# User Visit Certification and Visit Trace System Using Inaudible Frequency
## 3 User Visit Certification and Visit Tracing System Using Inaudible Frequency  
In this section, we explain a user visit certification and visit tracing system using inaudible high frequencies within the audible frequency. Our proposed system requires a speaker and personal computer (PC), smart device-based application, and server for user visit certification and visit tracing. The process flow between the proposed application and the server is shown in Figure 1.  
In Figure 1, the process flow of the proposed system for user visit certification and visit tracing is similar to that of existing QR codes. Sound data from speakers that generate trigger signals should be set up in a predefined protocol first to allow the smart device to recognize the signal. A PC or laptop connected to a speaker calculates a CRC value to generate a trigger signal (1) and encodes signal data (2) for signal output. The trigger signals created in this way are generated through speakers installed in the store (3), and trigger signals are generated at regular intervals during the time when people need to check their access to the store. The trigger signals that occur at this time use a high range of frequencies that people cannot hear, so they do not have a significant impact on facility managers located nearby [18]. When a user visiting a multi-use facility runs the proposed application  
Figure 1: The process flow for user visit certification and the visit tracing system. installed on a smart device, the application collects sounds around the visitor (1). The application performs a fast Fourier transform (FFT) analysis from the sound collected (2) and extracts valid frequency values by detecting the peak values of frequencies from the analysis results (3). The extracted frequency values are decoded into bit data according to the predetermined data frequency protocol (DFP) (4). Next, CRC checks confirm that an error exists in the decoded data (5), and if there is an error due to corruption, the application returns to 1 and performs the trigger signal analysis again. When accurate data without data deformation are received, the application sends trigger signal values, user location information, and user ID to the certification server. The certification server checks the data received (6) and stores each datum for use in tracing users' visits, epidemiological investigations, and store-specific visitor information retrieval (7). After checking the user's location information on the server, when the visit certification is approved, the information is delivered to the smart device to complete the user's visit certification (8). The detailed process flow of 6, 7, and 8 in Figure 1 is shown in Figure 2.  
In Figure 2, when the certification server receives a user ID from a smart device (1), it retrieves user information through a search in the user data table (2). The server then retrieves store information from the store information table (4) using the trigger signal value received (3). The server receives the user's location information from the smart device (5) and performs comparisons with the store location information obtained from 4 (6). If the user's location and store location information are located within a certain range, the certification server stores the visit certification data in the database (7) and approves the user's visit (8). If the user's location and store location information are outside the valid range, the server does not certify that the user has visited the store and refuses to approve the visit. If the visit is approved through the process shown in Figure 2, the server sends the application of the smart device a store contact, the store name, store global positioning system (GPS) data, and an approval time with the approval number. The application that receives the data stores the information in the SQLite database within the application and uses it to trace user visits and visit locations using Google Maps.