JP2005033599A - Coding device and coding method, and program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform rate control so that VBV underflow does not occur even when bit rate is changed, especially changed from low rate to high rate. <P>SOLUTION: Bit rate information is acquired in a step S1 and whether or not the bit rate is changed is decided in a step S2. When it is decided that the bit rate is not changed, initial buffer capacity of a virtual buffer is not adjusted. When it is decided that the bit rate is changed, occupancy amount of VBV is acquired in a step S3, the initial buffer capacity d(0) of the virtual buffer is calculated by d'(0)=d(0)×k1×(bit_rate(1)-bit_rate(0))/vbv_ocp on the basis of the occupancy amount of the VBV and bit rate before and after change and the initial buffer capacity of the virtual buffer is adjusted to a value made to correspond to the bit rate and VBV occupancy amount in a step S4. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an encoding device, an encoding method, and a program. In particular, when performing rate control in encoding, even if the bit rate is changed, VBV underflow is avoided and image degradation does not occur. The present invention relates to an encoding device, an encoding method, and a program.

  In digital broadcasting typified by satellite broadcasting, for example, a plurality of programs compressed by the MPEG (Moving Picture Experts Group) method are multiplexed with the transmission rate dynamically changed by a statistical multiplexing method. The As a result, it is possible to transmit a high-quality program as a whole by effectively using a limited transmission rate.

  That is, when multiple programs are multiplexed by the statistical multiplexing method, it is unlikely that locations where image quality deterioration is likely to occur at the same time in multiple programs will occur. In this program, a large number of rates are assigned to locations where image quality degradation is easily perceived, and the rate distribution at locations where image quality degradation is difficult to perceive is deleted, and the transmission rate of each program is dynamically changed. This eliminates the need to assign an unnecessarily high rate to parts where it is difficult to perceive image quality degradation, and assigns that rate to other programs, allowing transmission without degrading the overall image quality. Efficiency can be improved.

  By the way, when various programs are transmitted by changing the bit rate by statistical multiplexing in this way, the decoder (decoding device) side can decode continuous video without interruption (hereinafter referred to as seamless). It is necessary to control the bit rate on the encoder (encoding device) side. That is, on the encoder side, the bit rate is controlled so that a so-called VBV (Video Buffering Verifier) buffer (virtual buffer) that temporarily holds the data transmitted by the decoder does not overflow or underflow. There is a need to.

  In particular, in applications such as broadcasting or communication, unlike the case where information is recorded on a storage medium such as a DVD (Digital Versatile Disk), the decoder cannot control the reading of the bit stream. On the side, it is necessary to control the amount of data in the VBV buffer.

  To make it possible to change the bit rate seamlessly, the delay time τ until the decoder reads the corresponding data from the input buffer after writing the encoded data to the output buffer is always the same regardless of the bit rate. What is necessary is just to make it constant. If the bit rate is controlled in this way, constant transport stream data is held in the VBV buffer. Therefore, the input buffer can be prevented from overflowing or underflowing, and the bit rate can be controlled seamlessly.

  The delay time τ due to the VBV buffer becomes the largest when the bit rate of the transport stream is the lowest bit rate.

  In this way, when changing the bit rate seamlessly, such as when statistical multiplexing is performed, the delay time can be kept short even if the minimum bit rate is set low, taking advantage of statistical multiplexing, In order to improve the image quality, there is a technique for controlling the usable range of the VBV buffer according to the bit rate. (For example, refer to Patent Document 1).

JP-A-11-262008

  The maximum capacity, bit rate, and encoding delay of the VBV buffer when the usable range of the VBV buffer is controlled according to the bit rate will be described with reference to FIG.

  When the maximum capacity of the VBV buffer is vbv_size (0) and the bit rate when the encoding delay is τ is bit_rate (0), the following equation (1) is satisfied.

        τ = vbv_size (0) / bit_rate (0) (1)

  The condition for changing the seamless bit rate is that the delay time after changing the bit rate is always τ or less, and the VBV when changing the bit rate to bit_rate (1) lower than bit_rate (0). The buffer maximum value vbv_size (1) is set so as to satisfy the following expression (2).

          vbv_size (1) = bit_rate (1) x τ (2)

  That is, as shown in FIG. 2, when the bit rate is changed from bit_rate (0) to bit_rate (1), the maximum value of the VBV buffer is changed from vbv_size (0) to vbv_size (1). Is changed from bit_rate (1) to bit_rate (0), the maximum value of the VBV buffer is changed from vbv_size (1) to vbv_size (0).

  Here, when the bit rate is changed from bit_rate (1), which is a low bit rate, to bit_rate (0), which is a high bit rate, the VBV buffer occupies less at the bit rate change point A in FIG. However, since the bit rate suddenly increases, encoding is performed based on the previous picture, so that the amount of generated code increases and VBV underflow is likely to occur. In such a case, in order to avoid VBV underflow, the picture skip or macro block skip is performed in the decoder, and the image quality deteriorates.

  The present invention has been made in view of such a situation, and rate control is performed so that VBV underflow does not occur even when the bit rate is changed, particularly when the rate is changed from a low rate to a high rate. Is to be able to do.

  The encoding apparatus of the present invention includes a first detection unit that detects a bit rate of frame image data, a determination unit that determines quantization index data using a value of an initial buffer capacity of a virtual buffer, and a determination unit. Based on the determined quantization index data, a quantization unit that performs quantization, and an encoding unit that encodes the quantized coefficient data quantized by the quantization unit, the determination unit includes: When the bit rate is changed based on the detection result of the detection means, it is determined whether or not to adjust the initial buffer capacity of the virtual buffer.

