Non-parallel voice conversion (VC) is a technique for learning mappings between source and target speeches without using a parallel corpus. Recently, CycleGAN-VC  and CycleGAN-VC2  have shown promising results regarding this problem and have been widely used as benchmark methods. However, owing to the ambiguity of the effectiveness of CycleGAN-VC/VC2 for mel-spectrogram conversion, they are typically used for mel-cepstrum conversion even when comparative methods employ mel-spectrogram as a conversion target. To address this, we examined the applicability of CycleGAN-VC/VC2 to mel-spectrogram conversion. Through initial experiments, we discovered that their direct applications compromised the time-frequency structure that should be preserved during conversion. To remedy this, we propose CycleGAN-VC3, an improvement of CycleGAN-VC2 that incorporates time-frequency adaptive normalization (TFAN). Using TFAN, we can adjust the scale and bias of the converted features while reflecting the time-frequency structure of the source mel-spectrogram. We evaluated CycleGAN-VC3 on inter-gender and intra-gender non-parallel VC. A subjective evaluation of naturalness and similarity showed that for every VC pair, CycleGAN-VC3 outperforms or is competitive with the two types of CycleGAN-VC2, one of which was applied to mel-cepstrum and the other to mel-spectrogram.
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- We evaluated our method on the Spoke (i.e., non-parallel VC) task of the Voice Conversion Challenge 2018 (VCC 2018) .
- For each speaker, 81 utterances (approximately 5 minutes; which is relatively low for VC) were used for training.
- In the training set, there are no overlaps between source and target utterances; therefore, this problem must be solved in a fully non-parallel setting.
- We did not use any extra data, module, or time alignment procedures for training.
- B: Mel-cepstrum conversion by CycleGAN-VC2  + WORLD vocoder .
- V1: Mel-spectrogram conversion by CycleGAN-VC  + MelGAN vocoder .
- V2: Mel-spectrogram conversion by CycleGAN-VC2  + MelGAN vocoder .
- V3: Mel-spectrogram conversion by CycleGAN-VC3 + MelGAN vocoder .
- Female (VCC2SF3) → Female (VCC2TF1)
- Male (VCC2SM3) → Male (VCC2TM1)
- Female (VCC2SF3) → Male (VCC2TM1)
- Male (VCC2SM3) → Female (VCC2TF1)
Female (VCC2SF3) → Female (VCC2TF1)
Male (VCC2SM3) → Male (VCC2TM1)
Female (VCC2SF3) → Male (VCC2TM1)
Male (VCC2SM3) → Female (VCC2TF1)