# Experimental Evaluation of Display Field Communication Based on Machine Learning and Modern Design
## 5 Experimental Evaluation
### Performance According to Transmit Image Size  
Table 4 presents the parameter settings for all three images used to examine the performance of our DFC scheme. For easier understanding, we also show the areas of an image where the data are embedded. All three images were evaluated in the same experimental environment. Note that as the size of the image doubles, the number of embedded data bits also doubles keeping the area of the lower sub-band fixed from rows 5 to 15. Figure 6: Actual noise observed in a D2C link. (**a**) Noise expressed as a 3D function. (**b**) Noise as seen from the row axis. The red box indicates the rows where the data are embedded. The reason for using the same sub-band regions in all images lies in the frequency characteristics of the images. From the Fourier representations, we know that any signal can be decomposed into its multiple sinusoidal harmonics as shown in Figure 7. The original signal is depicted as a red square wave and the periodic functional components that compose it are depicted as blue sinusoidal waves of different frequencies. The main idea behind the Fourier transform is to express the signal as a sum of multiple signals, i.e., periodic functions of sine or cosine, each having a different frequency. By looking at the above concept from the viewpoint of image processing, each pixel of a frequency-domain image is considered to have a frequency component. That is, an image is a representation of multiple frequencies, where each frequency resides in each pixel. As shown in Figure 2, the upper part of the frequency-domain image contains the low-frequency components and the lower part of the image contains the high-frequency components. In our experiment, all the characteristics of the images are the same except for their size. As the image size increases, the pixels of the high-frequency coefficients increase to express the details of the image. Hence, whenever the size of the input image doubles, low-frequency components do not change while the image quality is improved by adding high-frequency components. Therefore, in the experiment, the lower sub-band regions in which the data are embedded remain the same (from rows 5 to 15) regardless of the size of the image. Figure 8 depicts the BER performance of the proposed scheme according to PSNR and transmit image size. In the case of \(128\times 128\) pixel Lena image (cf. Figure 8a), we can observe that the error rate of the red and green channels is zero for all the values of PSNR up to 50 dB. We can observe some BER in the case of the blue channel after 45 dB. Similar to Figure 8a, the error rate of the green channel of the \(256\times 256\) pixel Lena image was zero for all the values of PSNR. However, the difference can be observed in the case of the red and blue channels, which show a gradual increase in BER after 45 dB and 40 dB, \begin{table}
\begin{tabular}{c c c c} \hline
**Parameter** & **Image 1** & **Image 2** & **Image 3** \\ \hline Input Image & Lena color (\(128\times 128\) p) & Lena color (\(256\times 256\) p) & Lena color (\(512\times 512\) p) \\ No. of binary data bits & 250 & 500 & 1000 \\ No. of encoded data bits & 1268 & 2518 & 5018 \\ Lower sub-band position & rows 5 to 15 & rows 5 to 15 & rows 5 to 15 \\ Distance & 28 cm & 28 cm & 28 cm \\ Camera angle & \(0^{\circ}\) & \(0^{\circ}\) & \(0^{\circ}\) \\ Display rotation & \(0^{\circ}\) & \(0^{\circ}\) & \(0^{\circ}\) \\ \hline \end{tabular}
\end{table} 
Table 4: Experimental parameters for different image sizes. Figure 7: Original signal (red) and periodic functions (blue) composed of the weighted sums. respectively. Figure 8c depicts the error rate when the \(512\times 512\) pixel Lena image was used at the transmitter display. The red and green channels show errors after 45 dB PSNR. However, the blue channel is worse affected and started showing errors after 20 dB PSNR. Therefore, we can observe that the error rate is significant in the blue channel. Moreover, the overall error rate increases as the size of the image increases.