A pde-Based Analysis of the Spectrogram Image for Instantaneous Frequency Estimation
Abstract
:1. Introduction
2. Materials and Methods
2.1. Spectrogram of AM-FM Signals
2.2. The Proposed Method
2.2.1. Estimation Error
2.2.2. Chirp Rate Regularization
2.2.3. Crossing Point Detection
3. Results and Discussion
Some Remarks
- The proposed method requires two-component signals and prior knowledge of the presence of a non-separability region. A method for detecting the non-separability region, as the one proposed in [65], can be used as a preprocessing step, in case of constant amplitude signals. Furthermore, weakly amplitude modulation is assumed, as in most approaches.
- It is worth underlying that the proposed method could be easily generalized to MCS with modes if the TF non-separability regions are known and they are sufficiently separated. Indeed, under these assumptions, the analysis of a more complex signal reduces to the analysis of a two-component signal locally. This point will be investigated in-depth in future studies.
- The proposed model allows for IFs estimation, up to an integration constant, whose accuracy can affect the final result, as previously shown. However, it is worth pointing out that this error often causes the recovery of a characteristic different from the ridge, but still a characteristic. As a result, it contains significant information concerning IF. The integration constant can be directly estimated from spectrogram ridges belonging to the separability region. Also in this case, a prior TF localization of the non-separability region would solve the problem.
- The proposed procedure requires two frequency points for defining the linear system as in Equation (11), for each fixed u. The sensitivity to their selection has been numerically investigated. As shown in Figure 5 and Figure 6, involving too low characteristic curves can affect the final accuracy due to boundary effects, while characteristic at higher levels are generally more subjected to interference, resulting in inaccurate IFs estimate. For this reason, a good compromise can be achieved by selecting two frequency points close to the one where spectrogram concavity changes, as done in the presented simulations.
- As shown in the experimental results, the proposed method is robust to additive interference and cross-terms. The presence of spectrogram values close to zero, which occurs in case of strong amplitude modulation as well as destructive interference, could result in instabilities in the derivatives approximation, that can affect the final IFs estimation. However, it is worth highlighting that the proposed approach outperforms the state-of-the-art method in dealing with the critical case of destructive interference, as shown in Figure 7 and Figure 12. In addition, the proposed method has shown robustness to moderate noise.
- Spectrogram is widely used in practical applications because of its simplicity and computational benefit. It is well-known that, in case of MCS, spectrogram of close modes suffers from the presence of cross-terms that can affect each estimation concerning the signal. For this reason, more advanced and adaptive kernels with attenuated cross-terms, such as S-Transform or Locally Adaptive TF distribution, are often preferred in the literature. However, it is worth noticing that many real-life measurements, such as the ones concerning human gait classification and detection, precisely deal with spectrograms. That is why spectrogram processing is still of interest, today. In addition, cross-terms do not represent a limitation for the presented method, but a tool for estimating IF.
4. Conclusions
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
Abbreviations
AM | Amplitude Modulated |
CR | Chirp Rate |
FM | Frequency Modulated |
IA | Instantaneous Amplitude |
IF | Instantaneous Frequency |
MCS | Multicomponent Signal(s) |
RPRM | Ridge Path Regrouping Method |
STFT | Short-Time Fourier Transform |
TF | Time-Frequency |
TFD | Time-Frequency Distribution |
WSC | Weakened Separability Condition |
Appendix A
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Bruni, V.; Tartaglione, M.; Vitulano, D. A pde-Based Analysis of the Spectrogram Image for Instantaneous Frequency Estimation. Mathematics 2021, 9, 247. https://doi.org/10.3390/math9030247
Bruni V, Tartaglione M, Vitulano D. A pde-Based Analysis of the Spectrogram Image for Instantaneous Frequency Estimation. Mathematics. 2021; 9(3):247. https://doi.org/10.3390/math9030247
Chicago/Turabian StyleBruni, Vittoria, Michela Tartaglione, and Domenico Vitulano. 2021. "A pde-Based Analysis of the Spectrogram Image for Instantaneous Frequency Estimation" Mathematics 9, no. 3: 247. https://doi.org/10.3390/math9030247
APA StyleBruni, V., Tartaglione, M., & Vitulano, D. (2021). A pde-Based Analysis of the Spectrogram Image for Instantaneous Frequency Estimation. Mathematics, 9(3), 247. https://doi.org/10.3390/math9030247