QAudioInput Frequency and Amplitude

Hi,
I'm trying to measure the frequency of a sine given from a frequency-generator. The generator is connected to the PC via the audio-input.
I'm using a QAudioInput and a QIODevice to handle the data.
The frequency is measured by counting the zero-crossings in a defined time, this works until ~5khz.
I wonder, why the amplitude of the signal seems to lower with rising frequency:
Obove 5khz the data arriving in the QIODevice has values between 125 and 128, which makes it impossible, to count the zero-crossings correctly (It's normaly oscillating around 127).
The frequency generator does not lower the amplitude of the signal, i checked this with an oscilloscope.

Does somebody know, why QT seems to lower the amplitude?

Greetings

Hmm, this sounds more like a hardware problem to me. Is this an analog signal? If so, what are the details of the input card (sampling rate, bit depth, filters, ...)

Yes, it's an analog signal.
The sound input card is the one in my notebook: Intel 82801GBM ICH7-M - High Definition Audio Controller PCI

Very likely this is because of characteristic of your hardware audio input. Do you use line-in or microphone input?

Just a note - once you get your hardware issues sorted out, you might want to consider using a Fourier transform instead of counting zero crossings to obtain frequency.