The Creation of the Digitizer and TerraHub
In the early years of Silicon Audio Seismic, the team worked on a project with the Department of Energy (DOE) developing infrasound sensors. In creating the experimental design, it became glaringly difficult to connect all the analog sensors in an array. Gears shifted to creating a digital system and as a result, the inception of the digitizer began.
The motivation was simple: create an analog-to-digital converter (ADC) by sampling the output voltage of the sensor. A larger goal was to create a digital system that was not only low power but also tailored to the sensor. The result was to divide the system into two components. Near the sensor are the ADCs, clock, and control electronics. A digital link connects this to a remote Linux server called the TerraHub. The modular design of the digital system couples the digitizer to a sensor for a complete system calibration and offers an easy upgrade of Si-Audio analog-only seismometers to more-capable digital systems. Due to our analog sensor’s high dynamic range (DR), i.e., approx. 180 dB, a typical digitizer with a 24-bit ADC, offering approx. 138-dB DR, would not be able to record the full DR of our sensor. The solution – send each analog signal into two digitizers, one to capture the faintest of signals and the other to simultaneously capture very large signals faithfully. This “double digitizing” strategy is essential for capturing all the dynamic range that the seismometer has to offer. The digital seismometer we created has six on-board high-resolution ADCs.
The process to create the digital system can be broken into two phases. During Phase I, the initial focus was getting the digitizer that lives close to the sensor to store digitized data in an industry standard format. An integrated micro-SD card can be found on the digitizer circuit board for data storage. A second challenge was incorporating the Global Positioning System (GPS) to use as a time base reference. The digitizer code is written on bare metal and uses a Real-Time Operating System (RTOS). This is a multitasking code base to coordinate data sampling, storage, and communication on a small low-power processor. This concludes the end of Phase I: capturing the data, synchronizing with GPS (time reference), and storing the data on an accessible format.
Phase II revolved around the development of the TerraHub (we toyed around with different names for this system, and this is the only name that has stuck). The TerraHub uses custom spun distribution of the Linux operating system. This allows the operating system to build the code necessary for the custom-built low-power TerraHub and avoids the installation of unnecessary Linux applications to conserve space and processing power. As new features are added, new builds of Linux will be created to accommodate it. The TerraHub distributes time information to the digitizer. There is a single line maintained to the digitizer as a pulse-per-second (PPS) reference, which allows the digitizer to maintain a time reference to keep the clock going if there is a loss of the GPS server. Applications on the TerraHub are often multithreaded while on the digitizer, RTOS is used to achieve the appearance of multiprocessing. Below is a list of some features of the TerraHub:
- Running ring server for sample data distribution (not necessarily hardware but goes through the network SeedLink server)
- Dual USB ports for both storage and accessories like a Wi-Fi access point
- SD card for data storage
- Solar charge controller
- Network access via ethernet
- State-of-health (SOH) monitoring
Follow along for more updates of the digitizer and TerraHub as they come!