparatus according to the present invention. In the figure, the same components as those in FIG. In this embodiment, as shown in FIG. 10, the encoding control unit 200 ′ is replaced with a quantization step calculation mode switching unit 207, a picture unit code amount control unit 207, a switch SW1. And the switch SW2 are added, and the coding standard is controlled by the control for protecting the coding buffer (Coding Picture Buffer: CPB) capacity, which is a virtual buffer defined in advance (preventing underflow). This is an embodiment of a code amount control mode switching method for generating a bitstream that complies with the above and has a uniform image quality as compared with the conventional case. That is, this embodiment is obtained by adding a fixed bit rate code amount control in units of pictures and a code amount control mode switching method to the above-described embodiment described with reference to FIG.

  Next, the operation of this embodiment will be described with reference to FIGS. FIG. 11 is a flowchart of CPB underflow prevention processing in another embodiment of the moving picture coding apparatus according to the present invention. This flowchart is executed in units of pictures. FIG. 12 shows a change in the CPB sufficiency with time in this embodiment and a state of underflow prevention by the CPB sufficiency.

  The quantization step calculation mode switching unit 207 in FIG. 10 accumulates the code output from the variable-length encoding unit 104 in the CPB that is a virtual buffer provided therein and also encodes the excess / underencoded code amount through. The data is output to the calculation unit 201 or the picture unit code amount control unit 208 via the switch SW1. Further, the quantization step calculation mode switching unit 207 switches the code amount control mode (hereinafter referred to as rc_mode) between the variable bit rate (VBR) mode and the constant bit rate (CBR) mode based on the CPB sufficient amount that is the accumulated code amount of CPB. In order to switch, first, the maximum picture bit amount that is the image target code amount is set (step S101 in FIG. 11).

  Here, the default picture target (target code amount) is used only for the I picture immediately after the scene change. Otherwise, the generated code amount of the previous I picture is set as the maximum picture bit amount. When the maximum picture bit amount is MaxPictureBits, the maximum picture bit amount is expressed by the following formula. Here, T (I) is the target code amount of the I picture, and S (I) is the generated code amount of the previous I picture in the GOP.

(I) I picture immediately after scene change
MaxPictureBits = T (I) (23)
(Ii) Pictures other than I pictures immediately after a scene change
MaxPictureBits = S (I) (24)
Furthermore, in order to make room for the CPB sufficient amount by increasing the maximum picture bit amount MaxPictureBits in the above equation, a predetermined MaxPicBitsRatio (≧ 1.0) is multiplied.

MaxPictureBits = MaxPictureBits x MaxPicBitsRatio (25)
The maximum picture bit amount MaxPictureBits represented by the equation (25) is the maximum picture bit amount set in step S101.

  Subsequently, the quantization step calculation mode switching unit 207 determines whether or not the maximum picture bit amount MaxPictureBits is greater than or equal to the CPB sufficient amount (step S102 in FIG. 11). Is determined to be in the VBR mode (step S103 in FIG. 11). When the maximum picture bit amount MaxPictureBits is equal to or greater than the CPB full amount, it is determined that there is a possibility that an underflow may occur below the CPB full amount when the next picture is encoded in the VBR mode, The operation is switched to the CBR mode operation in which the code amount is controlled (step S104 in FIG. 11).

  By this switching, the switch SW1 in FIG. 10 is switched from the terminal a side to the terminal b side, and the switch SW2 is switched from the terminal c side to the terminal d side. In the CBR mode, the virtual buffer and the picture target are updated, and initial setting for code amount control is performed.

That is, if CpbFullness is the CPB sufficient amount, the code amount control mode (rc_mode) is
MaxPictureBits ≧ CpbFullness
In the VBR mode, the code amount control mode (rc_mode) is the CBR mode (step S104 in FIG. 11).

  When the code amount control mode is set to the CBR mode in step S104, or when it is determined in step S103 that the code amount control mode is not the VBR mode (CBR mode), the picture unit code amount control is performed. The unit 208 uses the GOP initial quantization step to initialize the virtual buffer d0 () (the picture type is shown in parentheses) based on the following equation (26) (step S105 in FIG. 11).

d0 (I) = initial QS (I) × r / QS_MAX
d0 (P) = Kp × d0 (I)
d0 (B) = Kb × d0 (I) (26)
In equation (26), initialQS (I) is the initial quantization step of the I picture, r is a reaction parameter indicating the response speed of the quantization step, and QS_MAX is the maximum value of the quantization step. Kp is the quantization step ratio of the P picture indicating the quantization step ratio of the I picture, and Kb is the quantization step ratio of the B picture.

