How do I know which spectrogram frames belong to which audio samples?

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I’ve been using this script:

spgram = torchaudio.transforms.Spectrogram(512, hop_length=32)
audio = spgram(audio)

to get the spectrogram of some stereo music audio. I expected that the resulting spectrogram has the shape [2, 257, audio.shape[1]/32] However, that’s not the case. For examples, an audio clip with size [2, 199488] (with sr=24576) yields a spectrogram with size [2, 257, 6241] (note that 199488/32=6234). Why is that? and how can I convert from frame location to sample location?

torchaudio pytorch audio
1 Answers

See center parameter.

whether to pad waveform on both sides so that the t-th frame is centered at time t x hop_length. (Default: True)

So, by default, the signal is padded with zeros. The padding length is probably (win_length - hop_length). This ends up making the result longer by (win_length - hop_length) / hop_length, which is 7 in your case.

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