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Fundamental relationships between subjective quality, user acceptance, and the VMAF metric for a quality-based bit-rate ladder design for over-the-top video streaming services

A quality-based bit rate ladder design for over-the-top video streaming services is presented. Following the design criterion of maximizing subjective quality under the constraint of minimizing storage costs, the bit rate ladder is defined by three parameters. The first parameter is the lowest VMAF score at which a video signal is on average subjectively indistinguishable from the original video signal. Following the international recommendation ITU-R BT.500, extensive subjective tests were carried out to evaluate the fundamental relationships between the subjective quality and the VMAF score using a 4K OLED TV environment. Based on the test results, this VMAF score is set to 95. The second parameter is the lowest VMAF score being accepted on average by more than 50 % of the users for watching video signals of free streaming services. Additional tests yield in setting this VMAF score to 55. The third parameter is the maximum difference of two VMAF scores, for which the associated subjective qualities are approximately the same on average. In a third test, this difference is determined to be 2. This results in an ideal bit rate ladder providing each video signal in 21 qualities associated to the VMAF scores 95, 93, …, 57, 55. This bit rate ladder design can be applied to complete video signals occurring in per-title encoding strategies or to individual scenes of video signals occurring in per scene or shot-based encoding strategies. Applications using less than 21 renditions for this range, may suffer from impaired subjective quality