  Subsequently, the picture unit code amount control unit 208 sets a GOP target code amount (GOP allocation code amount G) (step S106 in FIG. 12). In the case of the CBR mode, a picture target (target code amount) for reliable code amount control is calculated. At this time, since the calculation method of the GOP target code amount (GOP allocated code amount G) changes depending on whether the current picture belongs to the GOP immediately after the scene change, it is detected whether or not a scene change has occurred in units of pictures. The number of pictures constituting one GOP is a fixed number when no scene change occurs, but the picture in which a scene change has occurred becomes an I picture and is encoded as the first picture of a new GOP. Therefore, it is detected whether or not it is the GOP immediately after the coding order (transmission order) of the picture in which the scene change has occurred (hereinafter referred to as the GOP immediately after the scene change). The GOP immediately after this scene change is a GOP including the first picture of the new GOP (a picture that has become an I picture by detecting a scene change).

  (A) When the GOP is not a GOP immediately after a scene change, the remaining GOP code amount is controlled by the target bit rate ratio (rate_ratio) of the GOP. The target bit rate ratio (rate_ratio) of GOP is expressed by the following equation.

rate_ratio = Bitrate_For_Previous / Bitrate_For_Current (27)
In equation (27), Bitrate_For_Previous is the bit rate of the previous GOP, and Bitrate_For_Current is the target bit rate of the current GOP.

  The GOP target code amount calculated in step S106, that is, the GOP allocation code amount (G), is calculated based on the previous GOP generation bit rate (Coded_Bitrate) and the target bit rate ratio (rate_ratio) of equation (27) as follows: Is calculated by This suppresses a rapid bit rate decrease.

G = (Coded_Bitrate × N × rate_ratio / picture_rate) (28)
The remaining GOP code amount (R) is equal to the GOP allocated code amount (G).

R = G (29)
Next, the picture unit code amount control unit 208 sets a target code amount for each picture type (step S107 in FIG. 11). The CPB increase (P and B are half) (PictureIncBits) is set to the maximum (Tmax) so that the next picture is not involved in the encoding order (transmission order). However, PictureIncBits is the CPB increase amount in units of pictures, and Tmax is the maximum value of the target code amount for each picture type. There is a relationship of the following formula between them.

(A) For I picture Tmax = PictureIncBits (30)
(B) For P picture and B picture Tmax = PictureIncBits / 2 (31)
Further, the maximum value (Tmax) of the target code amount for each picture type is smaller than the previous same-type picture generation code amount (S) as shown in the following equation, and the target code for each picture type of the next picture is smaller. The maximum amount.

Tmax = min (Tmax, S) (32)
The reason for taking the smaller one is that CPB underflow can be prevented if the maximum value Tmax of the target code amount by picture type is set to a generated code amount (S) of the same level as the previous picture of the same type or less. Because.

  The maximum value Tmax of the target code amount for each picture type is determined based on the buffer increment (PictureIncBits) of the CPB in units of pictures and the previous generated picture amount (S) of the same type. Here, the buffer increment (PictureIncBits) in the picture unit in CPB is a value obtained by dividing the maximum bit rate (fixed value determined before encoding: “MaxBitRate”) by the picture rate (PR). It is represented by

PictureIncBits = MaxBitRate / PR (33)
The previous picture generation code amount (S) of the same type is defined only when there is no scene change. For example,
I (0), P (1), B (2), B (3), P (4), B (5), B (6), P (7), B (8), B (9), P (10), B (11), B (12), I (13) (however, the numbers in parentheses indicate the coding order)
In the case of encoding in this order, the picture generation code amount S of the same type before I (13) is the generation code amount of I (0). Similarly, for example, a picture generation code amount S of the same type before P (7) means a generation code amount of P (4). For example, a picture generation code of the same type before B (3) The quantity S means the generated code quantity of B (2). Here, since it is assumed that there is no scene change, the picture generation code amount S of the same type is different for pictures of different GOPs across the GOPs, such as I (0) with respect to I (13), regardless of the GOP configuration. It may indicate the amount of generated code.

  Thus, the target code amount T for each picture type set in step S107 is as follows. T (I) is the target code amount for I picture, T (P) is the target code amount for P picture, and T (B) is the target code amount for B picture.