  The determination unit may further detect the buffer occupancy, and when the bit rate is changed, the first bit rate before the change detected by the first detection unit, and the post-change Based on the second bit rate and the detected buffer occupation amount when the bit rate is changed, the initial buffer capacity value of the virtual buffer can be calculated to adjust the initial buffer capacity.

  Second detection means for detecting the encoding difficulty level of the frame image data can be further provided, and the determining means can further detect the buffer occupancy, and the bit rate can be changed. The first bit rate before the change detected by the first detection means, the second bit rate after the change, the buffer occupation amount when the detected bit rate is changed, and the second bit rate The initial buffer capacity value can be adjusted by calculating the initial buffer capacity value of the virtual buffer based on the encoding difficulty level of the frame image data detected by the detecting means.

  Second detection means for detecting the encoding difficulty level of the frame image data can be further provided, and the determining means can further detect the buffer occupancy, and the bit rate can be changed. In this case, the encoding difficulty level of the frame image data detected by the second detection means can be compared with a predetermined threshold value. When the encoding difficulty level is higher than the predetermined threshold value, The first bit rate before the change detected by the first detection means, the second bit rate after the change, the buffer occupation amount when the detected bit rate is changed, and the second bit rate detected by the second detection means The initial buffer capacity value can be adjusted by calculating the initial buffer capacity value of the virtual buffer based on the degree of difficulty of encoding the frame image data.

  In the encoding device of the present invention, when the bit rate of the frame image data is detected and the bit rate is changed, it is determined whether or not to adjust the initial buffer capacity of the virtual buffer, and the initial buffer capacity of the virtual buffer is determined. Quantization index data is determined using the value of, quantization is performed based on the quantization index data, and quantization coefficient data is encoded.

  The encoding method of the present invention includes a bit rate information acquisition step for acquiring information about a bit rate, and a bit rate of a picture constituting video data based on the information about the bit rate acquired by the processing of the bit rate information acquisition step. A determination step for determining whether or not the bit rate has been changed, and an adjustment step for adjusting the initial buffer capacity of the virtual buffer when the bit rate is determined to have been changed by the processing of the determination step.

  According to the program of the present invention, the bit rate of a picture constituting video data is changed based on the bit rate information acquisition step for acquiring information on the bit rate and the information on the bit rate acquired by the processing of the bit rate information acquisition step. A determination step for determining whether or not the bit rate has been changed by the processing of the determination step, and an adjustment step for adjusting the initial buffer capacity of the virtual buffer.

  In the encoding method and program of the present invention, information on the bit rate is obtained, and based on the information on the bit rate, it is determined whether or not the bit rate of the picture constituting the video data has been changed, and the bit rate is changed. If it is determined that the virtual buffer has been set, the initial buffer capacity of the virtual buffer is adjusted.

  According to the present invention, frame image data can be encoded. In particular, when a change in the bit rate is detected, the initial buffer capacity of the virtual buffer is adjusted as necessary, so that VBV buffer underflow can be prevented and image deterioration can be prevented. it can.

  Embodiments of the present invention will be described below. In order to clarify the correspondence between each means and each step of the invention described in the claims and the following embodiments, each means and each step will be described. The features of the present invention are described as follows by adding the corresponding embodiment (however, by way of example) within the parentheses. However, of course, this description does not mean that each means and each step are limited to those described.

  The encoding device according to claim 1 (for example, the encoding device 1 in FIG. 3 or the encoding device 41 in FIG. 5) includes first detection means (for example, FIG. 3 or FIG. 3) that detects the bit rate of the frame image data. The bit rate acquisition unit 31 in FIG. 5 and the initial buffer capacity value of the virtual buffer are used to determine quantization index data (for example, the virtual buffer determination unit 32 in FIG. 3 or the virtual buffer determination in FIG. 5). 62), quantization means for performing quantization based on the quantization index data determined by the determination means (for example, the quantization unit 15 in FIG. 3 or FIG. 5), and quantization by the quantization means Encoding means (for example, the VLC unit 16 in FIG. 3 or FIG. 5) for encoding the quantized coefficient data, and the determining means changes the bit rate based on the detection result by the first detecting means. If, characterized by determining whether to adjust the initial buffer capacity of the virtual buffer.

  In the encoding device according to claim 2, when the determination unit further detects the buffer occupancy and the bit rate is changed, the first bit rate before the change detected by the first detection unit (for example, , Bit_rate (1) in Expression (3)), the second bit rate after the change (for example, bit_rate (0) in Expression (3)), and the buffer occupation amount when the detected bit rate is changed ( For example, the initial buffer capacity is adjusted by calculating the value of the initial buffer capacity of the virtual buffer based on vbv_ocp) in Expression (3).

  The encoding apparatus according to claim 3 further includes second detection means (for example, the image difficulty level information acquisition unit 61 in FIG. 5) for detecting the encoding difficulty level of the frame image data, and the determination means includes a buffer. When the occupancy is further detected and the bit rate is changed, the first bit rate before the change (for example, bit_rate (1) in Expression (11)) detected by the first detection unit, and after the change The second bit rate (for example, bit_rate (0) in equation (11)), the detected buffer occupancy at the time of changing the bit rate (for example, vbv_ocp in equation (11)), and the second detection means The initial buffer capacity is adjusted by calculating the value of the initial buffer capacity of the virtual buffer based on the degree of difficulty in encoding the detected frame image data (for example, diff_info in Expression (11)).

