8
votes
Accepted
Spectrogram of a single tone complex signal has two dark lines?
Since this is a constant spectrogram, you could just as well have just averaged the |FFT|² and plotted that! (The most colorful way of visualizing things isn't always the optimal one; your signal ...
- 29k
7
votes
Accepted
How to interpret spectrogram correctly?
The problem is not the spectrogram parameters, these are correct since they only depend on what resolution you want in time and frequency domain. Also, the spectrogram interpretation is correct, there ...
- 186
6
votes
Accepted
Self studying, getting a quality spectrogram
Why are my peaks capped?
Your amplification gain is set to too high, or you are too close to the microphone. The amplifier is driven to its limits and it clips the output. Keep this recording and ...
- 10.5k
6
votes
Python audio analysis: which spectrogram should I use and why?
I recommend a Synchrosqueezed Continuous Wavelet Transform representation, available in ssqueezepy. Synchrosqueezing arose in context of audio processing (namely speaker identification), and there's ...
- 8,726
5
votes
Accepted
Spectrograms for neural nets
The "dimensions" of the spectrogram are not chosen based on where will the spectrogram be fed to but rather depend on your application. Therefore, it is key to understand the spectrogram itself first, ...
- 10.5k
5
votes
Converting mel spectrogram to spectrogram
Nowadays the easiest thing would be to use librosa for this task. It has the mel_to_stft function which does exactly what you want.
As others have mentioned, this ...
- 11k
5
votes
Understanding overlapping in STFT
I understand the concept of the STFT. In order to avoid spectral leakage, you use a hann window that overlaps by 50%.
I'm sorry but you have a misunderstanding of spectral leakage in addition to how ...
- 27.9k
4
votes
unexplainable aliases in spectrogram
Looking at the spectrogram, the prominent artifacts go up or down in frequency synchronously to the 138 MHz signal but have larger bandwidths. That is an indication that they are its harmonics, due to ...
- 13.4k
4
votes
How to interpret spectrogram correctly?
The answer from @orgeGT is quite detailed. It really looks like, to me, a pressure signal from a knocking gazoline engine, where you observe a wide band effect (the impulse part of the knock) and ...
- 31.6k
4
votes
Accepted
What is the instantaneous frequency of this signal
Concerning the comment from Juancho, we can assume that the time is normalized to e.g. 1s, meaning that $t=1$ corresponds to $1s$. Then, we dont have a problem with the units (this a very common ...
- 6,168
4
votes
Accepted
Is it possible to recover a waveform from spectrograms of magnitude *and* phase?
Real/imaginary or modulus/phase are two representations of a complex number that carry the same level of information. Then, a STFT is a redundant mapping from a space of functions over a 1D variable (...
- 31.6k
4
votes
Accepted
How to calculate the PSD from the complex calculated STFT?
In general, if you have complex spectrum and need PSD in dB the mathematical equation is
$$P_{xx} = 20\cdot\log_{10}|X_{x}|,$$
where $P_{xx}$ is your PSD in dB and $X_{x}$ is your complex STFT ...
- 86
4
votes
Accepted
Overall spectrum computation from Short-Time-Fourier-Transform
You're almost there.
Your data should be scaled with dBFS reference (see the "reference" box in cool edit pro).
Cool Edit Pro probably computes the average power spectrum:
Modify
...
- 4,706
3
votes
Accepted
Time-Frequency Analysis of Big Data - Data Size Reduction: averaging the most appropriate method?
So, first of all, CSV seems to me the least suitable format imaginable for this amount of data. It needs to be parsed, is memory hungry, and wastes precision, and isn't linearly addressable (ie. to ...
- 29k
3
votes
Accepted
Is this waterfall characteristic of QAM or OFDM (or something other) modulation?
The sharp rectangular shape of the spectrum, with the immensely flat top definitely says "OFDM, with suitable whitening/PAPR reduction"; what constellation is used on the individual carriers is a bit ...
- 29k
3
votes
Why is the DFT used for spectrograms rather than the DCT?
Let's define the N-point IDFT $y[n]$ of a signal $Y[f]$ as
$$\begin{align*}
y[n] &= \sum\limits_{f=0}^{N-1} Y[f] e^{j2\pi n\,\frac{f}{N}},& n\in \{0,\dots,N-1 \}\tag{1}
\end{align*}$$
The ...
- 29k
3
votes
Accepted
Meaning of concentric circles on the spectrogram
In Understanding FFT Overlap Processing, p. 10, it is suggested that uniform of constant windows, for certain hop patterns, with repeated pulses, could generate such artifacts.
- 31.6k
3
votes
Understanding overlapping in STFT
From my answer Short-time Fourier transform Tradeoffs
Since the frequency content of overlapping windows are displayed sequentially rather than averaged, I find talking about overlapping windows ...
- 368
3
votes
What is the spectrogram of an impulse response used for?
I think you are thinking of waterfall plots and/or cumulative spectral decay plots. They are spectograms in the widest sense of the word since their windowing function tends to be exponential. They ...
- 31
3
votes
Accepted
Spectrogram with square or non-square magnitude of STFT: power vs. magnitude
In the image you gave, the right-hand scale is in decibels (dB). So, essentially, the square turns into an affine scaling in the logarithm domain, which essentially yields the same image, at least the ...
- 31.6k
3
votes
How to calculate the PSD from the complex calculated STFT?
I normally use the Welch method to calculate and plot the PSD of a signal.
...
- 63
3
votes
Accepted
How to convert a spectrogram back to a signal
I am not a good Python coder, but did similar processing in Matlab in the past. The subject has been discussed in SE.DSP in several instances, for instance Librosa stft + istft - Understanding my ...
- 31.6k
3
votes
Accepted
Why does a strange frequency appear in this image?
If the sample rate of data fed to a DFT or FFT for spectrum analysis was around 24 KHz, then what you are seeing is just the complex conjugate negative frequency image of the low (2.5 kHz) frequency ...
- 35.1k
3
votes
Time Frequency Analysis by Frequency Contour Detection in Spectrogram
The OP is interested in detecting the presence of frequencies in the 155 to 165 Hz frequency band within a block of data (or any other defined frequency band). The spectrogram is useful to observe ...
- 48.2k
3
votes
Accepted
Does downsampling increase the resolution of frequencies?
Downsampling by a factor of $N$ in time-domain means that you throw away $N-1$ samples from $x[n]$ for every $N$ samples. In frequency domain this creates $N$ shifted copies of the original spectrum ...
- 2,571
3
votes
Python audio analysis: which spectrogram should I use and why?
From the information you've given, I'd use spectrogram A as the frequency data is spaced logarithmically. This is advantageous for pitch detection applications as it gives you a greater amount of ...
- 806
3
votes
Spectrum of FM signal?
Because frequency modulation is a nonlinear phenomenon, there's no good general-purpose answer to your question. You can take any specific baseband signal and figure out its modulated spectrum, but ...
- 11.7k
3
votes
What is really the Mel-filterbank?
As I am a newbie in DSP and I thought this article has a clear explanation, I did not try to answer your questions myself. But you said you have read it and thought these questions are still ambiguous,...
- 31
Only top scored, non community-wiki answers of a minimum length are eligible