GB/T 15845.5-1995 Technical requirements for audio-visual user terminals Video codec for p×64 kbit/s audio-visual services
Some standard content:
ICS.33.140
National Standard of the People's Republic of China
GB/T15845.5—1995
Technical requirements of audiovisual terminalsVideo codefor audiovisual services at PX64kbit/sPerformance requirements of audiovisual terminalsVideo codefor audiovisual services at PX64kbit/sPublished on December 13, 1995
State Administration of Technical Supervision
Implemented on June 1, 1996
National Standard of the People's Republic of China
Technical requirements of audiovisual terminalsVideo codefor audiovisual services at PX64kbit/sPerformance requirements of audiovisual terminalsVideo codefor audiovisual services atPX64kbit/sKAONiKAca-
GB/T15845.5—1995
This standard adopts the International Telegraph and Telephone Consultative Committee (CCITT) recommendation H.261 "Video Codec for PX64kbit/s Audiovisual Services" (1992 Edition) equivalently.
1 Subject content and scope of application
This standard specifies the technical requirements for video encoding and decoding of moving images in audiovisual services operating at a rate of P×64kbit/s, where P is 1 to 30.
This standard specifies the requirements for encoding and decoding of moving images in various audiovisual services using P×64kbit/s channels. 2 Reference standards
GB/T15845.1 Technical requirements for audio-visual user terminals Frame structure of 64-1920 kbit/s channels in audio-visual user terminal services CCIT T I.420 Basic user
-network interface
CCIR601 Studio digital television coding parameters 3 Technical requirements
3.1 Overview
The block diagram of the codec is shown in Figure 1.
Adoption instructions:
1J Except for the arrangement format in accordance with GB1.1, the technical content of Chapter 3 of this standard is completely identical to the CCIT T H.261 recommendation. Approved by the State Administration of Technical Supervision on December 13, 1995 and implemented on June 1, 1996
Source detector
Tide extraction alcohol device
3.1.1 Video input and output
GB/T15845.5-1995
External control
Video multiplexing device
) Video codec
Iodine display multiplexing encoder
L Video codec
Digital device
Burning device
Block diagram of video codec
Code data
Explanation
The source encoder specified in this standard is applicable to the encoding of pictures in the Common Intermediate Format (CIF) (see 3.2.1). The input and output television signals can be composite or component signals, analog or digital signals. The standards of these signals and the mutual conversion methods of public intermediate formats are not specified in this standard.
3.1.2 Digital input and output
The video encoder provides a self-generated digital bit stream, which will be combined with other possible signals (according to GB/T15845.1). The video decoder completes the reverse process. 3.1.3 Sampling frequency
The image is sampled at an integer multiple of the television line rate, and the sampling clock is asynchronous with the clock of the digital network. 3.1.4 Source coding algorithm
A method combining inter-frame prediction to reduce temporal redundancy and transform coding to reduce spatial redundancy of the residual signal is adopted. The decoder has motion compensation capability, allowing the use of motion compensation technology in the encoder. 3.1.5 Bit rate
This standard specifies that the main video bit rate used is between 40kbit/s and 2Mbit/s. 3.1.6 Transmission symmetry
The codec can be used for two-way or one-way visual communication. 3.1.7 Error Control
The transmitted bit stream contains a BCH (511, 493) forward error correction code, which is optional in the decoder. 3.1.8 Multipoint Operation
Contains the features required to support multipoint operation of the switch. 3.2 Source Encoder
3.2.1 Source Format
For non-interlaced (scanned) images, the source encoder operates at 30000/1001 per second (approximately 29.97 Hz frame rate), and the image frequency tolerance is ±50ppm.
GB/T15845.5—1995
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The image is encoded according to the brightness and color difference components (Y, CB and C). These components and the codes representing their sample values follow the provisions of CCIR601 recommendations.
