Perceptual Effects of Noise in Digital Video Compression

Abstract

We present results of subjective viewer assessment of video quality of MPEG-2 compressed video containing wide-band Gaussian noise. The video test sequences consisted of seven test clips (both classical and new materials) to which noise with a peak-signal-to-noise-ratio (PSNR) of from 28 dB to 47 dB was added. We used software encoding and decoding at five bit-rates ranging from 1.8 Mb/s to 13.9 Mb/s. Our panel of 32 viewers rated the difference between the noisy input and the compression-processed output. For low noise levels, the subjective data suggests that compression at higher bit-rates can actually improve the quality of the output, effectively acting like a low-pass filter. We define an objective and a subjective measure of scene criticality (the difficulty of compressing a clip) and find the two measures correlate for our data. For difficult-to-encode material (high criticality), the data suggest that the effects of compression may be less noticeable at mid-level noise, while for easy-to-encode video (low criticality), the addition of a moderate amount of noise to the input led to lower quality scores. This suggests that either the compression process may have reduced noise impairments or a form of masking may occur in scenes that have high levels of spatial detail.