How can I invert a MelSpectrogram with torchaudio and get an audio waveform?

Question

I have a MelSpectrogram generated from:

eval_seq_specgram = torchaudio.transforms.MelSpectrogram(sample_rate=sample_rate, n_fft=256)(eval_audio_data).transpose(1, 2)

So eval_seq_specgram now has a size of torch.Size([1, 128, 499]), where 499 is the number of timesteps and 128 is the n_mels.

I’m trying to invert it, so I’m trying to use GriffinLim, but before doing that, I think I need to invert the melscale, so I have:

inverse_mel_pred = torchaudio.transforms.InverseMelScale(sample_rate=sample_rate, n_stft=256)(eval_seq_specgram)

inverse_mel_pred has a size of torch.Size([1, 256, 499])

Then I’m trying to use GriffinLim:

pred_audio = torchaudio.transforms.GriffinLim(n_fft=256)(inverse_mel_pred)

but I get an error:

Traceback (most recent call last):
  File "evaluate_spect.py", line 63, in <module>
    main()
  File "evaluate_spect.py", line 51, in main
    pred_audio = torchaudio.transforms.GriffinLim(n_fft=256)(inverse_mel_pred)
  File "/home/shamoon/.local/share/virtualenvs/speech-reconstruction-7HMT9fTW/lib/python3.8/site-packages/torch/nn/modules/module.py", line 727, in _call_impl
    result = self.forward(*input, **kwargs)
  File "/home/shamoon/.local/share/virtualenvs/speech-reconstruction-7HMT9fTW/lib/python3.8/site-packages/torchaudio/transforms.py", line 169, in forward
    return F.griffinlim(specgram, self.window, self.n_fft, self.hop_length, self.win_length, self.power,
  File "/home/shamoon/.local/share/virtualenvs/speech-reconstruction-7HMT9fTW/lib/python3.8/site-packages/torchaudio/functional.py", line 179, in griffinlim
    inverse = torch.istft(specgram * angles,
RuntimeError: The size of tensor a (256) must match the size of tensor b (129) at non-singleton dimension 1

Not sure what I’m doing wrong or how to resolve this.

Asked By: Shamoon

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Source

Answer 1

Just from looking at the Torch Documentation.

The shape of the input to the Griffith reconstruction should be

(...,freq,frame). Here freq is n_fft/2 + 1 (presumably it omits the negative frequencies).

Therefore, if you did a 256 FFT than the shape of inverse_mel_pred should be [1,129,499] not [1,256,499]. To get this shape you should just omit all the negative frequency bins of each spectrogram in inverse_mel_pred. I don’t use Torch, but generally the bins are ordered from negative to positive frequencies (and Torch’s utilities are just wrappers for other tools so I am fairly sure it does the same). Therefore to get the desired shape:

inverse_mel_pred = inverse_mel_pred[:,128::,:]

Then pass it to GriffinLim just like you already did.

I might be off by one or so in the line above so make sure the shape is correct.

Answered By: James Kl

Answer 2

The input:

specgram (Tensor) has the shape (…, freq, frames), where freq is n_fft // 2 + 1

So if inverse_mel_pred has a size of torch.Size([1, 256, 499]), n_fft should (256 – 1) * 2 =510

Answered By: one

Answer 3

By looking at the documentation and by doing a quick test on colab it seems that:

When you create the MelSpectrogram with n_ftt = 256, 256/2+1 = 129 bins are generated
At the same time InverseMelScale took as input the parameter called n_stft that indicates the number of bins (so in your case should be set to 129)

As a side note, I don’t understand why you need the transpose call, since according to the doc and my tests

waveform, sample_rate = torchaudio.load('test.wav')
mel_specgram = transforms.MelSpectrogram(sample_rate)(waveform)  # (channel, n_mels, time)

already returns a (channel, n_mels, time) tensor
and InverseMelScale wants a tensor of shape (…, n_mels, time)

Answered By: emmunaf

Answer 4

Just for history, full code:

import torch
import torchaudio
import IPython

waveform, sample_rate = torchaudio.load("wavs/LJ030-0196.wav", normalize=True)

n_fft = 256

n_stft = int((n_fft//2) + 1)
transofrm = torchaudio.transforms.MelSpectrogram(sample_rate, n_fft=n_fft)
invers_transform = torchaudio.transforms.InverseMelScale(sample_rate=sample_rate, n_stft=n_stft)
grifflim_transform = torchaudio.transforms.GriffinLim(n_fft=n_fft)

mel_specgram = transofrm(waveform)
inverse_waveform = invers_transform(mel_specgram)
pseudo_waveform = grifflim_transform(inverse_waveform)

And

IPython.display.Audio(waveform.numpy(), rate=sample_rate)

IPython.display.Audio(pseudo_waveform.numpy(), rate=sample_rate)

Answered By: tttzof351

How can I invert a MelSpectrogram with torchaudio and get an audio waveform?

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