Project Update - 6
! I have the PDM MICs working now via USB Audio to a PC !
Cool.
Details:
First, I had audible artefacts, audible clicks in sound on PC (and clearly to see in Audacity as "jumps" in audio samples).
Knowing this issue - I could solve.
See details also here:
USB Audio issue
The PC USB Audio is a separate, independent and "asynchronous" clock, in relation to my MCU SAI4 clock used for PDM MICs. They differ in clock frequency, they drift - and no way to bring both independent clocks into "sync", with same speed or use one (PC Audio Clock) as master clock to generate the other (fixed coupled).
But using the "fractional multiplier" on PLL3, used for SAI4 (PDM MIC) clock and with some debug means (see waveform on GPIOs for timing relation), I could trim my system and all fine now.
Now I see the MICs, can analyze the MIC audio (with Audacity, using as USB Recording Device) etc.
Here, how it looks like when playing a 1 KHz tone on smartphone in front of the MICs.
I can now also analyze the spectrum, the sensitivity, noise level, see difference between left and right) ...
But:
I am not sure if VisionShield is good enough for my use case.
I have seen:
- one channel has different sensitivity as the other
- it is a bit obvious: one MIC sits directly in front of the ETH connector:
so, it gets a sound reflection, it is blocked a bit by this ETH connector. This can cancel out
the sound due to reflection from this ETH connector, the sound level is not the same between both channels! - not good for my use case (application): I want to have both MICs sensitive in the same way:
I want to track an object via sound and both MICs should give me same sound level when the source is centered in front of the MICs (but it does not, OK, I could try to "correct")
Never mind - great progress!
Even these (USB related) sound artefacts are not really an issue: I need just "artefact free" audio on PC (via USB) to evaluate the MICs, their parameters.
Later, when I do the MIC sound processing, to track my airplanes - it does not matter anymore,
because these artefacts are generated by the PDM MIC to USB (the clock issue) which I would not need anymore, later. So, the MIC sound seems to be clean internally.
BTW:
The debug of the timing was important. See, how the SAI4 clock (for PDM MICs) is related to the USB timing?
I did via dedicated GPIO output pins: toggle a GPIO pin when SAI4 frame (half-buffer) is ready and also another one when USB frame is ready (also half-frame, due to "DoubleBuffer" used).
Check the timing relation, the drift, and when a "wrong" buffer content would be sent (when they GPIO clocks "pass" each other).
This looked like this:
You see, that both clocks are drifting: they are not identical, SAI4 clock was a bit slower as the USB Audio clock. So, this "must" generate" an issue, a "buffer overrun/underrun" with audible artefacts (clicks).
Solved by adjusting SAI4 PLL clock via "fractional multiplier".
Fine tune the SAI4 clock frequency.
(Better: add a "Clock Recovery" and trim this value)
Next steps:
Now:
- start with audio analysis, track sound source (direction), generate signals to control the "mast motor" (yaw and tilt movement values, even w/o to use stepper motors to do so, mechanics at the end...)
- add camera vision: take pictures or shot movie for the object (airplane) in view
The biggest part is done and working (the audio). Cool!