Accuracy and correlation in stochastic computing

Abstract

This chapter begins by reviewing the sources of inaccuracy in stochastic computing, focusing on correlation, that is, dependencies among stochastic bitstreams. The measurement of correlation is considered, and the SCC metric is defined. The properties of correlation are then explored including some that have only been discovered recently. Correlation can be seen in two ways: either as corrupting a function f, or as changing f to a different, but potentially useful one. Therefore, to ensure that a stochastic circuit works as expected it is important to manage correlation appropriately. This can be done with correlation-controlling units, which must be used carefully to avoid unexpected functional changes and excessive hardware area or latency overhead. There are also cases where correlation has no effect at all (correlation insensitivity). Identifying such immunity to correlation can aid the design of stochastic circuits. Finally, design of stochastic number generators to provide specified levels of correlation is discussed.