‘Subjective Tests’ Show H.265 Codec’s Efficiency Even Better Than Expected, BBC Says

“Subjective tests” using human viewers to verify the compression efficiency of Ultra HD’s H.265 codec that previously had been estimated using only “objective metrics” have found better than expected bit-rate savings compared with the older H.264 codec. So said BBC R&D Engineer Rajitha Weerakkody and BBC Video Coding Project Leader Marta Mrak in a Thursday blog post that summarized the findings in a just-published IEEE research paper they both helped author. The overall average bit-rate saving achieved with H.265 compared with H.264 “for the same subjective quality” was found to be 59 percent, versus the 44 percent efficiency gain shown with objective quality metrics, Weerakkody and Mrak said. The tests also found the bit rate savings for larger picture sizes were higher than for smaller picture sizes, “which is a very encouraging sign for future UHD deployments,” they said. In the subjective tests, they said, viewers were shown “a carefully selected set of coded video sequences” in four different formats -- Ultra HD (3840x2160 and 4096x2048), 1080p (1920x1080), 720p (1280x720) and 480p (832x480), at frame rates varying between 30 and 60 Hz. Many of the tests were conducted at the BBC R&D labs “under controlled viewing environments,” in conformity with ITU recommendations on “visual quality assessment,” they said.