it is just two of two-level half bridge converter with additional diodes connected between the DC link. So the DC link mid point is no longer virtual. Lets assume the switches from top tp bottom as S1, S2, S3, S4. then, S1 and S3 makes one half bridge VSI (For positive side AC voltage) and S2 and S4 makes another VSI (For negative side AC output). the output terminal is between S2 ande S3. I hope this wil help....