Discussions

The recorded tones are played by human fingers while the synthetic ones are generated with the general excitation signals, there are more variations in the former. Therefore, there are some variances between the synthetic and recorded tones in the magnitude and the level of the spectra and the waveforms as shown in Fig. 6.2 and Fig. 6.3. But because the model has included most room acoustic characteristics of the pipa, the auditory timbres of the two are with a little difference. However, a minor common comment of the listening test subjects is about the timbre of higher pitch synthetic tones (660 Hz and 1109 Hz). Therefore, increasing the number of excitations could be a compromised solution if needed. Not only for the typical pluck tones, the method may be also helpful for other techniques.

Another one can be discussed is that there is a larger timbre variance between the synthetic and recoded push tones after the pitch sliding. Because of the remaining partials of initial pitch, the timbre of the recorded one is more saturated or fuller than that of the synthetic one after the pitch sliding. It’s unlike the nonlinear mixing partials occurring because of the longitudinal string vibration [34]. But to solve the issue, the string model still needs to be

divided to two paths for generating two types of partial somewhat similarly as the guqin model [11]. Or, the resonant characteristic of the pipa body should be investigated more so that it can preserve the sound reverberation. Besides, although the just-noticeable-difference (JND) for pitch change is typically around 1/10th of a semitone [35], we almost cannot distinguish the continuous frequency varying due to the short process time and the small interval of the pitch sliding. Therefore, each shift interval with a 0.5 delay length is enough for synthesis.

Regarding the wheel tone, since the attack transient is a very important characteristic for the timbre of this tone, continuing to search whether other ways to get better excitation signals may be required.

Finally, more samples should be provided to the subjects while doing the listening test.

Since there is only one damped tone sample, a larger standard deviation compared with others is shown in the statistic result.

Chapter 7

Conclusions and Future Works

The acoustic characteristics and a synthetic method of performing the traditional Chinese instrument pipa have been discussed in detail. The proposed synthesis model has demonstrated the important techniques of the instrument such as pluck with two termination types, push, pull and wheel successfully. The results can capture the prime features of the pipa and the insufficient parts are discussed as well. More important, the model has been proved efficient enough for computation and provides high flexibility to let people play simulated and even surreal pipa tones with this synthesis technique in real-time.

Of course, this prototype model can be modified according to the discussion in last section, and improved further to cover more physical phenomena no matter the straightforward nonlinearity characteristic of the playing dynamics or more complex mixing between the resonator and the strings. Besides, more play techniques should be included. In addition, as long as the excitation database is completed for all kinds of timbre, a complete pipa synthesizer based on the model can be achieved with an attached control interface [36], [37].

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