## 3 Experiments
### Used Instrumentation and Recording Arrangement
#### 3.2.2 PPG Signal Recording  
Generally, two principles of optical sensors (transmission or reflection) can be utilized in the PPG signal measurement. Both types consist of two basic elements: a transmitter (light source--LS) and a receiver (photo detector--PD). In the transmission mode, the LSs and PDs are placed on the opposite sides of the measured human tissue. In the reflection PPG sensor, the PDs and LSs are placed on the same skin surface. In this research, the optical sensors working on the reflection principle were used and the PPG signals were picked up from fingers [37]. For practical PPG signal recording, a previously developed wearable PPG sensor, PPG-PS1, was used. This also operates in a weak magnetic field with radiofrequency disturbance (in the scanning area of the running MRI device during patient examination) [38]. This PPG sensor realization is fully shielded, assembled only from non-ferromagnetic components, and based on the reflection optical pulse PPG sensor (Pulse Sensor Amped--Adafruit 1093 [39]). For data transmission to the control device, the wireless communication based on Bluetooth standard is utilized. Due to the 10-bit A/D converter implemented in the microcontroller of the whole PPG sensor, the absolute unipolar PPG signal representation lies in the range from 0 to 1024 (\(A_{\text{NR}}\) = 1024). This wearable sensor enables real-time PPG wave sensing and recording for the sampling frequencies from 100 to 500 Hz.  
The typical PPG cycle frequency corresponding to the HR of healthy adults is in the range 1 to 1.7 Hz (from 60 to 106 min\({}^{-1}\)) [37], so the \(f_{\text{S}}\) about 150 Hz is sufficient to fulfil the Shannon sampling theorem. In addition, the commercial wearable PPG sensors use typical sampling frequencies between 50 and 100 Hz. Using different \(f_{\text{S}}\) from the investigated range does not change the subsequently detected pulse period and the finally determined heart rate; only the precision of the systolic and systolic peaks decreases in the case of lower \(f_{\text{S}}\). For the purpose of this study the precise shape of peaks is less relevant, only the detected \(T_{\text{HP}}\) and \(W_{\text{SP}}\) parameters are necessary for HR and ORI calculation. As we statistically analyze the obtained HR and ORI values for final comparison in the fusion block, the statistical stability and credibility is most important for us. From the previously performed analysis, it follows that a decrease in the number of detected HR periods as a consequence of higher used \(f_{\text{S}}\) brings an incorrectness to the results of the statistical analysis due to too small a number of the processed values--the PPG signal is sensed in real-time by the data block samples from the internal memory of a wearable PPG sensor with sizes from 1 to 25 k [38]. This is the main reason why we use the \(f_{\text{S}}\) = 125 Hz for sensing of the PPG signal in our experiments.  
The optical part of the PPG sensor is fixed on a forefinger of the left hand by an elastic ribbon. The PPG signal pick-up is begun just before the start of the human voice phonation and the PPG sensing is finished immediately after the end of the phonation recorded by the microphone Mic2--see an arrangement photo in Figure 8 obtained during the \(MF_{1}\) measurement phase.