T (I) = R / (1.0 + N (P) ・ X (P) / (X (I) ・ Kp) + N (B) ・ X (B) / (X (I) ・ Kb))
T (I) = min (Tmax, max (T (I), Tmin)) (34)
T (P) = R / (Kp • X (I) / X (P) + N (P) + N (B) • Kp • X (B) / (Kb • X (P)))
T (P) = min (Tmax, max (T (P), Tmin)) (35)
T (B) = R / (1.0 ・ Kb ・ X (I) / X (B) + N (B) + N (P) ・ Kb ・ X (P) / (Kp ・ X (B)))
T (B) = min (Tmax, max (T (B), Tmin)) (36)
(B) In the case of a GOP immediately after a scene change In this case, the target code amount T (T (I), T (P), T (B)) for each picture type is calculated at the time of initial setting of the GOP. Update with in-screen activity square ratio.

T = T × Sqrt (CurIntAveACT / BasicActivity) (37)
However, in the above formula, CurIntAveACT is the activity of the current picture, and BasicActivity is the reference value (constant) of the activity. When the GOP is initially set, the GOP allocation code amount G in the equation (28) is set to the GOP reference target code amount (T_int) in the equation (1), and R = G according to the equation (29). = T_int) is substituted into Expressions (34) to (36) to obtain the target code amount by picture type T (T (I), T (P), T (B)) on the right side of Expression (37). It is done.

  Also, the CPB increase amount PictureIncBits for each picture is set to the maximum (Tmax) in the case of an I picture, and the maximum value (Tmax in the case of a P picture and a B picture so that the next picture is not involved in the encoding order (transmission order). ). That is, it is expressed by the following formula.

(A) For I picture Tmax = PictureIncBits (38)
(B) For P picture and B picture Tmax = PictureIncBits / 2 (39)
Furthermore, the target code amount T for each picture type is set to be larger than its minimum value (Tmin) and smaller than its maximum value (Tmax) as the target code amount T for each picture type of the next picture. .

T = min (Tmax, max (T, Tmin)) (40)
Thus, the target code amount for each picture type in step S107 in FIG. 11 is set. Subsequently, the picture unit code amount control unit 208 performs code amount control for each macroblock, thereby preventing CPB underflow (step S108 in FIG. 11). Here, the code amount control in units of macroblocks in step S108 may be a control method based on the target code amount for each picture type obtained as described above, and may be a method based on TM5, for example. However, other methods may be used.

  Here, as an example, a constant bit rate quantization step calculation method for each macroblock by the TM5 method is shown. Using the virtual buffers d0 (I), d0 (P), and d0 (B) set for each picture type according to equation (26), the quantization step is obtained by feedback control in units of macroblocks. Prior to encoding the j-th macroblock, the occupation amounts d [j] (I), d [j] (P), and d [j] (B) of the virtual buffer d0 for each picture type are obtained by the following equations. .

d [j] (I) = d0 (I) + B [j-1] − (T (I) × (j−1) / NumMB)
d [j] (P) = d0 (P) + B [j-1] − (T (P) × (j−1) / NumMB)
d [j] (B) = d0 (B) + B [j-1] − (T (B) × (j−1) / NumMB)
Here, B [j] is the amount of code generated when the j-th macroblock in the picture is encoded, and NumMB is the number of macroblocks in the picture. When j = 0, the above equation is as follows.

d [0] (I) = d0 (I)
d [0] (P) = d0 (P)
d [0] (B) = d0 (B)
Quantization steps Qstep [j] (I), Qstep [j] (P), and Qstep [j] (B) for I-picture, P-picture, and B-picture for the j-th macroblock are I-picture, P-picture, When each quantization parameter of the B picture is QP [j] (I), QP [j] (P), and QP [j] (B), the following equations are obtained.

QP [j] (I) = d [j] (I) × 51 / r
Qstep [j] (I) = 2 ^{((QP [j] (I) −4) / 6)} (41a)
QP [j] (P) = d [j] (P) × 51 / r
Qstep [j] (P) = 2 ^{((QP [j] (P) −4) / 6)} (41b)
QP [j] (B) = d [j] (B) × 51 / r
Qstep [j] (B) = 2 ^{((QP [j] (B) −4) / 6)} (41c)
After the code amount control in step S108, picture coding is performed by the same operation as that of the embodiment shown in FIG.

