## 2 Sampling and Recording of Revealing Emission Signals
Due to the finite accuracy of performance of the components of video systems of IT devices, the values of the synchronization parameters of images reconstructed from the revealing emission signals from video monitors may significantly, in the context of coherent summation requirements, differ from the catalog data defined by the VESA (Video Electronics Standards Association) guidelines [50], and even they can differ from one instance to the next of the same type of device. In addition, in the case of devices such as laser printers, the course of the printing process depends on other factors related to the operating conditions of the device, such as the current temperature of the printing system or the type of paper. Due to these factors, each attempt to recreate an image from the recorded revealing emission signal requires the determination of its imaging parameters. This determination can be made by the trial and error method, matching the appropriate values on a database of the catalog or archive data from previous surveys. However, these activities are labor-intensive and also require some experience from the operator. Video signals processed in the tracks of monitors, laser printers, or displays have a strictly defined time structure. In the case of screen monitors, both the VGA and DVI/HDMI standards (Digital Video Interface/High-Definition Multimedia Interface) maintain the principles of the so-called framing of images. Based on the information contained in [50], it is possible to provide the most important raster parameters of the screen for selected operating modes (Table 1). It is worth noting that the frequency values given in Table 1 are nominal values that define the mode according to the VESA standard. The formulas defining the exact values of these parameters and the values of the constants necessary to determine them can be found in [51]. Due to the finite accuracy of the components that make up the synchronization signal generation circuits, the actual values of their frequencies for specific graphics card systems may differ from the nominal ones. It is not always possible to directly measure the parameters of video signals in lines of graphic interfaces. For the above reasons, it is impossible to properly reconstruct an image based on catalog values. Figure 4: The reconstructed image (rasterized) in incorrect way, VGA interface signal as a source of reveal emission, \(640\times 480/60\) Hz mode, \(62.5\) MHz sampling rate, line length 1985. Figure 5: The reconstructed image (rasterized) in incorrect way, VGA interface signal as a source of reveal emission, \(640\times 480/60\) Hz mode, \(62.5\) MHz sampling rate, line length 1985, summed \(10\) times. In addition, during the recording of the real revealing emission signals using a physical analog-to-digital converter (DAC), only specific signal sampling frequency values can be used. For example, the 8-bit PDA-1000 converter card produced by Signatec, used in the Electromagnetic Compatibility Laboratory of the Military Communication Institute-State Research Institute (EMC Laboratory of MCI-SRI), offers a maximum sampling rate of 1 \(GS/s\), which may be reduced as a result of the application of card clock frequency division in the range from 2 to 1024. Assuming the perfect execution of both the transducer card and the graphics card elements, and assuming that the minimum sampling frequency should be at least equal to the \"pixel frequency\" value, for the previously selected monitor operation modes there are obtained appropriate (estimated) values of parameters of acquisition and rasterization of emission signals revealing (Table 2). The information provided in Table 2 shows that in the case of the rasterization of the signal obtained by sampling with the use of an ADC converter card, the line length of the reconstructed image is not expressed by the total number of pixels, which must be taken into account in the image reconstruction algorithm. In addition, the accuracy of reproducing this value is important in the process of coherent image summation.