Black = 16
Zero color difference = 128
White = 235
Peak color difference = 16 and 240
These values are nominal values, and the encoding algorithm works on inputs from 1 to 254. Two image scanning formats are specified as follows:
In the first format (CIF), the luminance sampling structure is 352 pixels per row arranged orthogonally, with 288 lines per picture in the vertical direction, and the sampling structure of each of the two color difference components is 176 pixels per row arranged orthogonally. There are 144 lines per picture in the vertical direction. The color difference samples should be arranged as shown in Figure 2 so that the boundaries of the blocks coincide with the boundaries of the luminance blocks. The image area covered by these pixels and lines has an aspect ratio of 4:3 and corresponds to the active part of the local standard television input. NOTE: The number of pixels per line is compatible for luminance and colour difference signals of 525 or 625 line video sources sampled at 6.75 MHz and 3.375 MHz respectively. These frequencies have a simple relationship to the frequencies recommended by CCITT 601. ××-××××
××××××
Figure 2 Arrangement of luminance and chrominance samples
Luminance samples:
Chroma samples:
Block edge.
The second format, quarter CIF (QCIF), has half the number of pixels and half the number of lines of the first format. All codecs must be able to operate with QCIF, and some codecs can also operate with CIF. The codec shall provide a method to limit the maximum picture rate of the encoder by having at least 0, 1, 2 or 3 non-transmitted pictures between transmitted pictures. It shall select this minimum picture interval and the CIF or QCIF format by external means (see, for example, GB/T 15845.1). 3.2.2 Video Source Coding Algorithm
Figure 3 shows the general form of the source coder, with the main units being prediction, block transform and quantization. The prediction error (inter-frame mode) or the input image (intra-frame mode) is divided into blocks of 8 pixels by 8 rows, which are then divided into those that are sent or not sent. The four luminance blocks are then combined with the two spatially corresponding color block differences to form a macroblock as shown in Figure 10 in Section 3.3.2.4. The mode and the criteria for selecting whether to send a block are not specified in this standard and can be changed dynamically as part of the coding control strategy. The block to be sent is transformed, the resulting transform coefficients are quantized and variable length coding is applied. 3
GB/T15845.5—1995
T—transformation, Q—quantization, P—image memory with variable delay for motion compensation, CC—coding control, F—loop filter, t—send or not flag; p—intra/interframe flag; q-quantization subscript of transform coefficient; qz—quantizer indication; f—on/off of loop filter; V—motion vector Figure 3 Source encoder
3.2.2.1 Prediction
Prediction is performed between images, and motion compensation (3.2.2.2) and spatial filtering (see 3.2.2.3) can be added. 3.2.2.2 Motion compensation
Interface
Motion compensation (MC) is an option in the encoder. The decoder receives a vector for each macroblock, and the horizontal and vertical components of these motion quantities have integer values not exceeding ±15. The vector is used for the four luminance blocks in this macroblock. Dividing the values of the motion vector components of the macroblock by 2 and discarding the decimal part gives the motion vectors of the two color difference components. A positive value of the horizontal or vertical component of the motion vector indicates that the prediction is made from pixels of the previous image that are spatially to the right or below the pixel to be predicted. The constraint on the motion vector is that all pixels referenced by the motion vector are in the coded image area. 3.2.2.3 Loop filter
The two-dimensional spatial filter can modify the prediction process. This filter (FL) only works on the pixels in the 8X8 block to be predicted. The filter can be separated into a one-dimensional horizontal function and a one-dimensional vertical function. Both one-dimensional filters are non-recursive, with coefficients of 0.25, 0.50, and 0.25. At the edge of the block, one of the taps will fall outside the block. At this time, the coefficients of the one-dimensional filter should be changed to 0, 1, and 0. All calculation results output by the two-dimensional filter are rounded to 8-bit integer values. All 6 blocks in a macroblock should switch the filter on/off according to the type of the macroblock (see 3.3.2.3 MTYPE).
