I just made a major improvement to my PC’s recording performance, and it may be of interest generally, since it applies to a very common sound chip, a conexant AC’97. I was measuring the frequency response using a direct wire connection, and noticed that it effectively rolled off abruptly at 8KHz, and had some odd behaviours that looked like AGC gain switching. I tried many things but couldn’t improve the behaviour.
Anyway, I was just browsing round the PC’s Start menu and found a never-before-used app called ‘smartaudio’. On opening, it showed a custom skin titled ‘Mic Enhancement’ and 3 options. The option selected was ‘Voice Recognition/Recording’. I selected ‘No Enhancement’ and immediately the recorded frequency response became smooth up to 20KHz.
So if you’ve got the same app on your machine (it doesn’t show up on the ‘add or remove programs’ list of installed progs by the way) you may well find it is screwing with your recordings.