# Signal Diversity for Laser-Doppler Vibrometers with Raw-Signal Combination
## 4 Discussion  
With the first experiment, we verified the functionality of the algorithm developed by [PERSON] [21] for deriving a combined signal from four velocity signals. By examining the resulting combined signal, we identified sources of potential errors, which limit the signal quality of the combined signal which has already been significantly improved. We derived an algorithm that calculated a combined signal based on four raw LDV signals. Subsequently, both algorithms were applied to the same measurement and revealed that the newly developed algorithm allows an improvement of the combined signal. For measurements with the first experimental setup, we were able to show that the mean noise level of the combined signal is reduced by an additional 15 dB (RBW = 1 Hz) in the frequency range up to 5000 Hz, when calculated with our algorithm (RawComb), compared to the algorithm from the literature (VeloComb), as shown in Figure 13b. For the second experimental setup the additional reduction was approx. 6 dB (RBW = 2.5 Hz). Based on the evaluations of the first and second experiments, we demonstrated the basic functionality of the developed algorithm for determining a combined signal from the raw signals of multiple channels. Specifically, it was shown that the combined demodulated signal is at least as good as the signal of the best individual channel. Depending on the disturbances and signal dropouts that occur, the signal quality, and accordingly the noise level, of the combined signal can be significantly better than that of a signal from a single channel. In the results of our measurements of the second experimental setup, as shown in Figure 16, a mean noise reduction in the frequency range up to 5000 Hz of more than 10 dB was achieved (RBW = 2.5 Hz). Figure 18: Frequency spectra of the four channels CH1–CH4 and the combined signal; poor alignment of the measurement heads; (RBW = 2.5 Hz). The results seem plausible and support the findings of [18; 21], as both algorithms for determining a combined signal significantly reduced the noise level caused by signal dropouts due to the laser speckle effect. The increased reduction of the noise level (up to 6 dB in the second experiment) by our algorithm (RawComb) compared to the algorithm from the literature (VeloComb) is reasonable, since our algorithm does not have the limitations shown in Section 3.1.1. The implementation of our findings could improve the signal quality in various applications involving measurements on rough or moving surfaces, where the signal quality is reduced by laser speckle effects and the resulting signal dropouts [7; 10]. A possible use case is medical applications, where LDVs are used for contactless measurements, such as monitoring cardiovascular activity [5; 23]. In such cases, it is not always possible to guarantee sufficient reflectivity of the skin and that the patient does not move, or the appropriate procedures involved in ensuring this require greater effort [23]. For this application, signal diversity could decrease noise contribution of laser speckle effects, or even eliminate the need for time-consuming preparation of the skin. For future research we are investigating additional real-world applications as well as the real-time capabilities of our algorithm and the impact of implementing peak-filtering algorithms in the individual signals before combining them. Furthermore, the question of to what extent the measurements can be compensated with respect to the angle of incidence needs to be addressed in order to obtain reliable and accurate measurements in real-world applications. Conceptualization, M.S., and C.R. ; methodology, M.S. and C.R. ; software, M.S. ; validation, M.S. ; formal analysis, M.S. ; investigation, M.S. ; resources, M.S. and C.R. ; data curation, M.S. ; writing--original draft preparation, M.S. ; writing--review and editing, M.S. and C.R. ; visualization, M.S. ; supervision, C.R. ; project administration, M.S. ; funding acquisition, M.S. and C.R.