Subjective and Objective Quality Assessment of the SoftCast Video Transmission Scheme

SoftCast-based linear video coding and transmission (LVCT) schemes have been proposed as a promising alternative to traditional video coding and transmission schemes in wireless environments. Currently, the performance of LVCT schemes is evaluated by means of traditional objective scores such as PSNR or SSIM. Nevertheless, since the compression is performed in a very different way from traditional coding schemes such as HEVC, visual artifacts are also quite different and deserve to be subjectively assessed. In this paper, we propose a subjective quality assessment of SoftCast, pioneer and standard of the LVCT schemes. This study aims to better understand the tradeoffs between the LVCT parameters that can be tuned to improve the quality. These parameters, including different GoP-sizes, Compression Ratios (CR) and Channel Signal-to-Noise Ratio (CSNR), are used to generate a dataset of 85 videos. A Double Stimulus Impairment Scale (DSIS) test is performed on the received videos to assess the perceived quality. Results show that the key characteristic of SoftCast, the linear relation between CSNR and PSNR, is also observed with the Mean-Opinion Scores (MOS), except at high CSNR where the quality saturates. In addition, Bjøntegaard model is used to quantify the trade-offs between CR, GoP-size and CSNR, depending on the intended application. Finally, the performance of objective metrics compared to the obtained MOS is evaluated. Results show that Multi-Scale SSIM (MS-SSIM), SSIM and Video Multimethod Assessment Fusion (VMAF) metrics offer the best correlation with the MOS values.

基于SoftCast的线性视频编码与传输（Linear Video Coding and Transmission，缩写LVCT）方案，已被提出作为无线环境下传统视频编码与传输方案的极具前景的替代方案。当前，LVCT方案的性能通常通过峰值信噪比（Peak Signal-to-Noise Ratio，PSNR）、结构相似性指数（Structural Similarity，SSIM）等传统客观指标进行评估。然而，由于其压缩机制与高效视频编码（High Efficiency Video Coding，HEVC）等传统编码方案大相径庭，所产生的视觉伪影也存在显著差异，因此有必要对其开展主观质量评估。本文针对作为LVCT方案先驱与标杆的SoftCast，开展了主观质量评估研究。本研究旨在深入探究可用于优化视频质量的LVCT可调参数间的权衡关系。本次研究所选用的可调参数包括不同的图像组（Group of Pictures，GoP）大小、压缩比（Compression Ratios，CR）以及信道信噪比（Channel Signal-to-Noise Ratio，CSNR），基于这些参数生成了包含85段视频的数据集。研究采用双刺激损伤标度（Double Stimulus Impairment Scale，DSIS）测试对接收视频开展主观感知质量评估。实验结果表明，SoftCast的核心特性——信道信噪比与峰值信噪比间的线性关系——在平均意见得分（Mean Opinion Score，MOS）中同样得到验证，仅在高信道信噪比场景下出现质量饱和现象，偏离该线性关系。此外，本研究采用比约恩特加德（Bjøntegaard）模型，针对不同应用场景量化了压缩比、图像组大小与信道信噪比三者间的权衡关系。最后，研究评估了各类客观指标与所得平均意见得分的相关性表现。结果显示，多尺度结构相似性指数（Multi-Scale SSIM，MS-SSIM）、结构相似性指数（SSIM）以及视频多方法评估融合（Video Multimethod Assessment Fusion，VMAF）这三类指标与平均意见得分的相关性最优。