# Experimental Demonstration of Single-Channel EEG Signal Using 32 x 32 Pixel OLED Screen and Camera
## 5 Experiments and Results
### Results Analysis and Discussion  
During the experimental work, the three different sets of experiments were carried out with three different cameras to examine the system performance. Figure 7 shows the image processing of the OLED screen at pixel size \(32\times 32\). The advantage of the VL-OCC system is that the system can be used both for online processing and saved later for processing purposes, depending upon the application. Presently, in the experimental work, the EEG signal transmitted, captured by the camera in video form, was processed offline using MATLAB to calculate BER and to determine the performance of the system and the effective link distance between the transmitter and the receiver. In order to overcome the delay, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Type of Camera** & **Thorlabs Camera** & **Android (Smartphone)** & **DSLR** \\ \hline Camera resolution & 8 MP & 5 MP & 18 MP \\ Camera frame rate & 30 fps & 30 fps & 30 fps \\ Auto focus & Manual & Manual & Manual \\ Data rate achieved & 2.8 kbps & 2.8 kbps & 2.8 kbps \\ Link distance & 2.25 m & 1.75 m & 5.5 m \\ Exposure mode & Rolling shutter & Rolling shutter & Rolling shutter \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Camera parameters. a pre-header of 40 bits using all 1 s was transmitted to ensure the start of the frames for successful transmission of VL-OCC system, to avoid delays and to maintain the synchronization between the transmitter and the receiver in all experiments. During the image processing in our experimental work, the two bits 0 and 1 were used, representing low and high levels, respectively. If the symbol or block was black, then it represented 0 bit and if the block was white it represented 1, as shown in the cropped in grey image in Figure 7. The cropped image was used to calculate the BER by comparing the bits transmitted and bits received. The transmitted information was in the form of video which was processed in MATLAB to calculate BER. A digital image consisted of rows and columns [\(r_{1}\),\(c_{1}\)], as shown in Figure 7a, where the transmitted bits formed an image using the OLED screen as transmitter, as per the pixel size of 32 \(\times\) 32. Depending upon the size of pixels, the number of bits transmitted per frame was determined. For instance, using a 32 \(\times\) 32 pixel OLED screen, 20 bits of useful information was transmitted per frame, thus requiring around 2.3 min of video to transmit a bit rate of 2.8 kbps. Figure 7b shows the grey scale image. In digital image processing black and white cells are represented with grey levels of 0-255. The grey image helps in determining BER by cropping the image, as shown in Figure 7c. The BER is also calculated by converting the image back to bits and comparing the actual bits transmitted and received. The BER formula:  
\[BER=\ \frac{Bits\ received}{Bits\ Transmitted}\]  
BER increases with increase in distance, as shown in Figure 8. In camera communication systems for EEG using VLC and a camera, the BER obtained to date is usually of the order of \(10^{-3}\). The BER values of the order \(10^{-4}\) achieved in the proposed system illustrates the successful transmission of EEG signal using the camera and OLED screen. This demonstrates the improved performance of the proposed system. The camera frame rate of 30 fps was fixed for all the respective cameras; however, the distances were varied. The graph in Figure 8 shows that the DSLR outperformed Thorlabs and the android smartphone camera. The error-free link distance covered by the DSLR camera was 5.5 m, whereas the Thorlabs camera was 2.25 m, and the smartphone camera was 1.75 m.  
Figure 7: Image processing: (**a**) Transmitted information captured by camera; (**b**) information converted to grey scale; (**c**) cropped image for BER calculation. Figure 9a,b shows the comparison of received EEG signal and transmitted EEG signal. The red line is the received signal, while the blue line is the transmitted signal.