Of MOS and men: bridging the gap between objective and subjective quality measurements in mobile TV

In this paper we explore the relation between subjective and objective measures of video quality. We computed objective MOS values from video clips using the video quality measuring tool VQM and compared it to the clips ’ subjective Acceptability scores. Using the ITU defined mapping (M2G) from MOS to binary Good or Better (GoB) values, we compared the M2G translated values to the clips ’ subjective Acceptability scores at various encoding bitrates (32-224kbps) and sizes (120x90, 168x126, 208x156 and 240x180). The results show that in the domain of mobile TV the ITU mapping M2G represents a serious overestimation of Acceptability. The mapping M2A, between MOS and Acceptability, that we suggest provides a significant improvement of 76 % in the root mean square error (RMSE) over M2G. We show that Acceptability depended on more than just the visual quality and that both content type and size are essential to provide accurate estimates of Acceptability in the field of mobile TV. We illustrate this gain in Acceptability predictions for the popular content type football (soccer). In terms of RMSE our content dependent mapping (M2Af) yielded an improvement of 39 % over M2A. Future research will validate the predictive power of our suggested mapping on other video material