  The encoding device according to claim 4 further includes second detection means (for example, the image difficulty level information acquisition unit 61 in FIG. 5) for detecting the encoding difficulty level of the frame image data. When the occupation amount is further detected and the bit rate is changed, the encoding difficulty level (for example, diff_info) of the frame image data detected by the second detection unit is compared with a predetermined threshold value, and the encoding difficulty level is determined. The first bit rate before the change (for example, bit_rate (1) in equation (11)) detected by the first detection means and the second bit rate after the change detected by the first detection means when it is higher than the predetermined threshold. (Eg, bit_rate (0) in equation (11)), detected buffer occupancy at the time of changing the bit rate (eg, vbv_ocp in equation (11)), and frame image data detected by the second detection means Encoding difficulty Degrees (e.g., Diff_info in equation (11)) based on, by calculating the value of the initial buffer capacity of the virtual buffer, and adjusting the initial buffer capacity.

  The encoding method according to claim 5 includes a bit rate information acquisition step (for example, step S1 in FIG. 4, step S21 in FIG. 6, or step S41 in FIG. 7) for acquiring information on the bit rate, Judgment step for judging whether or not the bit rate of the picture constituting the video data has been changed based on the information about the bit rate obtained by the processing of the information obtaining step (for example, step S2 in FIG. 4, step in FIG. 6) S22 or step S42 in FIG. 7) and an adjustment step for adjusting the initial buffer capacity of the virtual buffer when it is determined that the bit rate has been changed by the processing in the determination step, for example, step S4 in FIG. Step S24 of FIG. 7 or Step S46 of FIG. 7).

  Also in the program according to claim 6, the embodiment (however, an example) to which each step corresponds is the same as the encoding method according to claim 5.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  FIG. 3 is a block diagram showing a configuration of the encoding apparatus 1 to which the present invention is applied.

  The image rearrangement unit 11 rearranges each frame image of the sequentially input video data as necessary, or a macroblock configured by a luminance signal of 16 pixels × 16 lines and a color difference signal corresponding to the luminance signal Is generated and supplied to the calculation unit 12 and the motion detection unit 13.

  The motion detector 13 receives the input of the macroblock data, calculates the motion vector of each macroblock based on the macroblock data and the reference image data stored in the video memory 21, and as motion vector data, It is sent to the motion compensation unit 20.

  The calculation unit 12 performs motion compensation on the macroblock data supplied from the image rearrangement unit 11 based on the image type of each macroblock. Specifically, the calculation unit 12 performs motion compensation in the intra mode for the I picture, performs motion compensation in the forward prediction mode for the P picture, and bidirectional for the B picture. Motion compensation is performed in the prediction mode.

  Here, the intra mode is a method in which a frame image to be encoded is used as transmission data as it is, and the forward prediction mode is a prediction residual between a frame image to be encoded and a past reference image as transmission data. The bidirectional prediction mode is a method in which transmission residuals are prediction residuals between a frame image to be encoded and past and future reference images.

  First, when the macroblock data is an I picture, the macroblock data is processed in the intra mode. That is, the calculation unit 12 sends the macroblock of the input macroblock data as it is to the DCT (Discrete Cosine Transform) unit 14 as calculation data. The DCT unit 14 converts the input arithmetic data into a DCT coefficient by performing DCT conversion processing, and sends this to the quantization unit 15 as DCT coefficient data.

  The quantization unit 15 performs a quantization process on the input DCT coefficient data based on the quantization value Q supplied from the virtual buffer determination unit 32 of the quantization control unit 23 to obtain quantized DCT coefficient data. The data is sent to a VLC (Variable Length Code) unit 16 and an inverse quantization unit 17. Here, the quantization unit 15 controls the amount of generated code by adjusting the quantization step size in the quantization process according to the quantization value Q supplied from the virtual buffer determination unit 32. ing.

  The quantized DCT coefficient data sent to the inverse quantization unit 17 is subjected to an inverse quantization process with the same quantization step size as that of the quantization unit 15, and is converted into DCT coefficient data as an IDCT (Inverse Discreet Cosine Transform). ) Part 18. The IDCT unit 18 performs inverse DCT processing on the supplied DCT coefficient data, and the generated calculation data is sent to the calculation unit 19 and stored in the video memory 21 as reference image data.

  Then, when the macroblock data is a P picture, the calculation unit 12 performs a motion compensation process using the forward prediction mode for the macroblock data. When the macroblock data is a B picture, the calculation unit 12 performs the bidirectional prediction mode for the macroblock data. Performs motion compensation processing by.

  The motion compensation unit 20 performs motion compensation on the reference image data stored in the video memory 21 according to the motion vector data, and calculates forward prediction image data or bidirectional prediction image data. The calculation unit 12 performs a subtraction process on the macroblock data using the forward prediction image data or the bidirectional prediction image data supplied from the motion compensation unit 20.

  That is, in the forward prediction mode, the motion compensation unit 20 reads the reference image data by shifting the read address of the video memory 21 according to the motion vector data, and uses the reference image data as the forward prediction image data. It supplies to the calculating part 19. The calculation unit 12 subtracts the forward prediction image data from the supplied macroblock data to obtain difference data as a prediction residual. Then, the calculation unit 12 sends the difference data to the DCT unit 14.

  The forward prediction image data is supplied from the motion compensation unit 20 to the calculation unit 19, and the calculation unit 19 adds the forward prediction image data to the calculation data supplied from the IDCT unit 18, thereby referring to the calculation unit 19. Image data is locally reproduced and output to the video memory 21 for storage.

