[AudioI] stereo sound card on RPi4

[Matt Flax]

ok, that looks a little better, but still seems to be some spurious noise below the bandwidth of interest (two vertical red lines).
 

 
On 18/1/22 10:40 am, Bill Kunkle wrote:
 
 
 
Matt,
 

 
 
Thank you for your reply.
 

 
 
Yes, the stereo sound card and SD card with the OS were transferred from the RPi3 to the RPi4.  The RPi3 uses a 2.5 amp power supply.  The RPi4 uses a 3.0 amp power supply.  So the power supplies were different for each computer.
 

 
 
The bcm2835 has been blacklisted.  The stereo sound card is the only one that shows up using aplay -l
 

 
 
It appears that most of the noise is being created by the RPi4 power supply.  I used a micro USB to USB-C converter to connect the RPi3 power supply to the RPi4 and here is the result - see image.
 

 
 
The tests were done using WiFi.  At some point I will do another test with the RPi4 connected to ethernet to rule out any noise from the WiFi on the board.
 

 
 
Thanks for your help.  
 

 
 

 
 
 
 
 
On Mon, Jan 17, 2022 at 1:35 PM Matt Flax <matt at audioinjector.net <mailto:matt at audioinjector.net> > wrote:
 
 
 

This is a nice puzzle !

 

OK, I would imagine that the power supply would effect very low frequency noise and a bad power supply would possibly have spurious tonal spikes through the noise floor. You confirmed that the power supply didn't change the effects below.

 

Just restating what you said, the same sound card is being used, the same power supply and all other equipment remains the same. The only change is the Pi 3 becomes a Pi 4. The same operating system is running on both.

 

 

Please double check that the alsa mixer has no side paths for audio to sum together and that you are receiving audio from one source only in the codec mixing fabric.
 

 

There are two possibilities for these signal effects Either noise floor issues or signal corruption.
 

 

Is there a way to measure the noise floor of this system ? To do a sound card alone measurement,. you will need to plug the output to the input of the sound card - this will confirm that there is no noise floor problem.

 

How can we test for signal corruption ? Is it possible that the Pi4 is generating these spurious signals ?
 

 

Matt
 

 


 

 
On 17/1/22 1:50 pm, Bill Kunkle via People wrote:
 
 
 
I collect data from VLF radio stations for the Stanford SOLAR Center for detection of Sudden Ionospheric Disturbances (SID).
 
I've been using an RPi3 with the audioinjector zero sound card feeding the signal into the mic input.
 
I recently purchased the audioinjector stereo card and I am feeding the signal into the line in input.
 
With the stereo card on the RPi3 I can get very good signals above the noise floor as shown below:
 
 
 
I transferred the stereo card and the SD card to a RPi4 and I get this:
 
 
 
I've tried two RPi4's and two different power supplies (both from Canakit).
 
 
The RPi3 is working very well with the stereo card but I do not understand why the RPi4 does not work as it should.
 

 
 
Any thoughts would be appreciated.
 

 
 
Bill
 

 
 
 
 

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