# A Psychoacoustic-Based Multiple Audio Object Coding Approach via Intra-Object Sparsity
## 3 Performance Evaluation
### Subjective Evaluation  
The subjective evaluation is further utilized to measure the perceptual quality of decoded object signals, which consists of four MUltiple Stimuli with Hidden Reference and Anchor (MUSHRA) listening tests. Sennheiser HD600 headphone is used for playback. Note that for the first three tests, each decoded object generated by the corresponding approach is played independently without spatialization. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**The Index of the Bitrate Sub-Band**} & \multicolumn{4}{c}{\(r\)} \\ \cline{2-5}  & **1–13** & **14–26** & **27–39** & **40–51** \\ \hline
105.14 kbps & \(2^{-1.5}\) & \(2^{-1.0}\) & \(2^{-0.5}\) & \(2^{0.0}\) \\
112.53 kbps & \(2^{-1.5}\) & \(2^{-1.0}\) & \(2^{-1.0}\) & \(2^{-1.0}\) \\
120.7 kbps & \(2^{-1.5}\) & \(2^{-1.5}\) & \(2^{-1.5}\) & \(2^{-1.5}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The \(q_{\textit{stepsize}}\) allocation for three types of bitrates. Figure 5: The ODG score of four multi-track audio files, where each file correspond to three types of bitrates. (**a–d**) represent the results for 4 multi-track audio files. The first test is the lossless transmission case, aims to make a comparison between our proposed encoding framework and the SPA algorithm. Four group multi-track audio files used in previous experiments are also treated as test data in this section. Condition 'SPA' means the reference approach (the same as condition 'SPA-STFT' in Section 3.2) and condition 'Pro' means the proposed framework. The original object signal is served as the Hidden Reference (condition 'Ref') and condition 'Anchor' is 3.5 kHz low-pass filtered anchor signal. A total of 15 listeners participated in the test. Results are shown in Figure 6 with 95% confidence intervals. It can be observed that the proposed encoding framework achieves a higher score than the SPA approach with clear statistical significant differences. Moreover, the MUSHRA scores for the proposed framework achieve over 80 indicating 'Excellent' subjective quality compared to the Hidden Reference, which proves that the better perceptual quality can be attained compared to the reference approach. For lossy transmission case, the downmix signal encoded at 105 kbps via SQVH corresponds to 'Pro-105'. Condition 'SPA-128' means the reference approach whose downmix signal compressed at the bitrate of 128 kbps using the MPEG-2 AAC codec. Results are presented in Figure 7 with 95% confidence intervals. Obviously, our proposed encoding scheme has a better perceptual quality and a lower bitrate compared to the SPA approach. That is, when a similar perceptual quality is desired, the proposed method requires less total bitrate than the SPA approach. Figure 6: MUltiple Stimuli with Hidden Reference and Anchor (MUSHRA) test results for the SPA framework and the proposed framework with 95% confidence intervals. Figure 7: MUSHRA test results for the SPA method encoding at 128 kbps and the proposed approach at 105.14 kbps with 95% confidence intervals. Furthermore, we evaluate the perceptual quality of the decoded audio objects using our proposed approach, using MPEG-2 AAC to encode each object independently and using Spatial Audio Object Coding (SAOC). The MUSHRA listening test is employed with five conditions, namely, Ref, Pro-105, AAC-30, SAOC and Anchor. The downlink signal in condition 'Pro-105' is further compressed using SQVH at 105.14 kbps. Meanwhile, the side information can be compressed at about 90 kbps [19]. Condition 'AAC-30' is the separate encoding of each original audio object using the MPEG-2 AAC codec at 30 kbps, the total bitrate is almost the same as 'Pro-120' (30 kbps/channel \(\times\) 8 channels = 240 kbps). Condition 'SAOC' represents the objects are encoded by SAOC. The total SAOC side information rate of input objects is about 40 kbps (5 kbps per object), while the downlink signal generated by SAOC is compressed by the standard audio codec MPEG-2 AAC at the bitrate of 128 kbps.