问题
I have an audio sample of about 14 seconds in 8khz Sample Rate. Im using librosa to extract some features from this audio file.
y, sr = librosa.load(file_name)
stft = np.abs(librosa.stft(y, n_fft=n_fft))
# file_length = 14.650022675736961 #sec
# defaults
# n_fft =2048
# hop_length = 512 # win_length/4 = n_fft/4 = 512 (win_length = n_fft default)
#windowsTime = n_fft * Ts # (1/sr)
stft.shape
# (1025, 631)
Specshow :
librosa.display.specshow(stft, x_axis='time', y_axis='log')
[![stft sr = 22050][1]][1]
Now, i can understand the shape of the STFT
631 time bins = are 4 * ( file_length / Ts * windowsTime) #overlapping
1025 frequency bins = Frames frequency gap sr/n_fft.
so there are 1025 frequencies in 0 to sr/2(Nyquest)
what i cant understand is the different plot of two different sample rates with same ratios. 1 - 22050 as librosa default 2 - 8khz as sampling rate file
y2, sr = librosa.load(file_name, sr=None)
n_fft2 =743 # (same ratio to get same visuals for comparsion)
hop_length = 186 # (1/4 n_fft by default)
stft2 = np.abs(librosa.stft(y2, n_fft=n_fft2))
so ofc the shappe of stft will be different
stft2.shape
# (372, 634)
[![stft sr = 743][2]][2]
1. but why is the absoulte frequencies are not the same? its the same signal just not being oversampled so each sample is unique. what am i missing? is it the static y axis?
2. i couldnt understand the time bins values. im expecting bins in the number of frames when the first 1 is in the hop length and the second bin is windowTime from that point until the end of the file. but the units are wierd?
i want to be able to extract the magnitude of a specific Fbin in a specific time (frame) or additionally be able to sum some of those to get the magnitue for time RANGE.
Therefore, if i take stft[number of fBin] which is 1 row of 1025 fBins (stft[1025]) and look at it contents so stft[0] contains 630 points, which are exactly 630 time points for each frequency so each of the frames 1-1025 will have the same time points.
so if i take one sample which suits all the other fbins as well ( same time points) which is stft[0] i would be able to choose time frame and fBin and get the spcific magnitude:
times = librosa.core.frames_to_time(stft2[0], sr=sr2, n_fft=n_fft2, hop_length=hop_length)
fft_bin = 6
time_idx = 10
print('freq (Hz)', freqs[fft_bin])
print('time (s)', times[time_idx])
print('amplitude', stft[fft_bin, time_idx])
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[1]: https://i.imgur.com/OeKzvrb.png
[2]: https://i.imgur.com/ALtba5F.png
回答1:
Question 1:
You need to specify the sampling rate when using specshow:
librosa.display.specshow(stft, x_axis='time', y_axis='log', sr=sr)
Otherwise the default value (22,050 Hz) will be used (see docs).
Question 2:
librosa.core.frames_to_time does not take stft[0] as argument, which would be the frequency bins of the first frame. Instead, it takes number of frames as first argument.
Imagine you have an audio signal with sr=10000 Hz. Then you run an STFT over it using n_fft=2000 and hop_length=1000. Then you get one frame per hop and the hop is 0.1s long, because 10000 samples correspond to 1s and 1000 samples (1 hop) therefore correspond to 0.1s.
stft[0] is not a frame number. Instead the first stft is of shape (1 + n_fft/2, t) (see here). This means the first dimension is the frequency bin and the second dimension is the frame number (t).
The total number of frames in stft is therefore stft.shape[1].
To get the length of the source audio, you could do:
time = librosa.core.frames_to_time(stft.shape[1], sr=sr, hop_length=hop_length, n_fft=n_fft)
来源：https://stackoverflow.com/questions/57058875/stft-understanding-using-librosa