AIC23 / WM8731 audio codec noise problem

Hello,

I am using the Cyclone II DSP Development Board. I want to test the audio codec (TLV320AIC23B) which is as far as I know from the programming point of view identically with the WM8731 codec.

For testing my VHDL code purpose, I just want to capture the line in input and send the data directly back to the DAC.

So far it works, but if I plug out the analog line in input, can hear a lot of noise. I already tried different transfer modes (DSP mode as slave and I2S mode as master).

If I transfer just zeros to the DAC the noise is gone. Of course there is some thermal noise involved, but this could hardly cause the strong noise, I got.

Any ideas?

Thanks,

Daniel