Proposal for Compactification of CaloCell Objects

All ATLAS calorimeters together have 187652 cells. In the reconstruction software of ATLAS for each cell an object of type CaloCell with 195 bits of information for energy, time, gain and quality is created. Saving all the calorimeter cells with the LCG POOL persistency service consumes in excess of 1 MB (the 1 MB is of course dependent on the persistency service used because POOL does a gzip equivalent). In this proposal we are investigating algorithms to compactify the calorimeter data as much as possible without harming the resolution. Using a cubic root compactification method for the energy and a logarithmic compactification method for the time for the ~10% of the cells with calculated time the average size per cell could be reduced to 17.6 bits. This note describes the "lossy" algorithms to achieve this goal and their implementation in the ATLAS reconstruction software.