## 2 Materials and Methods
### Apparatus Description
Due to the absence of a physical 3.5 mm audio connector on the device, we were compelled to use a USB sound card jointly with a USB-C to USB-A adapter. An inexpensive external USB 2.0 sound card dongle manufactured by Delock was utilized for the sound acquisition. Observing the available information on the two piezos provided in Table 1, we can draw out some conclusions from it. First, the PZT-5 has a larger piezo crystal affixed to it and a higher current rating. Consequently, it should provide a stronger signal, which has been experimentally confirmed. The PZT-5 has a higher resonant frequency, so it should be more sensitive to higher frequencies than the FT-50T-3A1, which has a lower resonant frequency and therefore should be more sensitive to lower frequencies. It is important to acknowledge that there exist other piezo devices of higher cost that may exhibit superior performance and have larger physical dimensions. However, for the specific objectives of this study, it is reasonable to infer that the selected piezo devices will provide satisfactory results. By incorporating a smartphone into our set-up, we were able to broaden the scope of our research and gather more data. In the experimental setup, a Samsung smartphone was employed. To be more specific, we utilized the Galaxy S9+ model. The device operates on the Android 9 (Pie) operating system and has the One UI version 1.0 user interface. Due to the aforementioned limitations imposed by the S Pen Remote SDK, the top of the stylus features an ArUco marker [36]. The main purpose of the smartphone is to record a video documenting the process of writing. Using the ArUco marker from the video, we \begin{table}
\begin{tabular}{l c c} \hline \hline
**Model** & **FT-50T-3A1** & **PZT-5** \\ \hline \hline Shape & Round & Square \\ \hline Metal plate material & Brass & Elastic Steel \\ \hline Contact material & Silver & Copper \\ \hline Metal plate dimensions [mm] & \(\Phi\)50 & \(65\times 37\) \\ \hline Metal plate thickness [mm] & 0.18 & 0.25 \\ \hline Metal plate area [mm\({}^{2}\)] & 1963 & 2405 \\ \hline Metal plate volume [mm\({}^{3}\)] & 353.34 & 601.25 \\ \hline Ceramic dimensions [mm] & \(\Phi\)25 & \(50\times 30\) \\ \hline Ceramic thickness [mm] & 0.27 & 0.2 \\ \hline Ceramic area [mm\({}^{2}\)] & 490.9 & 1500 \\ \hline Ceramic volume [mm\({}^{3}\)] & 132.54 & 300 \\ \hline Total thickness [mm] & 0.45 & 0.45 \\ \hline Operating temperature range [\({}^{\circ}\)C] & \(-\)20 to 70 & \(-\)20 to 70 \\ \hline Resonant frequency [kHz] & 3.2 & 28 \\ \hline Maximum voltage [V] & 15 & 12 \\ \hline Resonant impedance [\(\Omega\)] & \(\leq\)200 & 1002 \\ \hline Static capacity [nF] & 4.2 & 102 \\ \hline Maximum current [mA] & 3 & 8 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics comparison of the piezo models used as sound vibration and pressure input sensors. were able to determine the inclination of the stylus. As illustrated in Figure 3, the holder for attaching the marker to the stylus was produced using a 3D printer. The built-in camera sensor on the particular model is capable of capturing high-quality videos at 4K resolution and 30/60 frames per second (FPS), 1080p resolution and 30/60/240 FPS, and 720p resolution and 960 FPS. In addition to using the camera sensor, the smartphone, similar to the tablet, is used for the supplementary capture of touch screen pressure. In this case, a second, smaller piezo sensor is utilized and is also positioned beneath the tablet computer. We have further enhanced the capabilities of our system by including an additional component, a smartwatch. In particular, we leveraged the integrated accelerometer and gyroscope sensors from the Samsung Gear S3 Frontier wearable. This model runs the 4.0.0.7 version of the wearable platform based on the Tizen operating system. The device is equipped with an integrated LSM6 DSL iNEMO 6 DoF inertial measurement unit (IMU) [37] manufactured by STMicroelectronics. The system-in-a-package (SiP) comprises a 3D digital accelerometer and a 3D digital gyroscope.