Systems combination in large vocabulary continuous speech recognition

We present a review of the most significant advances in the field of system combination towards reducing word error rates (WER) in large vocabulary continuous speech recognition (LVCSR). We have mainly focused on Recognizer Output Voting Error Reduction (ROVER), confusion network (CN) and minimum frame word error rate (fWER) based combination along with the latest improvements. Despite lot of progress witnessed in this field, some challenges still remain in enhancing the performance of LVCSR. We suggest in this paper some directions that may lead to a lower WER within the framework of system combination.