DPTC -- an FPGA-based trace compression

Abstract: Recording of flash-ADC traces is challenging from both the transmission bandwidth and storage cost perspectives. This paper presents a configuration-free lossless compression algorithm which addresses both limitations, by compressing the data on-the-fly in the controlling field-programmable gate array (FPGA). Thus the difference predicted trace compression (DPTC) can easily be used directly in front-end electronics. The method first computes the differences between consecutive samples in the traces, thereby concentrating the most probable values around zero. The values are then stored as groups of four, with only the necessary least-significant bits in a variable-length code, packed in a stream of 32-bit words. To evaluate the efficiency, the storage cost of compressed traces is modeled as a baseline cost including the ADC noise, and a cost for pulses that depends on their amplitude and width. The free parameters and the validity of the model are determined by comparing it with the results of compressing a large set of artificial traces with varying characteristics. The compression method was also applied to actual data from different types of detectors, thereby demonstrating its general applicability. The compression efficiency is found to be comparable to popular general-purpose compression methods, while available for FPGA implementation using limited resources. A typical storage cost is around 4 to 5 bits per sample. Code for the FPGA implementation in VHDL and for the CPU decompression routine in C of DPTC are available as open source software, both operating at multi-100 Msamples/s speeds.