  In the bidirectional prediction mode, the motion compensation unit 20 reads the reference image data by shifting the read address of the video memory 21 according to the motion vector data, and uses this as the bidirectional prediction image data. It supplies to the calculating part 19. The calculation unit 12 subtracts bidirectional prediction image data from the supplied macroblock data to obtain difference data as a prediction residual. Then, the calculation unit 12 sends the difference data to the DCT unit 14.

  Bidirectional prediction image data is supplied to the calculation unit 19 from the motion compensation unit 20, and the calculation unit 19 adds the bidirectional prediction image data to the calculation data supplied from the IDCT unit 18, thereby referring to the calculation unit 19. Image data is locally reproduced and output to the video memory 21 for storage.

  Thus, the image data input to the encoding apparatus 1 undergoes motion compensation prediction processing, DCT processing, and quantization processing, and is supplied to the VLC unit 16 as quantized DCT coefficient data. The VLC unit 16 performs variable length coding processing based on a predetermined conversion table for the quantized DCT coefficient data, and sends the variable length coded data obtained as a result to the buffer 22. The buffer 22 buffers the supplied variable length encoded stream for a predetermined time and then outputs it.

  The output variable length encoded stream is multiplexed with, for example, an audio stream or another video stream as necessary, divided by a predetermined process, and packetized by adding a header or the like. Then, an MPEG transport stream composed of a plurality of packet data is transmitted through a predetermined data transmission path.

  The quantization control unit 23 controls quantization in the encoding device 1. The bit rate information acquisition unit 31 acquires a bit rate value or information indicating the bit rate value, and supplies the acquired value to the virtual buffer determination unit 32. The virtual buffer determination unit 32 constantly monitors the accumulation state of the variable-length encoded data stored in the buffer 22, and adjusts the initial buffer capacity of the virtual buffer based on whether or not the bit rate has been changed. To do. The virtual buffer determination unit 32 determines a target code amount based on the calculated initial buffer capacity of the virtual buffer, determines a quantization step size for generating the target code amount, and supplies the quantization step size to the quantization unit 15 It is made to do.

  As described with reference to FIG. 2, in the encoding device 1, when the bit rate is changed from bit_rate (0) to bit_rate (1), the maximum value of the VBV buffer is vbv_size (0) to vbv_size (1). ) And the bit rate is changed from bit_rate (1) to bit_rate (0), the maximum value of the VBV buffer is changed from vbv_size (1) to vbv_size (0).

  In particular, when the bit rate is changed from a low rate to a high rate, the difference between the bit rate changes is larger, and the smaller the buffer occupancy at the rate change, the more likely VBV underflow occurs. When the change in the bit rate is detected, the unit 32 determines the initial buffer capacity of the virtual buffer using the following equation (3), determines the quantization step size for generating the target code amount, Is supplied to the conversion unit 15.

    d ′ (0) = d (0) × k1 × | bit_rate (1) −bit_rate (0) | / vbv_ocp (3)

  In Equation (3), k1 is a predetermined constant obtained empirically or experimentally, bit_rate (1) is the bit rate after change, and bit_rate (0) is the bit rate before change. Yes, vbv_ocp is the VBV occupation amount when the bit rate is changed.

  Further, when the virtual buffer determination unit 32 does not detect the change in the bit rate, the virtual buffer determination unit 32 controls the quantization as usual. That is, the virtual buffer determination unit 32 determines the generated code amount when the generated code amount of the actually generated macroblock is larger than the target generated code amount based on the VBV buffer size corresponding to the bit rate and the buffer occupation amount information. In order to reduce the quantization step size, the quantization step size is increased. When the actual generated code amount is smaller than the target generated code amount, the quantization step size is decreased to increase the generated code amount.

  That is, the virtual buffer determination unit 32 obtains the buffer occupancy of the virtual buffer by assuming the transition of the accumulation state of the variable-length encoded data stored in the VBV buffer provided on the decoder side, and obtains the quantized value Q is calculated and supplied to the quantization unit 15.

  The buffer occupancy d (j) of the virtual buffer in the jth macroblock is expressed by the following equation (4), and the buffer occupancy d (j + 1) of the virtual buffer in the j + 1th macroblock is The buffer occupation amount d (j + 1) of the virtual buffer in the j + 1-th macroblock is expressed as the following equation (6) by subtracting the equation (5) from the equation (4). The

      d (j) = d (0) + B (j−1) − {T × (j−1) / MBcnt} (4)

  Here, d (0) is the initial buffer capacity, B (j) is the number of encoding generation bits in the j-th macroblock, MBcnt is the number of macroblocks in the picture, and T is the target generation in units of pictures. Code amount.

      d (j + 1) = d (0) + B (j) − (T × j) / MBcnt (5)

      d (j + 1) = d (j) + {B (j) -B (j-1)}-T / MBcnt (6)

  Therefore, the virtual buffer determination unit 32 substitutes the buffer occupation amount d (j + 1) and the constant r shown in the equation (7) into the equation (8), thereby quantizing the index data of the macroblock (j + 1). Q (j + 1) is calculated and supplied to the quantization unit 75.

r = (2 × br) / pr (7)
Q (j + 1) = d (j + 1) × (31 / r) (8)
Here, br is a bit rate and pr is a picture rate.

  The quantization unit 15 determines the quantization step size in the next macroblock based on the quantization value Q, and quantizes the DCT coefficient data by the quantization step size.

  As a result, the quantization unit 15 quantizes the DCT coefficient data with the optimum quantization step size for the target generated code amount of the next picture calculated based on the actual generated code amount of the previous picture. can do.

