MPEG-4提出了视频对象的概念，产生了基于对象的速率控制问题，编码的视频对象可以是同步进行也可以是异步进行，后者在低比特率条件下更具有优越性，实验结果也证实了这点，针对异步视频对象编码，提出 一种速率控制算法，以达到恒定目标比特率，该算法首先根据在一个短的时间段内，一个视频对象的同一类型编码比特数近似相等的假设来估计某时刻总的编码比特数，然后采用全局分配原则，将目标比特数分配到编码的视频对象中，以获得视频对象近似恒定的失真比，另外，还提出了一种后处理技术，并用以克服异步对象的解码合成图象的失真，实验结果表明，该算法能够对异步视频对象进行有效的比特控制，并具有较高的主客观质量。

The object based rate control problem arises along with the video object concept proposed by MPEG 4. The video objects can be coded in synchronous or asynchronous way, but latter one has more advantages in low bit rate coding and it is also proved by the experiments of this paper. This paper proposes a new rate control algorithm which can achieve a constant bit rate when coding asynchronous video objects. Firstly, the algorithm estimates the total target bits at one time assuming that the video plane from the same video objects of the same coding type and close in time are coded with a similar number of bits. Then the algorithm to allocate the target bits to the coding video objects at the time in order to achieve constant distortion ratios between pairs of video object planes adopts an allocation policy called constant distortion ratios. This paper also proposes a post processing technology to solve the problems of composition picture quality brought by asynchronous video objects. The experimental results show that the algorithm can efficiently control the coding bits of asynchronous video objects and achieve high subject and object quality.