# Signal Diversity for Laser-Doppler Vibrometers with Raw-Signal Combination
## 3 Results
### Comparison of the Algorithms  
With our modified algorithm a combined velocity signal is calculated (with \(\alpha=5\)) from the same measurement and from the first experimental setup as before. The combined signal from the raw channels, the demodulated velocity signals of the individual channels and the combined signal from the velocity signals is shown in Figure 11.  
The resulting velocity signal from our algorithm is significantly less noisy. The peaks of the combined signal, in comparison to the combined signal derived from the demodulated velocity signals, are mostly gone. Figure 12 additionally shows the displacement signals derived from the velocity signals. The offset resulting from the difference between the actual carrier frequency and the carrier frequency assumed for demodulation was compensated for each signal.  
The functionality of the algorithms is evident in both combined signals--because most of the time just one channel is affected by signal dropouts, this time segment can be replaced in the combined signal by the other channels.  
Figure 11: Comparison of the demodulated velocity signals of the channels CH1–CH4 and the combined signals (from [21] “VeloComb” and from our algorithm “RawComb”); zoomed sections only for combined signals to illustrate less noise; (RBW = 2.5 Hz).  
Figure 12: (**a**) Comparison of the demodulated displacement signals of the four channels CH1–CH4 as well as the combined signals derived from the old and new algorithms; (RBW = 2.5 Hz). (**b**) Magnified section of the combined signals from (**a**) to illustrate the difference.  
The shaker's vibration frequency of 100 Hz is visible in all signals. For the combined signal from the old algorithm, a slight offset can still be seen compared to the combined signal from the algorithm developed by us. The cause of this offset can be explained by the disturbances visible in the velocity signal in Figure 11.  
The difference between the algorithms can also be shown by the significantly lower noise in the frequency spectrum, as shown in Figure 13.  
The relatively large amplitude of the shaker at 100 Hz is detected with a similar amplitude in all four channels as well as in the combined signals. Because of the signal dropouts, forced by our setup, in the individual channels, the noise level is significantly higher.  
For CH1 and CH2, the amplitude at 100 Hz is just slightly above the noise level, making the detection of an unknown frequency unrealistic. Generally, only higher amplitudes can reliably be detected in the individual channels, that are affected by signal dropouts.  
In order to estimate the noise reduction more accurately, Figure 13b shows the smoothed frequency spectra of the combined as well as one individual velocity signal. For this measurement, the algorithm, that calculates the combined signal from the velocity signals (VeloComb), reduces the mean noise level in the frequency range up to 5000 Hz by 17 dB. Our algorithm that calculates the combined signal from the raw signals (RawComb) decreases the mean noise level by an additional 15 dB. Therefore, only the algorithm using the raw signals will be described in the following parts of this article, as it consistently achieves better results.  
All previously shown results used measurements from the first experimental setup, which had the main objective of generating a reliable, reproducible signal as a basis for developing and testing the presented algorithm. To test our algorithm further, the results from the second experiment with only one active channel, which is much closer to real world applications, are presented in the following section.