# Perceptual Quality of Audio-Visual Content with Common Video and Audio Degradations
## 7 Perceptual Experiment 2 (Audio-Only)  
For this experiment, the perceived quality responses were collected for a set of audio-visual video clips degraded with audio distortions only. The experiment had the goal of recreating four common streaming audio degradations from the TCD-VoIP dataset [30]: Background noise, Clipping, Chop, and Echo. These degradations were inserted into the audio components of a set of audio-visual sequences. The goal was to analyze the effect of such degradations on the perceived audio-visual quality. Forty (40) source stimuli (out of the entire 140 stimuli pool) were considered to build an audio-visual dataset, replicating the sequence processing method used in the TCD-VoIP dataset that was taken as a reference for this experiment. For each degradation type, four (4) single test conditions were selected and presented as a particular HRC. The selection of these test conditions was empirical, aiming to cover the quality range of the TCD-VoIP dataset. This resulted in sixteen (16) HRCs organized according to the type of degradation, as described in Table 8. Additionally, four test conditions without degradations were used as anchors (ANCs) to help participants establish the range of quality used in the experiment. All twenty (20) test conditions (16 HRCs plus 4 ANCs) were replicated for all forty (40) source stimuli, resulting in eight hundred (800) test stimuli. These 800 sequences compound the second part of the UnB-AV dataset [47]. The sub-set is labelled as UnB-AV-Experiment-2, and it is available for download along with the entire UnB-AV dataset. Figure 7 presents the average quality scores obtained for each HRC (i.e., the \(\text{MQS}_{\text{HRC}}\)), along with the individual scores given by each participant of Experiment 2. Results are organized according to the type of audio distortion. For each HRC, the \(\text{MQS}_{\text{HRC}}\) value is represented by the larger dot in the middle. It can be observed that, contrary to what was observed in Experiment 1, most of the responses gathered were disperse along the quality scale. Moreover, there is more agreement among participants for quality scores given to the anchors conditions (ANC1, ANC2, ANC3 and ANC4). This suggests that there was  
less agreement regarding the different audio distortions when compared to the agreement observed in Experiment 1 for the different visual distortions. In Figure 8a,b, the Mean Quality Score (\(\text{MQS}_{\text{HRC}}\)) and Mean Content Score (\(\text{MCS}_{\text{HRC}}\)) are presented. For each HRC, a confidence interval of 95% is included, plus all HRCs are grouped according to Background Noise, Chop, Clip, and Echo distortions. Each HRC is assigned with a combination of parameters according to the type of distortion (see Table 8). The \(\text{MQS}_{\text{HRC}}\) values are between 1.5 and 3.9, depending on the types of distortions and their degradation levels. With the exception of Echo distortions, the \(\text{MQS}_{\text{HRC}}\) range occupied only about 30% of the scale. This means that participants had difficulties distinguishing the quality levels for different types of audio distortions. Naturally, this effect was different for different types of audio distortions.