  Thus, the quantization unit 15 can quantize the buffer 22 so that the buffer 22 does not overflow or underflow according to the data occupancy of the buffer 22, and the decoder side VBV buffer performs quantization so as not to overflow or underflow. Quantized DCT coefficient data can be generated.

  In the conventional processing described with reference to FIG. 2, the maximum value of the VBV buffer is determined by the product of the delay time τ and the bit rate. In this case, the bit rate is particularly low. When changing from a high rate to a high rate, the difference in bit rate change is larger, and the smaller the buffer occupancy at the rate change, the more likely VBV underflow occurs. In such a case, in order to prevent VBV underflow, it is necessary to detect a change in the bit rate and adjust the initial buffer capacity of the virtual buffer using the above-described equation (3).

  With reference to the flowchart of FIG. 4, the virtual buffer adjustment process 1 executed in the encoding device 1 of FIG. 3 will be described.

  In step S 1, the bit rate information acquisition unit 31 acquires the bit rate information and supplies it to the virtual buffer determination unit 32. The bit rate information may be numerical data of the bit rate itself or corresponding information indicating the bit rate. The bit rate information acquisition unit 31 may acquire the bit rate information from an external device. When the bit rate information is multiplexed with the video data, the bit rate information acquisition unit 31 extracts the bit rate information from the video data. May be.

  In step S2, the virtual buffer determination unit 32 determines whether the bit rate has been changed based on the supplied bit rate information. If it is determined in step S2 that the bit rate has not been changed, the initial buffer capacity of the virtual buffer is not adjusted, so the process returns to step S1 and the subsequent processes are repeated.

  When it is determined in step S2 that the bit rate has been changed, in step S3, the virtual buffer determination unit 32 acquires the VBV occupation amount vbv_ocp.

  In step S4, the virtual buffer determination unit 32 uses the equation (3) to determine the initial buffer of the virtual buffer based on the VBV occupation amount vbv_ocp acquired in step S3 and the bit rate before and after the change supplied in step S1. The capacity d ′ (0) is calculated, the initial buffer capacity of the virtual buffer is adjusted to a value according to the bit rate and the VBV occupation amount, the process returns to step S1, and the subsequent processes are repeated.

  If there is a risk of VBV underflow by adjusting the initial buffer capacity of the virtual buffer so that it becomes a value according to the bit rate and VBV occupation amount when the bit rate is changed by such processing Since the amount of generated code can be suppressed, even if the difference in bit rate change is large and the buffer occupancy at the time of rate change is small, VBV underflow can be prevented in advance, thereby skipping pictures And image quality deterioration due to occurrence of macroblock skip or the like can be prevented.

  In the encoding apparatus 1, for example, when the virtual buffer is managed for each picture type as in TM5 (Test Model 5), the virtual buffer determination unit 32 first displays each picture that appears after the bit rate is changed. In type, adjust the initial buffer capacity of the virtual buffer.

  In the above-described processing, even if it is detected in step S2 that the bit rate has been changed from the high rate to the low rate, it is detected that the bit rate has been changed from the low rate to the high rate. Even so, it was explained that the initial buffer capacity of the virtual buffer is adjusted according to the bit rate and VBV occupancy, but VBV underflow is likely to occur because the bit rate is changed from low rate to high rate Therefore, the initial buffer capacity of the virtual buffer may be adjusted according to the bit rate and the VBV occupation amount only when the bit rate is changed from the low rate to the high rate. Note that even if the initial buffer capacity of the virtual buffer is adjusted when the bit rate is changed from the high rate to the low rate, the image quality deteriorates compared with the case where the initial buffer capacity of the virtual buffer is not adjusted. If necessary, the initial buffer capacity of the virtual buffer may be adjusted only when the bit rate is changed from the high rate to the low rate.

  By the way, if the picture to be encoded is an image that is easy to encode, the amount of code generation does not increase, so the initial buffer capacity of the virtual buffer is reduced even though there is almost no possibility of underflow of the VBV buffer. If the code generation amount is suppressed by adjustment, the image is deteriorated more than necessary.

  Therefore, by detecting the encoding difficulty level of the image, in addition to the bit rate and VBV occupation amount, the initial buffer capacity of the virtual buffer corresponding to the encoding difficulty level is calculated, or the encoding difficulty level is predetermined. In the case where the value is lower than the above value, it is possible to prevent the image from being deteriorated more than necessary in an image that is easy to encode by not adjusting the initial buffer capacity of the virtual buffer. .

  FIG. 5 is a block diagram illustrating a configuration of the encoding device 41 that can adjust the initial buffer capacity of the virtual buffer according to the bit rate, the VBV occupation amount, and the encoding difficulty level.

  Note that portions corresponding to those in FIG. 3 are denoted by the same reference numerals, and description thereof will be omitted as appropriate. That is, the encoding device 41 in FIG. 5 has the same configuration as the encoding device 1 described with reference to FIG. 3 except that a quantization control unit 51 is provided instead of the quantization control unit 23. The quantization control unit 51 is different from the virtual buffer determination unit 32 in that a virtual buffer determination unit 62 is provided and an image difficulty level information acquisition unit 61 is newly provided. This has the same configuration as that of the quantization control unit 23.

  The image difficulty level information acquisition unit 61 acquires or calculates image difficulty level information such as IntraAC or ME residual information, and supplies it to the virtual buffer determination unit 62.

  When the image difficulty level information is included in the supplied video data, the image difficulty level information acquisition unit 61 extracts the image difficulty level information from the video data. Further, the image difficulty level information may be supplied from another device.