  When the quantization step calculation mode switching unit 207 in FIG. 10 determines that the maximum picture bit amount is less than the CPB sufficient amount in step S102 in FIG. 11, the code amount control mode (rc_mode) is set to the variable bit rate ( VBR) mode, the switch SW1 is connected to the terminal a side and the switch SW2 is connected to the terminal c side, and the processing of the picture unit code amount control unit 208 described above is not performed. That is, since there is a surplus in the CPB sufficient amount, the process returns to encoding in the VBR mode (step S109 in FIG. 11).

  The processing of FIG. 11 will be further described with reference to FIG. When there is no scene change, I (0), P (1), B (2), B (3), P (4), B (5), B (6), ... (however, in parentheses When the I (0) picture is encoded in the VBR mode, the code generated by the fixed value CPB increment (PictureIncBits) indicated by the diagonal line rising to the right is generated. The quantity S (I (0)) increases.

Here, the value of the generated code amount S (I (0)) times the maximum picture bit amount (MaxPicBitsRatio) indicated by 301 in FIG. 12 is the CPB underflow warning line indicated by the dotted line 302 in FIG. When the amount (CpbFullness) falls below the CPB underflow warning line, the CPB underflow is warned and the CBR mode operates in the CBR mode. In the example of FIG. 12, the CPB underflow warning line 302 falls below the generated code amount of the P (4) picture. At this time, Equation (24), Equation (25),
S (I (0)) × MaxPictureBits ≧ CpbFullness
Under the conditions, in the VBR mode, the code amount control mode is switched to the CBR mode (step S104 in FIG. 11).

  Subsequently, the target code amount T (B (5)) of the B (5) picture is obtained from the equations (32) and (36) from the maximum value Tmax (B ( 5)), the B (3) picture generation code amount S (B (3)) of the same type before the B (5) picture (this corresponds to S in the equation (32)), and T (B ( 5)), the smallest one is selected. Here, if Tmax (B (5)) is selected, this is PictureIncBits / 2 from equation (31).

  That is, in this case, the target code amount T (B (5)) of the B (5) picture does not exceed a value that is 1/2 times the increment of CPB (PictureIncBits). In addition, the generated code amount can be suppressed to about 1/2 times or less of the increment of CPB for each picture (PictureIncBits) by the macroblock control. As shown in FIG. 12, the CPB sufficient amount is equal to the CPB underflow warning line. 302 and above.

  Even if it falls below the underflow warning line 302 at the time of encoding picture B (5), the next picture B (6) is also encoded at about 1/2 times the increase in CPB, so this is repeated. In any case, the CPB sufficiency is above the underflow warning line 302. Therefore, according to the present embodiment, CPB underflow can be prevented.

  According to the embodiment described with reference to FIGS. 10 to 12 above, in the VBR mode, the switch SW1 is connected to the terminal a side and the switch SW2 is connected to the terminal c side, and the above described with reference to FIG. When the maximum picture bit amount is equal to or larger than the CPB full amount while controlling the quantization unit 103 in the variable bit rate quantization step and performing the quantization parameter control in units of GOPs, the switch SW1 is connected to the terminal b side. And the switch SW2 is connected to the terminal d side so as to be in the CBR mode, and the quantization unit 103 is controlled by a fixed bit rate quantization step output from the picture unit code amount control unit 208. VBR control is possible while maintaining the reserved buffer capacity, so a bitstream that conforms to the coding standard is generated with higher image quality than before. It is possible.

  Note that the present invention is not limited to the above-described embodiment, and the present invention may cause a computer to realize the functions of the above-described apparatus by a program. This program may be read from a recording medium and loaded into a computer, or may be transmitted via a communication network and loaded into a computer. Further, the present invention can be applied to a compression method of a signal that performs quantization / encoding, and can be applied not only to a moving image signal but also to an audio signal.

