1. Introduction The measurement of sound velocity in a transmission medium is of great significance in acoustic testing. How to more accurately measure the sound velocity of a medium has always been a major concern in the field of acoustics. The measurement accuracy of ultrasonic ranging equipment and the accuracy of material thickness measurement in non-destructive testing are all related to the accuracy of sound velocity measurement. The key to accurately measuring the sound velocity of a medium is to accurately measure the time. Since L is known according to the sound velocity calculation formula c=L/Δt, the accuracy of Δt directly affects the accuracy of the sound velocity measurement. Using the TDC-GP1 circuit from ACAM as a timing tool, it can provide results accurate to ps. From the perspective of time measurement accuracy alone, this circuit can meet the requirements of high-precision sound velocity measurement. 2. Structural Characteristics of TDC-GP1 With the development of semiconductor technology, the design and production of high-precision time interval measurement circuits and systems have become possible. The TDC-GP1 is a high-precision time interval measurement circuit developed by ACAM in Germany, providing time interval measurement with a resolution of 250 ps for dual channels or 125 ps for a single channel. The TDC-GP1, manufactured using 0.8 μm CMOS technology, is a general-purpose high-precision time-to-digital converter with an actual resolution of 30 ps to 300 ps. Packaged in a 44-pin TQFP, the TDC-GP1 contains registers, a TDC measurement unit, an RLC measurement unit, a 16-bit arithmetic logic unit (ALU), and an 8-bit processor interface unit. The registers can be configured to operate in different modes depending on the application. The pin functions of the TDC-GP1 are shown in Table 1. The internal block diagram is shown in Figure 1. The TDC-GP1 has two ALUs. The first ALU converts the measurement results in the coarse value register into unsigned integers for subsequent ALU arithmetic operations; this ALU does not require a clock. The second 16-bit sequential ALU performs deviation correction on the measurement results based on the register settings and performs multiplication on the calibration values. Each ALU has an independent clock, and the clock frequency can be adjusted, completing all the above tasks in just 4 μs. The TDC-GP1 offers three modes for users to select: two ranges and adjustable accuracy. In range 1, the time interval between each STOP pulse and the start pulse in both channels, as well as the time interval between STOP signals, can be measured. However, in range 1, the measurement range is only 7.6 μs. To increase the measurement range, the circuit has a 16-bit prescaler with a maximum range of 60 ns to 200 ms, which is range 2. In the experiment, range 2 was used, and its signal timing is shown in Figure 2. In this range, only the time interval between the START and each STOP pulse of one channel can be measured (channel selection is achieved through control register 2), and the time interval between STOP pulses cannot be directly measured. After the START signal enters, the time difference between this signal and the rising edge of the next calibration clock is quickly measured inside the circuit, i.e., tFC1. Then, the counter starts working, obtaining the number of working cycles of the prescaler, i.e., counter. At this time, the internal measurement unit of the circuit is reactivated to measure the time difference between the rising edge of the first pulse of the input STOP signal and the rising edge of the next calibration clock, denoted as tFC2. tFC3 is the time difference between the rising edge of the second pulse of the STOP signal and the rising edge of the calibration clock, tcal1 is one calibration clock cycle, and tcal2 is two calibration clock cycles. According to Figure 2, the time interval between the first pulse of the START signal and the first pulse of the STOP signal can be obtained: period represents the calibration clock cycle, and counter represents the count value of the prescaler. 3. Implementation Scheme and Software Design The electrical principle of the high-precision sound velocity measurement device based on TDC-GP1 is shown in Figure 3. The power supply, crystal oscillator, and other peripheral circuits are omitted in the figure. During measurement, the settings of each register are as follows: reg7=0x00; reg0=0x58; reg2=0x21; reg7=0x02; reg11=07H. The circuit must be initialized before each measurement. The measurement result is read from the result register, with each read address being 0x00, and the address pointer automatically increments by 1. Since it is calibration data, each result is stored in two registers. TDC-GP1 has eight 16-bit registers, therefore, in measurement mode 2, the time difference between four STOP pulses and four START pulses can be recorded. Eight registers are used to store data in a loop. After the eighth register records data, the fifth data is stored in the first register, overwriting the original register contents. In the experiment, the distance between the transducers is 13 cm. An ADAC842 is used to control the transmission signal to transducer 1. The received signal from transducer 2 is amplified and compared before entering the STOP pin of the TDC-GP1. After measuring the time delay, the data is sent out through the serial port and recorded in the mydata.dat file. The simplified software flow is shown in Figure 4. The data in the mydata.dat file consists of four hexadecimal numbers, which need to be converted to decimal. Matlab can easily perform the conversion and calculate the speed of sound. The distance between the transducers is L, and the converted time delay data is LΔt. The speed of sound is LΔt. The experimental results are very accurate. In the VC++ serial port receiving program, the serial port settings are as follows: where length is the distance between the two transducers and velocity is the measured speed of sound. 4. Conclusion The TDC-GPI type circuit offers time interval measurement accuracy on the order of hundreds of picoseconds, facilitating precise sound velocity measurement. Based on the TDC-GPI and a high-efficiency microcontroller, the author designed a high-precision sound velocity measurement device. Trial operation shows that the measurement accuracy meets practical requirements, the response time is relatively fast, and it can meet the needs of practical applications, especially suitable for applications requiring rapid or dynamic measurements.