[wdmaudiodev] mic gain range missing MSB?

  • From: Marc Lindahl <marc@xxxxxxxxxx>
  • To: wdmaudiodev@xxxxxxxxxxxxx
  • Date: Fri, 9 Feb 2007 10:51:41 -0500

Maybe a little OT but I'm hoping some expertise on this list can address this... quirk?


We were looking at the gain range of the mic input on a standard AC97 type of codec on a stock mobo, using winXP pro, and it looks like the MSB of the gain control register is never been used.

Register Definition btw :
D16 : Mute
D6 : Fixed 20dB Gain (Mic Boost)
GN[4:0] 'Gain programmable from -34.5dB to 12dB in 1.5dB increments providing 32 levels.'
 
However our measurements are showing a 23-ish-dB range, not the listed 46.5dB, so the windows mixer value seems to ignore the available msb.  Default value is 0x8008, so that would mean the default range of gain is between -10.5dB and 12dB. The msb is for values from -12dB down to -34.5.

I'm wondering if this is a bug or a feature :) Anyone have some input? (forgive the pun!)

M
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