3.2.2.4 Transformation
GB/T15845.5—1995
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A separable 8X8 two-dimensional discrete cosine transform is first used to process the block to be sent. The output of the inverse transform is limited and varies between 256 and 255 and is represented by 9 bits. The transfer function of the inverse transform is: f(a,9) = 1/42c(u)C(0)F(u,0)cos[p(2a + 1)u/16Jcos[p(2g + 1)0/16]（1）
where, ay=0,1,2....7,
-spatial coordinates in the pixel domain;
-transform domain coordinates,
c(u）=1/V2 when u=0, otherwise c(u）=1
C()=1/~2 when v=0 otherwise C()=1.
Note: In the block to be exchanged, =0 and 3=0 refer to the leftmost and topmost pixels of the image block respectively. This standard does not specify the calculation method of the calculation transform, but the inverse transform should meet the tolerance requirements specified in Appendix A (Supplement). 3.2.2.5 Quantization
There is one quantizer for the internal DC coefficients and 31 quantizers for the other coefficients. In a macroblock, all coefficients except the internal DC coefficients are quantized using the same quantizer. The decision level is not specified. The internal DC coefficients are linearly quantized transform values with a step size of 8 and no dead zone. Each of the other 31 quantizers is usually also linear, but with a central dead zone around zero (point) and an even value between 2 and 62 steps.
The reconstruction level is determined in 3.3.2.4.
Note: If the quantization step size is too small, the entire dynamic range of the transform coefficients cannot be represented. 3.2.2.6 Clipping of the reconstructed image
In order to prevent quantization distortion of the transform coefficient amplitudes from causing computational overflows in the encoder and decoder loops, clipping must be inserted. Clipping shall be applied to the reconstructed image, which is the sum of the prediction value and the prediction error that is continuously corrected during the encoding process. The limiter will limit pixel values less than 0 or greater than 255 to 0 and 255 respectively. 3.2.3 Coding Control
Several parameters can be varied to control the rate at which coded video data is generated, including source coder preprocessing, quantizer, block valid block criteria and temporal subsampling. How these quantities are proportionally related in the overall control strategy is outside the scope of this standard. Temporal subsampling will be achieved by discarding frames if necessary. 3.2.4 Forced Refresh
This function is achieved by forcing the use of the intra-frame mode of the coding algorithm. No refresh pattern is defined. To control the accumulation of inverse transform mismatch errors, at least one forced refresh should be issued for each macroblock up to 132 times. 3.3 Video Multiplexing Coder
3.3.1 Data Structure
Unless otherwise specified, the most significant bit is sent first, that is, bit 1 (Bit 1) or the leftmost bit in the code table of this standard. Unless otherwise specified, all unused or spare bits are set to "1". The remaining bits can only be used after CCITT specifies their functions.
3.3.2 Arrangement of video multiplexing
Video multiplexing is arranged in a hierarchical structure with 4 layers, from the top to the bottom: picture,
group of blocks (GOB)
macroblock (MB);
picoblock.
The system of the video multiplexing encoder is shown in Figure 4. 5
Figure Business Limit
Block Group Layer
3.3.2.1 Picture Layer
GB/T15845.5—1995
MRA STUFFING
TOCEFF
Indicates fixed length
PSPARE
GSPARE
Figure 4 System Diagram of Video Multiplexing Encoder
Indicates variable length
GOB LATER
MBLAYE
The data of each picture consists of a header and the GOB data following it. The picture layer structure is shown in Figure 5. The header of the discarded picture is not sent.
Picture Start Code (PSC)
PSPARE
Figure 5 Picture Layer Structure
20-bit codeword, value 00000000
Time Reference (TR)
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5-bit number, with 32 possible values, formed by increasing the value of the previously sent header by 1 plus the number of pictures (at 29.97 Hz) that have not been sent since the last transmission, using only the 5 least significant bits to complete the calculation. Type Information PTYPE
Information about the entire picture is:
Split screen indication, "0" for no, "1" for yes. bit2:
bit3:
bit5 and 6:
Extra Insertion Information (PEI)
GB/T15845.5—1995
Document camera indication, "0" not, "1" yes. Freeze image release, "0" not, "1" yes. Source format, "0" is QCIF, "1" is CIF. Spare.
