All voiced obstruents in Mediaeval Times became unvoiced in Era when Zhongyuan Rhymes was written.
中古声母中的全浊音到了《中原音韵》时代全部变读为相应的清音声母。
2
But voiced obstruent devoicing in Wuyu is not an individual phenomenon. In dialects of Jinhua and Quzhou, nominally voiced obstruents can not uncommonly be recorded as voiceless on sets.
但全浊声母清化在吴语里已不是个别现象，金华、衢州一带方言的所谓全浊声母不少也可以记作清声母。
3
The prenasalized stops in the southern Min languages evolved from the nasals of Middle Chinese and thus are historically different from the voiced obstruents found in Wu and Xiang languages.