Statistical multiplexing using MPEG-2 video encoders

This paper presents a system for statistical multiplexing of several compressed video programs using MPEG-2-compatible video encoders. We propose a new external joint rate control algorithm to dynamically distribute the channel bandwidth among the program encoders such that the video quality is approximately equal in all programs. In our algorithm, the bit rate of each encoder is updated on the basis of the relative complexities of the programs measured at boundaries of groups of pictures (GOPs) and whenever scene changes are detected. The proposed algorithm requires no external preprocessing of the input video sources. Furthermore, as compared with previous work in this area, our algorithm is not restricted to operate only with encoders having the same GOP structure. Thus, the GOP boundaries at the different encoders need not be synchronized. Bit rate changes take place only at GOP boundaries, allowing the encoders to operate at a constant bit rate within GOPs. Overall, this results in a piecewise variable bit rate compression for each of the encoders. We also describe a strategy for decreasing the reaction delay of the system for scene changes. Experimental results show that the proposed multiprogram video compression

system results in good picture quality with no external preprocessing, despite its relative simplicity.

1. Introduction

In typical broadcast systems, such as in direct broadcast satellite (DBS) applications, multiple video programs are encoded in parallel, and the digitally compressed bitstreams are multiplexed onto a single, constant bit rate channel. The simplest approach to this multiprogram encoding is to divide the available channel bandwidth equally among all programs. This method has the disadvantage that at any instant in time, the resulting quality of the video programs is uneven because of the different scene content of the programs and changes of scene content over time. The explanation for this result lies in the rate-distortion theory [1]. To achieve equal video quality (i.e., equal distortion) for all programs, the available channel bandwidth should be distributed unevenly among the programs, namely, in proportion to the information content (e.g., complexity) of each of the video sources. Thus, the objective of statistical multiplexing is to dynamically distribute the available channel bandwidth among the video programs in order to maximize the overall picture quality of the system. This is achieved by using a joint rate-control algorithm that guides the operation of the individual encoders based on a continuous monitoring of the scene content of each of the video sources.

Basically, two different approaches can be distinguished for joint rate control: the feedback approach and the lookahead approach. In the feedback approach [2-4], statistical measurements of video complexity are generated by the encoders as a by-product of the compression process. The statistics from all encoders are compared and used to control the bit allocation for the subsequent video. In the look-ahead approach [3, 5], the complexity statistics are computed by preprocessing all video programs prior to encoding. These statistics are then used to more accurately predict the bit rate allocation needed for optimum compression of the video sources in the ratedistortion sense. Finding the best statistics to describe the complexity of a program is a challenging task. In the feedback approach, the statistics are limited primarily to coding-related parameters. The look-ahead approach provides more freedom of choice, but at the price of extra computational complexity and additional cost. In either case, the main feature of the statistical multiplexing (statmux) system is that each encoder will produce a variable rate bitstream [6].

In this paper, we propose a solution for statistical multiplexing of MPEG-2 compressed video programs [7]. In particular, an external joint rate control algorithm is proposed that dynamically allocates bit rates for the program encoders using the feedback approach. In our algorithm, the bit rate for a given program encoder is updated only at boundaries of groups of pictures (GOPs), or when a scene change is detected in the given program. This strategy allows the encoders to operate in a constant bit rate mode within the GOPs, resulting in piecewisevariable bit rate bitstreams. In contrast to previously published works in this area [2-6], the MPEG encoders in the proposed system are not required to have identical GOP structures. GOP boundaries may occur at arbitrary times in each encoded bitstream. Furthermore, for scene changes, a new GOP is started dynamically at or near the beginning of each new scene, ensuring quick reaction to video complexity changes. Because of these features, a channel buffer and a corresponding buffer control feedback loop are required in the proposed system.

In Section 2 we describe the proposed multiprogram video compression system. The joint rate control algorithm is presented in Section 3. The strategy for joint rate control in the event of scene change is described in Section 4. Determination of the minimum channel buffer size and the corresponding channel buffer control algorithm is given in Section 5. Finally, in Section 6, we present the experimental results obtained by computer simulations of the proposed system, followed by conclusions.