When set to 1, this bit indicates that there is an optional data segment behind. Picture layer spare information (PSPARE)
0/8/16.-bit
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If PEI is set to "1", it is followed by 9 bits, of which 8 bits are data bits (PSPARE) and the 9th bit is the PEI bit, which indicates whether there is a 9-bit structure behind, and so on. The use of PSPARE is for further study. 3.3.2.2 Group of Blocks Layer
Each frame of an image is divided into a number of groups of blocks (GOBs). A group of blocks consists of an image area of 1/12CIF or 1/3QCIF (see Figure 6). A GOB is associated with an area of 176 pixels by 48 lines of Y and 88 pixels by 24 lines of each of the spatially corresponding C and C.
The data of each GOB consists of a GOB header followed by macroblock data, and its structure is shown in Figure 7. Between the image start codes, each group of blocks number is transmitted once in sequence. Even if there is no macroblock data in a group of blocks, the group of blocks number must be transmitted. The group of blocks number is shown in Figure 6.
Group start code (GBSC)
GQUANI
Arrangement of GOBs in a picture
GSTARE
Figure 7 Group hierarchy
0000000000001
16-bit codeword, value 0000
Group number (GN)
The 4 bits indicating the group position are represented by the binary code of the group number in Figure 6. Group numbers 13, 14 and 15 are reserved for future use. Group number 0 is used for PSC.
Quantizer information (GQUANT)
5-bit fixed-length codeword, which indicates the quantizer used for this group before any subsequent MQUANT replaces it. The codeword is the natural binary code of the QUANT value (see Section 3.3.2.4), which is half the quantization step size and ranges from 1 to 31. Extra Insertion Information (GEI)
When this bit is set to "1", it indicates that there are optional data segments to follow. Block Group Spare Information (GSPARE)wwW.bzxz.Net
0/8/16..bit
GB/T15845.5—1995
If GEI is set to "1", the following 9 bits consist of 8 bits of data (GSPARE) and another GEI bit. The GEI bit indicates whether there are 9-bit groups to follow, and so on. The use of GSPARE is for further study. Note: If the future specification of GSPARE does not limit the last GSPARE data bit, confusion with the start code may occur. 3.3.2.3 Macroblock layer
Each GOB is divided into 33 macroblocks, as shown in Figure 8. A macroblock is associated with 16 pixels by 16 rows of Y, and spatially corresponds to each 8 pixels by 8 rows of Cs and CR. 1
Figure 8 Arrangement of macroblocks in GOB
The data of a macroblock consists of an MB header followed by image block data, as shown in Figure 9. The presence or absence of MOUANTMVD and CBP is indicated by MTYPE.
Macroblock address (MBA)
MQUANT
Structure of macroblock layer
Variable length
Block Data
The position of a macroblock in a block group is indicated by a variable length codeword, and the transmission order is shown in Figure 8. The MBA of the first macroblock transmitted in a GOB is the absolute address. For subsequent macroblocks, the MBA is the absolute address difference between this macroblock and the previous transmitted macroblock. The code table of MBA is given in Table 1.
This table contains the codewords used for bit filling (MBAstuffing) immediately following the GOB header or the coded macroblock. This codeword should be discarded in the decoder.
The VLC of the start code (startcode) is also shown in Table 1. Table 1 Variable length code table for macroblock addressing
0000111
0000110
00001011
010101
010010
0100011
0100001
00000111
00000110
010111
MBA is always included in the transmitted macroblock.
GB/T15845.5—1995
Continued Table 1
MBAStuffing
Start code
If there is no information in the part of the picture where the macroblock is located, the macroblock is not sent. Type information (MTYPE)
Variable length
0011011
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The variable length codeword gives the relevant information of the macroblock and the information of which data units exist in the macroblock. The macroblock type, the units contained and the VLC codeword are listed in Table 2.
