Experiments with Digital Video Playback

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Gerber, Richard
Gharai, Ladan
In this paper we describe our experiments on digital video applications, concentrating on the static and dynamic tradeoffs involved in video playback. Our results were extracted from a controlled series 272 tests, which we ran in three stages. In the first stage of 120 tests, we used a simple player-monitor tool to evaluate the effects of various static parameters: compression type, frame size, digitized rate, spatial quality and keyframe distribution. The tests were carried out on two Apple Macintosh platforms: at the lower end a Quadra 950, and at the higher end, a Power PC 7100/80. Our quantitative metrics included average playback rate, as well as the rate's variance over one-second intervals. The first set of experiments unveiled several anomalous latencies. To track them down we ran an additional 120 tests, whose analysis led us to find the locus of the system's bottlenecks. They also let us conclude that a software-only solution was sufficient for good video playback on the systems under observation - provided that the operating system is tuned accordingly. In the next step we attempted to achieve this goal, by implementing our own video playback software and accompanying device-level handlers. Our emphasis was on achieving a controlled, deterministic coordination between the various system components. An additional set of 32 experiments were carried out on our platforms, which showed significant improvements in our quantitative performance measurements, as well as in visual quality. (Also cross-referenced as UMIACS-TR-95-103)