摘要
伴随着无线通信技术的迅速发展以及无线网络带宽的不断提高,在移动环境下为用户提供多媒体通信业务正在成为可能。视觉是人类最重要的信息载体,因此视频通信也成为多媒体业务的核心。另一方面,由于无线信道具有误码率高等特点,如何在无线环境中实时传输高质量的视频面临着巨大的挑战,成为人们的研究热点。
视频播放具有严格的实时性要求,因此,网络就必须为视频传输提供足够的带宽﹑有保障的延时和误码率。为了获得可接收的重建视频质量,视频传输至少需要28Kbps左右的带宽。而视频传输对时延非常敏感,为了保证视频能够连续流畅的播放,视频数据必须及时到达接收端,否则播放过程会中断,造成播放质量的严重下降。而使无线移动网络无法提供可靠的服务质量。
对于无线移动信道来说,网络的可用带宽是有限的。由于多径﹑衰落﹑时延扩展﹑噪声影响和信道干扰等原因,无线移动通信不仅具有带宽波动的特点,而且信道误码率高,经常会出现连续的﹑突发性的传输错误。无线信道可用带宽与传输速率的时变特性,使得传输的可靠性大为降低。
基于无线视频通信面临的挑战,本文在对新一代视频编码国际标准H.264/AVC研究的基础上,主要在提高抗误码扩散性能和降低H.264编码复杂度以及如何在DSP环境下实现H.264进行了研究。
为减少H.264中P帧模式选择的时间,本文提出一种优化算法,将视频序列分为背景区域和活动区域来处理,同时采用7种模块中的16×16、16×8、8×16、8×8以及4×4的模块,实验结果表明,该算法可以显著减少编码时间,提高编码效率。
在对H.264中扩展档次中所特有的SP/SI帧技术研究的基础上,提出一种SP帧与I帧相结合的抗误码扩散方案CSWI。实验证明,CSWI方法抗误码扩散性能比单一的使用SP帧取代I帧更好,而且能有效的控制视频编码码率。
相对于以往视频标准,H.264/AVC中采用了多参考帧技术,针对低码率视频通信的特点,对H.264/AVC中的多参考帧技术的编码效率和抗误码性能进行了测试,实验表明,在低码率视频通信中,减少参考帧的数目能有效地提高编码效率和抗误码性能。
在对TMS320DM642硬件结构和X264研究的基础上,实现了基于H.264的DSP视频编码,通过海康威视第三代视音频压缩卡DS-4008HC对实时视频信号进行采集、压缩编码,并对压缩码流进行JM86解码得出基于H.264的DSP编码器可以对视频信号进行实时编码。同时针对不同的视频源进行编码,并对编码码流进行分析,指出运动估计是后续研究的一个重点。
With the rapid development of wireless communication technology and the increase of wireless networks bandwidth , it is possible to transmit multimedia content in mobile circumstance. Video communication is the kernel of multimedia operation because vision is the most important information media of human being. On the other hand, the transmission of video content over wireless networks poses big technical challenge duo to the specific characteristics of wireless channel such as high bit error rate, it becomes a hotspot for the research!
Video display needs well timeliness, this requires the sufficient bandwidth, guaranteed time lag and the bit error rate. In order to replay the video , we need at least 28 Kbits bandwidth. Video transmission is very sensitive to the time lag, we must transmit the video signal from the sending terminal to the receiving terminal, or the video will not display glibly and continuously, so the quality of the video will drop. But mobile network couldn’t provide credible services.
For the wireless channels, the available bandwidth is limited. Due to the multipath effect, multipath delay, multipath fading and some other reasons, the mobile communication has the characteristics that its bandwidth is undulate and high bit error rate, so some continuous and gusty error of transmission usually appears. The reliability of the transmission decrease because of the available bandwidth and the transmission rate are time-variant.
Based on the challenge of the wireless video communication, this paper dose some research on the H.264 , especially on how to well control error diffuseness and drop the complexity of coding as well as how to realize the H.264 coding with DSP.
In order to reduce the time of the P-frames mode decision in H.264, this paper proposes an improved algorithm, to divide the video sequence into the background areas and the motional areas, and meanwhile to use the blocks: 16×16、16×8、8×16、8×8 and 4×4 of seven blocks,the experiment results indicate that this method could save lots of time for coding and increase the coding efficiency.
We do some research on the SP-frames and further on we bring forward a new thought CSWI that which can well control error diffuseness, and simulation results prove that it is better than the way of single using SP-frames to replace I-frames. Compared with the previous video standards, H.264/AVC use multi-Reference frame technology, for the features of low bit rate video communication, we test the coding efficiency and Error resilient performance of the multi-Reference frame technology in H.264. Experiments show that reducing the number of the reference frames can effectively improve the coding efficiency and error resilient performance for the low bit-rate video communication.
Based on the research of the hardware of the TMS320DM642 and the X264, we make use of the DSP to encoding the video signals. By means of collecting, compressing, and decoding through JM86, we know that this method could real-time encoding the video signals. In this coding technique,motion estimation takes up most of the time, so the optimization of motion estimation is a key problem for the improvement of coding efficiency.
引文
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