Before revision:  The main problem we focus on in this paper is the frequent label permutation switching of permutation invariant training (PIT) . For N-speaker separation, there would be N! possible label permutations. How to stably select correct label permutations is a long-standing problem .
After revision:  The main problem we focus on in this paper is the frequent label permutation switching of permutation invariant training (PIT) approach, although the frequent label assignment switching happening during PIT training remains to be a problem when better convergence speed and achievable performance are desired .