  When the image difficulty level information is an ME residual, the ME residual is information obtained when motion is detected, and thus the image difficulty level information may be supplied from the calculation unit 12. The ME residual is calculated on a picture-by-picture basis, and is a total value of luminance difference values between the previous picture and the next picture.

  When the image difficulty level information is intra AC, the image difficulty level information acquisition unit 61 may calculate the intra AC based on the macroblock data supplied from the image rearrangement unit 11.

  Intra AC is a parameter defined as the sum of the variance values of video data for each DCT block in the DCT processing unit in the MPEG system, and indicates the complexity of the video, the difficulty of the picture design and the compression Correlate with data volume. In other words, the intra AC is the sum in the screen of the absolute value sum of the pixel value of each pixel minus the average value of the pixel values for each block in DCT block units. The intra AC is expressed by the following equation (9).

・・・（９）

... (9)

・・・（１０） Further, in Expression (9), Expression (10) is established.

... (10)

  When detecting a change in the bit rate, the virtual buffer determination unit 62 determines the initial buffer capacity of the virtual buffer using the following equation (11), and determines the quantization step size for generating the target code amount. Thus, the signal is supplied to the quantization unit 15.

d ”(0) = d (0) × k2 × diff_info × | bit_rate (1) −bit_rate (0) | / vbv_ocp
(11)

  In Equation (11), k2 is a predetermined constant obtained empirically or experimentally, diff_info is the image difficulty level, bit_rate (1) is the changed bit rate, bit_rate ( 0) is the pre-change bit rate, and vbv_ocp is the VBV occupation amount when the bit rate is changed.

  Alternatively, when detecting a change in the bit rate, the virtual buffer determination unit 62 compares the image difficulty level with a predetermined threshold value obtained empirically or experimentally, and if the image difficulty level is greater than the predetermined threshold value (predetermined value) Only when the encoding difficulty level is higher than the value of (2), based on the equation (11), determine the initial buffer capacity of the virtual buffer, determine the quantization step size for generating the target code amount, You may make it supply to the quantization part 15. FIG.

  When the virtual buffer determination unit 62 does not detect a change in the bit rate, the virtual buffer determination unit 62 performs the same rate control as the conventional one and controls the quantization process of the quantization unit 15.

  Next, the virtual buffer adjustment processing 2 executed in the encoding device 41 in FIG. 5 will be described with reference to the flowchart in FIG.

  In step S <b> 21, the bit rate information acquisition unit 31 acquires the bit rate information and supplies it to the virtual buffer determination unit 62. The bit rate information may be numerical data of the bit rate itself or corresponding information indicating the bit rate. The bit rate information acquisition unit 31 may acquire the bit rate information from an external device. When the bit rate information is multiplexed with the video data, the bit rate information acquisition unit 31 extracts the bit rate information from the video data. May be.

  In step S22, the virtual buffer determination unit 62 determines whether or not the bit rate has been changed based on the supplied bit rate information. If it is determined in step S22 that the bit rate has not been changed, the initial buffer capacity of the virtual buffer is not adjusted, so the process returns to step S21 and the subsequent processes are repeated.

  When it is determined in step S22 that the bit rate has been changed, in step S23, the virtual buffer determination unit 62 acquires the VBV occupation amount vbv_ocp, and the image difficulty level information acquisition unit 61 acquires the image difficulty level information diff_info. Obtained and supplied to the virtual buffer determination unit 62.

  In step S24, the virtual buffer determination unit 62 uses the equation (11) based on the VBV occupation amount vbv_ocp and the image difficulty level information diff_info acquired in step S23, and the bit rate before and after the change supplied in step S21. Then, the initial buffer capacity d ″ (0) of the virtual buffer is calculated, and the initial buffer capacity of the virtual buffer is adjusted to values according to the bit rate, the VBV occupation amount, and the image difficulty level. Return to, and the subsequent processing is repeated.

  By such processing, when the bit rate is changed, in addition to the bit rate and VBV occupation amount, the initial buffer capacity of the virtual buffer is adjusted so as to become a value according to the image difficulty level, and the generated code amount VBV underflow can be suppressed without unnecessarily degrading the image even if the difference in bit rate change is large and the buffer occupancy at the rate change is small. Occurrence can be prevented in advance, and image quality deterioration due to occurrence of picture skip or macro block skip can be prevented.

  In the above-described processing, even when it is detected in step S22 that the bit rate has been changed from the high rate to the low rate, it is detected that the bit rate has been changed from the low rate to the high rate. Even so, although it has been described that the initial buffer capacity of the virtual buffer is adjusted according to the bit rate, VBV occupation amount, and image difficulty level, the bit rate is low because VBV underflow is likely to occur. Since the rate is changed from high rate to high rate, only when the bit rate is changed from low rate to high rate, the initial buffer capacity of the virtual buffer depends on the bit rate, VBV occupancy, and image difficulty It is good also as what adjusts. Even if the initial buffer capacity of the virtual buffer is adjusted when the bit rate is changed from the high rate to the low rate, the image quality is deteriorated as compared with the case where the initial buffer capacity of the virtual buffer is not adjusted. Therefore, if necessary, the initial buffer capacity of the virtual buffer may be adjusted only when the bit rate is changed from the high rate to the low rate.

  Next, the virtual buffer adjustment processing 3 executed in the encoding device 41 in FIG. 5 will be described with reference to the flowchart in FIG.

