Question about i.Mx27 + wm8974: lots of noise

[Jie Jiang <jjinfo@nudt.edu.cn>]

Hi all,

I have been developing alsa driver for wm8974 + i.MX27.By now, the
playback channel works well. But there is lots of noise in my record
file. The input audio is from aux input. I have configured wm8974 so
that ONLY the aux input goes into ADC. (The INPPGAMUTE bit is 1  and the
MICP2BOOSTVOL is 0.) And I have enabled the Speaker Line Bypass switch
so that I can verify that the aux input is correct and the bypass output
is very clear. That means that there is nothing wrong with the ADC
input. But the wav file (generated by command "arecord -d 20 -c 1 -f
S16_LE -r 8000 -t wav foobar.wav") contains both the aux input audio and
lots of noise, which makes the quality of the record file even
unacceptable. 

Since the playback works well, the register setting of wm8974 and mx27
ssi seems to be correct. 

What's the possible reason causing so much noise? Can the digital filers
of ADC be helpful to eliminate recording noise and improve the quality
of record file?

Regards,
Jie