Absolute Category Rating (ACR)

ACR is a category judgment method where the test sequences are presented one at a time and are rated independently on a category scale. (This method is also called Single Stimulus ⁴² Method.) Subjects are asked to rate the quality of the presentation based on the level of the quality they have in their opinion for it after viewing or listening it. This phase is named the voting time. The time pattern for the stimulus presentation can be illustrated by Figure 4.5. The voting time should be less than or equal to 10 seconds. The five-level scale for rating overall quality is the most used scale, see Table 4.1. If higher discriminative power is required, a nine-level scale may be used as shown in Table 4.2. There is another variant of this scale which is the 11-point scale, depicted in Figure 4.3. Finally, there is a general scale, which is the continuous quality scale that is depicted in Figure 4.7.

Table 4.1: ITU 5-point quality scale

Grading value	Estimated Quality
5	Excellent
4	Good
3	Fair
2	Poor
1	Bad

  

Table 4.2: ITU 9-point quality scale

Grading value	Estimated Quality
9	Excellent
8
7	Good
6
5	Fair
4
3	Poor
2
1	Bad

Figure 4.3: Eleven-point quality scale. 10 score for the sequence(s) that is identical to the reference one. Similarly, 0 score is for the sequence that has no similarity with the reference.

  

Figure 4.4: ITU 5-point impairment scale

Grading value	Estimated impairment level
5	Imperceptible
4	Perceptible, but not annoying
3	Slightly annoying
2	Annoying
1	Very annoying

Figure 4.5: Stimulus presentation timing in ACR method.

Figure 4.6: Stimulus presentation timing in DCR method.