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，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。目前，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 Index Measure，SSIM）等传统客观指标进行评估。然而，由于其压缩方式与高效视频编码（High Efficiency Video Coding，HEVC）等传统编码方案存在显著差异，其视觉失真特征也截然不同，因此有必要开展主观质量评估。 本文针对作为LVCT方案先驱与标准代表的SoftCast，提出了主观质量评估方法。本研究旨在更深入地理解可用于优化视频质量的LVCT可调参数间的权衡关系。研究中通过调整不同的图像组（Group of Pictures，GoP）大小、压缩比（Compression Ratio，CR）以及信道信噪比（Channel Signal-to-Noise Ratio，CSNR）等参数，构建了包含85段测试视频的数据集。随后，针对接收端视频开展了双刺激损伤量表（Double Stimulus Impairment Scale，DSIS）主观评测实验，以评估视频的主观感知质量。 实验结果表明，SoftCast的核心特性——CSNR与PSNR间的线性关联，在平均主观得分（Mean Opinion Score，MOS）中同样得到验证，仅在高CSNR场景下质量趋于饱和。此外，本文采用伯恩特加德（Bjøntegaard）模型，结合目标应用场景量化了CR、GoP大小与CSNR三者间的权衡关系。最后，评估了各类客观质量评价指标与所得MOS值的相关性，结果显示多尺度结构相似性（Multi-Scale Structural Similarity Index Measure，MS-SSIM）、SSIM以及视频多方法评估融合（Video Multimethod Assessment Fusion，VMAF）指标与MOS值的相关性表现最优。