too noisy signal on frequency between 2k-7kHz

[pixtop]

Hello, I'm trying to use amodem, but I'm encountering issues with the calibration process.

It appears that the calibration fails to achieve a value above 0.99 for frequencies between 2kHz and 7kHz, whereas frequencies above 7kHz (i.e., 8kHz, 9kHz, and 10kHz) calibrate successfully. I have tested this across different bitrates.

I'm running amodem through an audio cable between two machines on Ubuntu 22.04.

Below, you’ll find the output of the calibration command as well as a recording of the calibration signal:

audio.raw

[LaurenceLong]

This might be your sound device or volume setting issue, try to lower down your volume

[ke4ahr]

If it's too loud, the harmonics will cause issues; the higher ones are probably being masked by the Nyquist limit and/or anti-alias filters.

Any other issue is likely to be due to something in the signal path, e.g. capacitance or inductance which is causing an issue at that frequency. It's also possible that the audio output is a balance or unbalanced source, and there is DC on the output which is causing issues due to the input impedance and/or input loads of the computer.

I'd also look at equalization and the audio paths themselves to make sure you're not dealing with broken hardware. You can change the sample rate in the code or config and see if the results change as well.

I think it's more likely that the volume is too loud based on the RMS values vs Peak. If you have ten carriers and feed them into a 100W amplifier, you don't get 10W per carrier out of the amplifier. You get 9.0 - 9.9 W at best.