  In step S <b> 41, the bit rate information acquisition unit 31 acquires the bit rate information and supplies it to the virtual buffer determination unit 62. The bit rate information may be numerical data of the bit rate itself or corresponding information indicating the bit rate. The bit rate information acquisition unit 31 may acquire the bit rate information from an external device. When the bit rate information is multiplexed with the video data, the bit rate information acquisition unit 31 extracts the bit rate information from the video data. May be.

  In step S42, the virtual buffer determination unit 62 determines whether the bit rate has been changed based on the supplied bit rate information. If it is determined in step S42 that the bit rate has not been changed, the initial buffer capacity of the virtual buffer is not adjusted, so the process returns to step S41 and the subsequent processes are repeated. If it is determined in step S42 that the bit rate has been changed, in step S43, the image difficulty level information acquisition unit 61 acquires the image difficulty level information diff_info and supplies it to the virtual buffer determination unit 62.

  In step S44, the virtual buffer determination unit 62 determines whether the image difficulty level information diff_info supplied in step S43 is higher than a predetermined threshold. If it is determined in step S44 that the image difficulty level information diff_info is lower than a predetermined threshold, the VBV buffer underflow does not occur in an image that is easy to encode, so the initial buffer capacity of the virtual buffer is not adjusted. The processing returns to step S41, and the subsequent processing is repeated.

  If it is determined in step S44 that the image difficulty level information diff_info is higher than a predetermined threshold value, in step S45, the virtual buffer determination unit 62 acquires the VBV occupation amount vbv_ocp.

  In step S46, the virtual buffer determination unit 62 determines the VBV occupation amount vbv_ocp acquired in step S45, the image difficulty level information diff_info supplied in step S43, and the bit rate before and after the change supplied in step S41. The initial buffer capacity d ″ (0) of the virtual buffer is calculated by equation (11), and the initial buffer capacity of the virtual buffer is adjusted to a value corresponding to the bit rate, the VBV occupation amount, and the image difficulty level. The processing returns to step S41, and the subsequent processing is repeated.

  By such processing, when the bit rate is changed, only when the image difficulty level is high, the initial buffer capacity of the virtual buffer is set so that the value depends on the bit rate, the VBV occupation amount, and the image difficulty level. Since the adjustment is made, it is possible to suppress the amount of generated code as needed while preventing image deterioration of an image that is easy to encode, the difference in bit rate change is large, and the buffer at the time of rate change Even if the occupation amount is small, by preventing the occurrence of VBV underflow, it is possible to prevent image quality deterioration due to the occurrence of picture skip or macro block skip.

  In the processing described above, even when it is detected in step S42 that the bit rate has been changed from the high rate to the low rate, it has been detected that the bit rate has been changed from the low rate to the high rate. Even so, although it has been described that the initial buffer capacity of the virtual buffer is adjusted according to the bit rate, VBV occupation amount, and image difficulty level, the bit rate is low because VBV underflow is likely to occur. Since the rate is changed from high rate to high rate, only when the bit rate is changed from low rate to high rate, the initial buffer capacity of the virtual buffer depends on the bit rate, VBV occupancy, and image difficulty It is good also as what adjusts. Even if the initial buffer capacity of the virtual buffer is adjusted when the bit rate is changed from the high rate to the low rate, the image quality is deteriorated as compared with the case where the initial buffer capacity of the virtual buffer is not adjusted. Therefore, if necessary, the initial buffer capacity of the virtual buffer may be adjusted only when the bit rate is changed from the high rate to the low rate.

  In the encoding device 41 as well, for example, when the virtual buffer is managed for each picture type as in TM5 (Test Model 5), the virtual buffer determination unit 62 first displays each bit rate after changing the bit rate. In the picture type, the initial buffer capacity of the virtual buffer is adjusted.

  In the present invention, for example, 15 frames are divided into three types of intra-frame encoded image (I picture), inter-frame forward prediction encoded image (P picture), or bidirectional predictive encoded image (B picture). When specifying which image type to process, and encoding a frame image according to the image type (I picture, P picture, or B picture) of the specified frame image In addition, as a low delay coding, each frame image is set to a P picture. For example, within the frame size of 45 macroblocks in the horizontal direction and 24 macroblocks in the vertical direction, from the upper part of the frame image to 2 macroblocks and 45 macroblocks in the horizontal direction. When the area is set as one intra slice part and all others are set as inter slice parts It can also be applied. In the case of performing low delay encoding, for example, the intra slice portion may be formed with regions of various other sizes, such as a region corresponding to one vertical macroblock and 45 horizontal macroblocks.

  Furthermore, the present invention is applicable to the case where encoding processing is performed by specifying the type of predictive encoding (intra macroblock or inter macroblock) for each macroblock, or when encoding such as all-intra is performed. Can also be applied.

  Further, in the above-described embodiment, the case where the present invention is applied to the encoding device 1 or the encoding device 41 as an encoding device that performs compression encoding by the MPEG method has been described. However, the present invention is not limited to this. Instead, the present invention may be applied to encoding devices using other various image compression methods.

  Further, the present invention is applied not only to the encoding device 1 or the encoding device 41 configured as an independent device, but also to an encoding unit having a similar configuration included in, for example, a transcoder or an editing device. Is possible.

  The series of processes described above can be executed by hardware, but can also be executed by software. In this case, for example, the processing executed by the encoding device 1 is configured by a personal computer 101 as shown in FIG.

  In FIG. 8, a CPU (Central Processing Unit) 111 performs various processes according to a program stored in a ROM (Read Only Memory) 112 or a program loaded from a storage unit 118 to a RAM (Random Access Memory) 113. Execute. The RAM 113 also appropriately stores data necessary for the CPU 111 to execute various processes.

