# Audio Time Stretching Using Fuzzy Classification of Spectral Bins
## 4 Evaluation  
To evaluate the quality of the proposed TSM technique, a listening test was conducted. The listening test was realized online using the Web Audio Evaluation Tool [35]. The test subjects  
Figure 7: An example of the proposed transient preservation method. **(a)** shows the original audio, consisting of a solo violin overlaid with a castanet click. Also shown are the modified samples with TSM factor \(\alpha=1.5\), using (**b**) the standard phase vocoder, and (**c**) the proposed method. were asked to use headphones. The test setup used was the same as in [28]. In each trial, the subjects were presented with the original audio sample and four modified samples processed with different TSM techniques. The subjects were asked to rate the quality of time-scale modified audio excerpts using a scale from 1 (poor) to 5 (excellent). All 11 subjects who participated in the test reported having a background in acoustics, and 10 of them had previous experience of participating in listening tests. None of the subjects reported hearing problems. The ages of the subjects ranged from 23 to 37, with a median age of 28. Of the 11 subjects, 10 were male and 1 was female. In the evaluation of the proposed method, the following settings were used: the sample rate was 44.1 kHz, a Hann window of length \(N=4096\) was chosen for the STFT analysis and synthesis, the synthesis hop size was set to \(H_{\mathrm{s}}=512\), and the number of frequency bins in the STFT was \(K=N=4096\). The length of the median filter in the frequency direction was 500 Hz, which corresponds to 46 bins. In the time direction, the length of the median filter was chosen to be 200 ms, but the number of frames it corresponds to depends on the analysis hop size, which is determined by the TSM factor according to (10). Finally, the transient detection threshold was set to \(t_{d}=10^{-4}=0.00010\). In addition to the proposed method (PROP), the following techniques were included: the standard phase vocoder (PV), using the same STFT analysis and synthesis settings as the proposed method; a recently published technique (harmonic-percussive separation, HP) [28], which uses harmonic and percussive separation for transient preservation; and the elastique algorithm (EL) [36], which is a state-of-the-art commercial tool for time and pitch-scale modification. The samples processed by these methods were obtained using the TSM toolbox [37]. Eight different audio excerpts (sampled at 44.1 kHz) and two different stretching factors \(\alpha=1.5\) and \(\alpha=2.0\) were tested using the four techniques. This resulted in a total of 64 samples rated by each subject. The audio excerpts are described in Table 1. The lengths of the original audio excerpts ranged from 3 to 10 s. The processed audio excerpts and Matlab code for the proposed method are available online at [[http://research.spa.aalto.fi/publications/papers/applsci-ats/](http://research.spa.aalto.fi/publications/papers/applsci-ats/)]([http://research.spa.aalto.fi/publications/papers/applsci-ats/](http://research.spa.aalto.fi/publications/papers/applsci-ats/)). To estimate the sound quality of the techniques, mean opinion scores (MOS) were computed for all samples from the ratings given by the subjects. The resulting MOS values are shown in Table 2. A bar diagram of the same data is also shown in Figure 8.  
As expected, the standard PV performed worse than all the other tested methods. For the _CastViolin_ sample, the proposed method (PROP) performed better than the other methods, with both TSM factors. This suggests that the proposed method preserves the quality of the transients in the modified signals better than the other methods. The proposed method also scored best with the _Jazz_ excerpt. In addition to the well-preserved transients, the results are likely to be explained by the naturalness of the singing voice in the modified signals. This can be attributed to the proposed phase propagation, which allows simultaneous preservation of the tonal and noisy qualities of the singing voice.