fix automatic (and unwanted) audio filter when recording on (some) C-Media audio chips

today i finally discovered why the fuck the sound chip in my pc (a C-Media CMI9761A in a VIA southbridge) was recording badly with Fraps (
since i'm not an expert on audio stuff, i wasn't able to recognize the problem with its proper name, so it took me a bit before i could understand what kind of issue there was, and thus finding the correct terms to google up.

now, the problem is that when recording, the volume jumps up and down silly. i noticed it when i was testing out Fraps.
i've also contacted Fraps support ppl and they kindly said that Fraps does not alter or filter the audio in any way, so i figured the problem was somewhere else.
after some searching, i learned that the audio chip does something called compression or expansion which is also used to filter out background noise when recording or whatever. the net has all the answers; no need to do a lesson here.

in the audio config cpl there is nowhere to disable this filter, and many people complained about it in the C-Media official forums.
in one particular post, one of the tech support guys showed some interesting registry settings for the driver, in this hive:
HKLM\SYSTEM\CurrentControlSet\Control\Class\guid\index\Settings (where guid and index may vary)

thanks to this info, i was able to find this setting: RemoveDcOffset
when set to 1, the filter is active and the volume goes evil. to disable it, set it to 0

after recording from the line-in and the wave-mix sources, all went ok and the volume staid stable.
here some comparison with a sound played 5 times, first with the filter and then without: