Software filtering and online reconstruction in H1

In spring 1992, the H1 detector at the HERA electron proton collider at DESY came into operation. The high bunch crossing rate of 10 MHz and the total number of 270,000 electronic channels, are placing demanding requirements on the trigger, the data logging and event reconstruction. This note describes a software filtering, data logging and event reconstruction system for the H1 experiment. An event rate of typically 40 Hz (4 MB/s) is processed by a parallel array (“filter farm”) of 34 MIPS R3000 RISC processors. Fast online event reconstruction and filtering algorithms reduce this rate to 5-8 Hz (<500 kB/s). The data logging and the full event reconstruction are performed on a multiprocessor SGI Challenge computer connected via FDDI links to the data acquisition system. An Ampex mass storage device is used to store raw as well as reconstructed data