With the development of computer network and multimedia technology, the application of digital video is more and more extensive, such as DVD, network conference service, digital TV and so on. In these video processing and communication, effective video coding is its key technology.
H.264 / AVC is the latest generation of video coding standards jointly proposed by the ITU-T video coding expert group and the ISO / IEC moving image expert group. H.264 has many excellent performances [1]: higher compression ratio, compared with existing encoding standards (H.263, MPEG-4 Simple Profile), under the same video quality, can save about 50% of the code stream , The image quality is better, the adaptability is wider, and it can better meet various applications such as real-time (video conference) and non-real-time (storage, broadcasting, etc.). Realizing the real-time coding of H.264 on DSP has great engineering significance and economic value.
1 Introduction to H.264 standard and DSP platform
1.1 Introduction to the H.264 standard
H.264 is the latest video coding standard jointly developed by ITU-T and ISO / IEC. It was proposed by ITU-T in 1997 and the final draft standard was formed in March 2003. It contains many of the latest research results in the field of video compression, mainly using the following technology [1]:
(1) The coding is divided into coding layer VCL (Video Coding Layer) and transmission layer NAL (Network AbstracTIon Layer). Separating the coding layer and the transmission layer is conducive to the expansion of H.264.
(2) H.264 uses intra-frame prediction in the spatial domain. There are two prediction modes: intra16 & TImes; 16 and intra4 & TImes; 4. Among them, intra16 & TImes; 16 has four prediction methods, and intra4 × 4 has nine prediction methods.
(3) For inter-frame prediction, a prediction mode is added, a total of seven prediction modes. The prediction block can be subdivided into 4 × 4 from 16 × 16.
(4) Increase the number of reference frames to make the prediction more accurate.
(5) Put the deblocking filter in the coding loop to improve the subjective quality of the image.
(6) B frame can be used as a reference frame, and the decoding order of images is separated from the display order.
(7) Use integer coefficient transformation to increase the transformation speed.
(8) New entropy coding methods such as CAVLC and CABAC are used to improve the coding effect.
(9) The bitstream's ability to resist bit errors is improved. For example, if the encoded data is divided, a frame of image can be flexibly divided into several slices.
1.2 DSP platform based on Blackfin533
Blackfin533 is a high-performance video processing chip in the Blackfin series from Analog Devices. Its maximum frequency can reach 600MHz, and it can process 1200M times of multiplication and addition operations per second. There are a large number of special instructions for video, which can process multiple instructions in parallel.
In general, Blackfin533 is divided into two parts: kernel and system interface. The core refers to the processor, L1 memory, event controller, core timer, etc .; the system interface refers to the SPORT interface, PPI interface, SPI interface, external storage controller, DMA controller, and external resources that interface with them, etc.
The schematic diagram of the Blackfin533 development platform is shown in Figure 1. The analog video signal output by the camera is converted into a digital signal by the 7113 video chip. This signal enters Blackfin533 from the PPI interface of Blackfin533, and the compressed code stream is transmitted to the PC by the PCI bridge. This system is started by Flash, and the original image, reference frame and other variables in the encoding process are stored in SDRAM.
Figure 2 shows the video input module of the H.264 encoding system. The 7113 chip reads the analog signal output from the camera from the video terminal and outputs the digital signal to the Blackfin533 through the parallel port. Blackfin533 configures 7113 through I2C bus to output YUV mode, ITU656 mode and enhanced ITU656 mode. 
Figure 1 Blackfin533 platform overall framework 
Figure 2 Video input module
 
Figure 3 H.264 encoding module diagram
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