[AudioI] Audio Injector Zero

[Matt]

On 7/3/21 8:30 am, Bill Kunkle via People wrote:
> 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/ 
> <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.
> Zero.PNG
>
>
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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