## 3 Common-Mode Noise and Signal Processing Method for Reconstruction of PC Display
### Reconstruction of PC Display-Image  
#### 3.2.1 Examination on Undesired Beats Accompanying with the Common-Mode Noise  
The smallest characters that could be displayed on a PC monitor of the measurement system without any defects were the alphabetic characters with the size of \(9\times 9\) dots. It is very meaningful in information security to reproduce these characters from the common-mode noise. Thus, we set these characters as the target objects for reconstruction.  
At first, three one-dot vertical lines (black/white/black/white/black lines on a white background) were displayed on the monitor to examine the response of common-mode noise for a standard display image. The waveforms for the normal-mode and the common-mode noise currents are shown in Figure 4. The observed display images are shown in the insets. The coaxial parallel series cable was used for the signal cable, and the red color signal line was selectively probed at a parallel part for the normal-mode and at a coaxial part for the common-mode, respectively. The monitor was set at a resolution of \(800\times 600\) pixels. The normal-mode or the common-mode pulse wave due to the vertical synchronizing signal was used as a trigger for the oscilloscope. In the normal-mode,  
Figure 3: Dependences of the common-mode noise level on the distance from the monitor of the signal cables: (**a**) coaxial parallel series cable, (**b**) only coaxial cable.  
the noise current value changed by several mA between high (corresponding to the white color) and low (corresponding to the black color) levels. In this case, the one-dot width corresponded to 25 ns (40 MHz). Dull responses for the up- and the down-steps may have been caused by the bandwidth of 50 MHz in this measurement system. The normal-mode waveform was regarded to be almost equal to the monitor signal except for the dullness. On the other hand, the change reduced to about 2 mA, and the waveform was accompanied by complicated beats in the common-mode. Such beats were not observed for the reference signals from a pulse generator with the same current probe and oscilloscope used in this measurement. Therefore, these beats were interpreted to be generated intrinsically from the measured PC system itself.  
The above beats needed to be removed from the common-mode noise waveform to reconstruct the display images with a high resolution of one-dot. Thus, in the next step, we examined details of the beats by analyzing the responses for the up- and the down-step signals. A half-white and half-black image, as shown in Figure 1, was used for up- or down-step signals. The up- and the down-step responses were upper and lower symmetry. The beats seemed to appear only just after the steps where the display images changed steeply, as shown in Figure 5. This indicated that the beats were generated as step responses by a quadratic transfer function, which was caused by the electromagnetic induction and the capacitive coupling in the PC system.  
**Remark 1**.: _The superimposed beats accompanying the common-mode noise for the image, where the up-step and the down-step repeat continuously (horizontal dot-line), affect the common-mode noise waveform most seriously. The mentioned above three one-dot vertical lines are a simple example of such images._  
Figure 4: Waveforms of the noise level for: (**a**) normal-mode, (**b**) common-mode.