I have been using a Raspberry Pi with a USB sound card to monitor very low frequency signals (+/- 25 kHz) for sudden ionospheric disturbances.  It receives the VLF signals from an antenna which is connected to a preamp which then sends the signal to the microphone input of the sound card.  This is the SuperSID program as explained here: http://solar-center.stanford.edu/SID/sidmonitor/

The sound card needs to be capable of a 96000 Hz sample rate.

I decided to switch to an Audio Injector Zero that I had purchased and it seems to work far better than the USB sound card.  I have the signal from the preamp connected to the microphone input on the Zero (circle on the board).  I am curious as to why the strength of the signal that is visible on the Pi does not change when I decrease the level under Capture in the alsamixer.  Also, the control for Mic Boost can only be changed to full on or full off using the arrow keys.

Just curious as to why this is.  It seems to be working very well.

Great to hear about your project - it is awesome when people report in with their various projects. Looks like a very interesting area of research. Curiously, the frequencies we are talking about here are above the frequency threshold of hearing ! People loose temporal locking at around 4 kHz and complete sound loss above around 20 kHz.

Explaining the lack of fine grain microphone gain control :

Attached is an image of the block diagram of the wm8731 codec. It shows the line inputs and microphone input on the left side. The line inputs go through the volume gain stage. The microphone input goes through a 0 dB or 20 dB boost selection option. This would be why you observe no gain changes when you change the gain level of the soundcard's input mixer.

If anyone in the future does have interest in a finer gain control on the microphone input, try using the softvol ALSA plugin.

Matt

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