Interframe+MC
Interframe+MC
Interframe+MC
Interframe+MC+FIL
Interframe+MC+FIII
Interframe+MC+FIL
MQUANT
Note: ① "X" means that the item exists in this macroblock. VLC table of MTYPE
TCOEFF
0000000001
② It is possible to use the filter for non-motion compensated macroblocks, as long as its type is specified as MC+FIL, but with zero loss. ③MC indicates motion compensated prediction, and FIL indicates loop filtering. MTYPE is always included in the transmitted macroblock. Quantizer (MQUANT)
The existence of MQUANT is only indicated by MTYPE. A 5-bit codeword indicating the quantizer used for this macroblock and subsequent macroblocks in this block group before any subsequent MQUANT substitution. The MQUANT codeword is the same as GQUANT.
Motion Vector Data (MVD)
Variable length
Motion vector data is present in all motion compensated (MC) macroblocks. The MVD is obtained by subtracting the previous macroblock vector from the current macroblock vector. When calculated in this way, the previous macroblock vector is considered to be zero in the following three cases: a.
MVD is calculated for macroblocks 1, 12, and 23;
MVD is calculated for macroblocks whose difference indicated by MBA is not 1; b.
The MTYPE of the previous macroblock is not motion compensated. 9
GB/T15845.5—1995
MVD consists of two variable length codewords, the first one is the horizontal component and the second one is the vertical component. The variable length codewords are listed in Table 3. Each VLC codeword in Table 3 represents two differences, and only one of the differences will produce a VLC table of a macroblock vector MVD that falls within the allowed range
-16&16
-15&17
-14&18
-13&19
-11&21
-10&22
10&—22
11&-21
12&-20|| tt||13&-19
001111
001110
001101
001100
Coded Block Pattern (CBP)
15845.51995
CBP from
VLC table
Variable length
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The presence of CBP is indicated by MTYPE. This codeword gives a pattern number, which indicates those blocks in the macroblock that have at least one transform coefficient to be sent. The pattern number is given by equation (2): 11
Calculate MVD for macroblocks 1, 12, and 23;
Calculate MVD for macroblocks whose difference indicated by MBA is not 1; b.
The MTYPE of the previous macroblock is not motion compensated. 9
GB/T15845.5—1995
MVD consists of two variable-length codewords, the first one is the horizontal component and the second one is the vertical component. The variable-length codewords are listed in Table 3. Each VLC codeword in Table 3 represents two differences, and only one of the differences will produce a VLC table of a macroblock vector MVD that falls within the allowed range
-16&16
-15&17
-14&18
-13&19
-11&21
-10&22
10&—22
11&-21
12&-20|| tt||13&-19
001111
001110
001101
001100
Coded Block Pattern (CBP)
15845.51995
CBP from
VLC table
Variable length
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The presence of CBP is indicated by MTYPE. This codeword gives a pattern number, which indicates those blocks in the macroblock that have at least one transform coefficient to be sent. The pattern number is given by equation (2): 11
Calculate MVD for macroblocks 1, 12, and 23;
Calculate MVD for macroblocks whose difference indicated by MBA is not 1; b.
The MTYPE of the previous macroblock is not motion compensated. 9
GB/T15845.5—1995
MVD consists of two variable-length codewords, the first one is the horizontal component and the second one is the vertical component. The variable-length codewords are listed in Table 3. Each VLC codeword in Table 3 represents two differences, and only one of the differences will produce a VLC table of a macroblock vector MVD that falls within the allowed range
-16&16
-15&17
-14&18
-13&19
-11&21
-10&22
10&—22
11&-21
12&-20|| tt||13&-19
001111
001110
001101
001100
Coded Block Pattern (CBP)
15845.51995
CBP from
VLC table
Variable length
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The presence of CBP is indicated by MTYPE. This codeword gives a pattern number, which indicates those blocks in the macroblock that have at least one transform coefficient to be sent. The pattern number is given by equation (2): 11
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