Report Number: CSL-TR-96-701
Institution: Stanford University, Computer Systems Laboratory
Title: Characterization of Quality and Traffic for Various Video Encoding Schemes and Various Encoder Control Schemes
Author: Dalgic, Ismail
Author: Tobagi, Fouad A.
Date: August 1996
Abstract: Lossy video compression algorithms, such as those used in the H.261, MPEG, and JPEG standards, result in quality degradation seen in the form of digital tiling, edge busyness, and mosquito noise. The encoder parameters (typically, the so-called quantizer scale) can be adjusted to trade-off encoded video quality and bit rate. Clearly, when more bits are used to represent a given scene, the quality gets better. However, for a given set of encoder parameter values, both the generated traffic and the resulting quality depend on the scene content. Therefore, in order to achieve certain quality and traffic objectives at all times, the encoder parameters must be appropriately adjusted according to the scene content. Currently, two schemes exist for setting the encoder parameters. The most commonly used scheme today is called Constant Bit Rate (CBR), where the encoder parameters are controlled to achieve a target bit rate over time by considering a hypothetical rate control buffer at the encoder's output which is drained at the target bit rate; the buffer occupancy level is used as feedback to control the quantizer scale. In a CBR encoded video stream, the quality varies in time, since the quantizer scale is controlled to achieve a constant bit rate regardless of the scene complexity. In the other existing scheme, called Open-Loop Variable Bit Rate (OL-VBR), all encoder parameters are simply kept fixed at all times. The motivation behind this scheme is to presumably provide a more consistent video quality compared to CBR encoding. In this report, we characterize the traffic and quality for the CBR and OL-VBR schemes by using several video sequences of different spatial and temporal characteristics, encoded using the H.261, MPEG, and motion-JPEG standards. We investigate the effect of the controller parameters (i.e., for CBR, target bit rate and rate control buffer size, and for OL-VBR, the fixed quantizer scale) and video content on the resulting traffic and quality. We show that with the CBR and OL-VBR schemes, the encoder control parameters can be chosen so as to achieve or exceed a given quality objective at all times; however, this can only be done by producing more bits than needed during some of the scenes. In order to produce only as many bits as needed to achieve a given quality objective, we propose a video encoder control scheme which maintains the quality of the encoded video at a constant level, referred to as Constant Quality VBR (CQ-VBR). This scheme is based on a quantitative video quality metric which is used in a feedback control mechanism to adjust the encoder parameters. We determine the appropriate feedback functions for the H.261, MPEG, and motion-JPEG standards. We show that this scheme is indeed able to achieve a constant quality at all times; however, the resulting traffic occasionally contains bursts of relatively high-magnitude (5-10 times the average), but short duration (5-15 frames). We then introduce a modification to this scheme, where in addition to the quality, the peak rate of the traffic is also controlled. We show that with the modified scheme, it is possible to achieve nearly constant video quality while keeping the peak rate within 2-3 times the average.