It is a block diagram of one embodiment of a moving picture coding apparatus of the present invention. 3 is a flowchart for explaining the operation of a coding excess / deficiency code amount calculation unit in FIG. It is a flowchart for operation | movement description of the buffer fullness rate calculation part in FIG. It is a flowchart for operation | movement description of the code distribution rate calculation part in FIG. 3 is a flowchart for explaining the operation of a target bit rate calculation unit in FIG. 1. 3 is a flowchart for explaining the operation of a target code amount calculation unit by image type in FIG. 1. 3 is a flowchart for explaining the operation of an image type-specific quantization step calculation unit in FIG. 1. It is a figure showing an example of the relationship (linear) of the buffer fullness rate and code allocation rate in the code allocation rate calculation part in FIG. It is a figure showing the other example of the relationship (nonlinear) of the buffer fullness rate and code allocation rate in the code allocation rate calculation part in FIG. It is a block diagram of the principal part of other embodiment of the moving image encoder of this invention. It is a flowchart for operation | movement description of the principal part of FIG. It is a figure which shows the mode of the underflow prevention by the change of CPB sufficiency with time in other embodiment of this invention, and CPB sufficiency.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Moving image encoding part 101 Prediction part 102 Orthogonal transformation part 103 Quantization part 104 Variable length encoding part 105 Inverse quantization part 106 Inverse orthogonal transformation part 107 Image memory part 200,200 'Encoding control part 201 Encoding excess or deficiency Code amount calculation unit 202 Buffer filling rate calculation unit 203 Code allocation rate calculation unit 204 Target bit rate calculation unit 205 Target code amount calculation unit by image type 206 Quantization step calculation unit by image type 207 Quantization step calculation mode switching unit 208 Picture Unit code amount control unit SW1, SW2 switch

Claims (2)

A prediction signal is generated in a predetermined encoding unit by a prediction unit from a moving image signal to be encoded and a reference image signal, and a difference signal between the prediction signal and the moving image signal is orthogonally converted by an orthogonal conversion unit. An orthogonal transform coefficient is generated, and the reference image signal is generated by a reference image signal generation unit from a signal obtained by quantizing the orthogonal transform coefficient based on an external quantization step, and a variable length code is generated by a variable length encoding unit. And an encoding unit that outputs an encoded signal,
In accordance with an excess / deficiency code amount that is a difference between a generated code amount for each predetermined image group coding unit in the variable length coding unit and a predetermined reference target code amount, a predetermined length in the variable length coding unit is determined. A moving image encoding apparatus comprising: an encoding control unit that variably controls the quantization step supplied to the encoding unit such that a generated code amount for each image group encoding unit becomes the reference target code amount. And
The encoding control unit
An excess / deficiency code amount calculation means for calculating the excess / deficiency code amount by subtracting the reference target code amount for each image group coding unit from the generated code amount for each image group coding unit in the variable length coding means. When,
A buffer fullness rate which is a ratio of the buffer fullness obtained by integrating the excess / deficient code amount and the allowable buffer size of the buffer for accumulating codes for each image group coding unit in the variable length coding unit A buffer filling rate calculating means for calculating;
When the buffer filling rate is 0 or more and the excess / deficiency code amount is 0 or more, a code amount distribution rate is calculated from the buffer filling rate according to a first nonlinear function, and the buffer filling rate is 0 or more. When the excess / deficiency code amount is negative, the code amount distribution rate is calculated from the buffer filling rate according to a second nonlinear function. When the buffer filling rate is negative, the code amount distribution rate is set to 0. Code amount distribution rate calculating means, and
Target code amount calculating means for calculating a target code amount of a current image group coding unit from the code amount distribution rate calculated by the code amount distribution rate calculating means and the excess / deficiency code amount;
Quantization for calculating the quantization step from the ratio between the target code amount of the current image group coding unit calculated by the target code amount calculation means and the target code amount of the previous image group coding unit Step calculating means, wherein the first nonlinear function is a function for increasing the code amount allocation rate in a nonlinear manner according to an increase in the buffer filling rate, and the second nonlinear function is the buffer filling rate. The moving picture coding apparatus is a function that reduces the code amount distribution rate in a non-linear manner in accordance with an increase in. The encoding control unit
Image target code amount calculating means for calculating an image target code amount included in a current image group encoding unit calculated from the target code amount;
Fixed bit rate quantization step calculating means for calculating a fixed bit rate quantization step in a predetermined encoding unit according to the image target code amount;
A buffer capacity monitor for accumulating codes output from the variable length coding means in a predetermined buffer and monitoring whether the accumulated code quantity of the buffer is below a predetermined capacity related to the image target code quantity Means,
When the buffer capacity monitoring means determines that the stored code amount of the buffer is less than the predetermined capacity, the fixed bit rate quantization step calculated by the fixed bit rate quantization step calculating means Is output to the encoding unit, and when it is determined that the accumulated code amount of the buffer exceeds the predetermined capacity, the quantization step of the variable bit rate calculated by the quantization step calculation unit is performed. The moving picture coding apparatus according to claim 1, further comprising: a quantization step switching output means for outputting to the coding unit.

Images