What is the JND Scale

The most traditional objective ways of evaluating quality of digital video processing system (e.g. digital encoder) are calculation of the signal-to-noise ratio (SNR) and peak signal-to-noise ratio (PSNR) between the reference and processed videos. PSNR is the most widely used objective video quality metric. However, PSNR reflects the absolute difference between two sequences and ignores the human brain's ability to compensate for degraded video quality.

The Video Quality Expert Group (VQEG) created a specification for subjective video quality testing and submitted it to the governing body as ITU-R BT.500 Recommendation. This recommendation describes methods for subjective video quality analysis where a group of human testers analyze the video sequence and grade the picture quality. The grades are combined, correlated and reported as Mean Opinion Score (MOS). More information is proved here.

The heuristic, nominal values for MOS when a 1-5 scale is used are listed below:

• 4.4-5.0 – Very Satisfied
• 4.0-4.3 – Satisfied
• 3.0-3.9 – Some Users Satisfied
• 2.0-2.9 – Many Users Dissatisfied
• 1.0-1.9 – Most Users Dissatisfied

Perceptual video assessment techniques are mathematical models that approximate results of subjective quality assessment. They take into account the human visual system (HVS) and measure the video's contrast, luminance, foreground/background, blocking, blurring, etc. The algorithm scales each of these measurements creating a value that reflects the perceived score of each frame (or field). If the algorithm is well designed, this number increases (or decreases) as the video quality increases.

This number is then correlated with the MOS data collected. Several organizations have collected subjective data - VQEG, CRC, EBU, etc. - but until recently none of them have made their studies public (the University of Texas did with their LIVE database in September 2009).

If you want to determine whether an audience can see a difference between 2 video sequences, then the JND scale (often referred to as Picture Quality Rating/PQR) can be used. PQR/JND is defined in T1.TR.75.2001.

JND Score - Based on the the Number of Experts

JND Score	Experts	Percentage	Description
0	2	50%	If you ask 2 experts which video is better, they cannot agree.
1	4	75%	3 pick one sequence and 1 picks the other sequence
2	8	87.5%	7 pick one sequence and 1 picks the other sequence
3	16	93.75%
4	32	96.875%
5	64	98.437%
6	128	99.219%
9	1024	99.902%	1023 pick one sequence and 1 picks the other sequence

Video Clarity created many different test cases and scored the results. The following is a graph created using the uncompressed football sequence as the source and a 15Mbps MPEG-2 compression as the processed. The areas with better scores are I-frames as discussed here. In general, this video is considered broadcast quality.



Using ClearView, you can automatically reject frames that fall under your perceived video quality thresholds. Setting these thresholds depends on the goals of your organization. We made the following chart as a guideline that we use internally as an example.

JND	Description
13+	Videos are probably not aligned. View our alignment application note.
10.0-12.99	Unwatchable
7.0-9.99	Annoying
2.0-6.99	Broadcast Quality
0.01-1.99	Production Quality
0	No Defects