  The CPU 111, the ROM 112, and the RAM 113 are connected to each other via the bus 114. An input / output interface 115 is also connected to the bus 114.

  The input / output interface 115 includes an input unit 116 including a keyboard and a mouse, an output unit 117 including a display and a speaker, a storage unit 118 including a hard disk, and a communication unit 119 including a modem and a terminal adapter. It is connected. The communication unit 119 performs communication processing via a network including the Internet.

  A drive 120 is connected to the input / output interface 115 as necessary, and a magnetic disk 121, an optical disk 122, a magneto-optical disk 123, a semiconductor memory 124, or the like is appropriately mounted, and a computer program read from them is loaded. If necessary, it is installed in the storage unit 118.

  When a series of processing is executed by software, a program constituting the software executes various functions by installing a computer incorporated in dedicated hardware or various programs. For example, a general-purpose personal computer is installed from a network or a recording medium.

  As shown in FIG. 8, this recording medium includes a magnetic disk 121 (including a floppy disk) and an optical disk 122 (including a floppy disk) that are distributed to supply a program to a user separately from the apparatus main body. CD-ROM (compact disk-read only memory), DVD (including digital versatile disk), magneto-optical disk 123 (including MD (mini disk) (trademark)), or semiconductor memory 124 In addition, the program is configured by a ROM 112 storing a program, a hard disk included in the storage unit 118, and the like supplied to the user in a state of being incorporated in the apparatus main body in advance.

  In the present specification, the steps for describing a program include not only processes performed in time series in the order in which they are included, but also processes executed in parallel or individually, even though they are not necessarily processed in time series. Is included.

It is a figure for demonstrating the maximum capacity | capacitance of a VBV buffer, a bit rate, and an encoding delay. It is a figure for demonstrating the buffer capacity | capacitance in case a bit rate is changed. It is a block diagram which shows the structural example of the encoding apparatus to which this invention is applied. 10 is a flowchart for explaining virtual buffer adjustment processing 1; It is a block diagram which shows the other structural example of the encoding apparatus to which this invention is applied. 10 is a flowchart for explaining virtual buffer adjustment processing 2; 10 is a flowchart for explaining virtual buffer adjustment processing 3; It is a block diagram which shows the structure of a personal computer.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 Encoder, 11 Image rearrangement part, 12 Computation part, 13 Motion detection part, 14 DCT part, 15 Quantization part, 16 VLC part, 17 Inverse quantization part, 18 IDCT part, 19 Computation part, 20 Motion compensation Unit, 21 video memory, 22 buffer, 23 quantization control unit, 31 bit rate information acquisition unit, 32 virtual buffer determination unit, 41 encoding device, 51 quantization control unit, 61 image difficulty level information acquisition unit

Claims (6)

In an encoding device for encoding frame image data,
First detection means for detecting a bit rate of the frame image data;
Determining means for determining quantization index data using the value of the initial buffer capacity of the virtual buffer;
Quantization means for performing quantization based on the quantization index data determined by the determination means;
Encoding means for encoding the quantized coefficient data quantized by the quantization means, and
The determination unit determines whether or not to adjust an initial buffer capacity of the virtual buffer when the bit rate is changed based on a detection result by the first detection unit. apparatus. The determination means further detects the buffer occupancy, and when the bit rate is changed, the first bit rate before the change detected by the first detection means and the second bit after the change. 2. The initial buffer capacity is adjusted by calculating a value of an initial buffer capacity of the virtual buffer based on a rate and a detected buffer occupancy at the time of changing a bit rate. Encoding device. A second detection means for detecting a degree of difficulty in encoding the frame image data;
The determination means further detects the buffer occupancy, and when the bit rate is changed, the first bit rate before the change detected by the first detection means and the second bit after the change. The initial buffer capacity value of the virtual buffer is calculated based on the rate, the buffer occupancy at the time of changing the bit rate, and the encoding difficulty level of the frame image data detected by the second detection means The encoding apparatus according to claim 1, wherein the initial buffer capacity is adjusted. A second detection means for detecting a degree of difficulty in encoding the frame image data;
The determining means further detects the buffer occupancy, and when the bit rate is changed, compares the encoding difficulty of the frame image data detected by the second detecting means with a predetermined threshold, When the encoding difficulty level is higher than the predetermined threshold, the first bit rate before the change detected by the first detection means, the second bit rate after the change, and the detected bit rate Based on the buffer occupancy at the time of change and the encoding difficulty level of the frame image data detected by the second detection means, a value of the initial buffer capacity of the virtual buffer is calculated, and the initial buffer capacity The encoding device according to claim 1, wherein the encoding device is adjusted. In an encoding method of an encoding device for encoding frame image data,
A bit rate information acquisition step of acquiring information on the bit rate;
A determination step of determining whether or not the bit rate of a picture constituting the video data has been changed based on information on the bit rate acquired by the processing of the bit rate information acquisition step;
An encoding method comprising: an adjusting step of adjusting an initial buffer capacity of the virtual buffer when it is determined that the bit rate has been changed by the processing of the determining step. A program for causing a computer to execute processing for encoding frame image data,
A bit rate information acquisition step of acquiring information on the bit rate;
A determination step of determining whether or not the bit rate of a picture constituting the video data has been changed based on information on the bit rate acquired by the processing of the bit rate information acquisition step;
An adjustment step of adjusting an initial buffer capacity of the virtual buffer when the bit rate is determined to have been changed by the processing of the determination step.

Images