How to post articles in different charset for different group?

[Brep]

Hello, there.

How can I setup to post articles in different charset
for different group. Say, I have subscribed 3 groups:

cn.bbs.comp.linux
tw.bbs.comp.linux
gnu.emacs.gnus

The question is when I posting articles in these 3 groups,
all of these articles are encoded by my default coding system, 
say, 'gb2312' for example. 

I'd like using different charset for different group, 
because there's a favorite charset for most of the 
subscribers of one particular group. Say, the subsribers
from cn.bbs.comp.linux prefer 'gb2312', the subsricbers from
tw.bbs.comp.linux prefer 'big5', the subscribers from
gnu.emacs.gnus prefer 'us-ascii'. 

Now what can I do?

-- 
Zhang Wei, or Brep 
<brep@newsmth.org>