Audio acquisition and analysis with ARM Cortex-M0 and I2S microphone


Hi Clemens, how much program space (%) does this use? … I think I looked at this as well, but there was hardly anything left if used in conjunction with TTN.


Clemens, do you have the hardware still available? I would be curious if you could reproduce the observation that I2S.end() stalls. I looked back at all the things I have done so far to scrutinize microphone output and see that in those tests I put begin()s I2S always in the setup() function. Only when combining it with LMIC, I started using I2S.end(). And only with this microphone (ICS43432) I run into this issue.


Hi Andreas, I am not yet convinced that this is the same issue, but that could be because I don’t see deep enough in (for me) muddy water.

By collecting data like this:

for (int j=0; j<DATA_SIZE/64; j++) get_audiosample(&data[j*64]);

And only doing processing on this sample later, I thought I escaped from the issue that there would be too much time between subsequent calls to onI2SReceive(). My reason for doing this is that I wanted the complete sample to be contiguous (and thereby not to introduce weird frequencies). Then I go on to proceed with lengthy stuff, such as the FFT analysis. And afterwards I collect a new sample via the above line of code.

This loop of collecting and doing an FFT is repeated a number of times. If the issue would have been the same as described in the post (i.e. not emptying a buffer in time), I would have expected for it to arise within that loop. However it only does upon calling I2S.end().

I tried to insert a bunch of Serial.println() statements in that function in i2s.cpp and one by one removing the last one. With the function full of println() statements, it gets through. At some point, after removing a few, it gets caught in the following line of code inside i2s.cpp:


Which both rely on getting out of a loop in SAMD21_I2Sdevice.h:


… I have no clue what that bit does and who/what sets it.

Then again, I know that this kind of debugging is risky, as it introduces different timings.

A completely different thought just occurred to me: with the microphone in the beehive, I use a low sampling frequency (3125 Hz) and a relatively small DATASIZE=256 (because that gives me FFT bins of ~12 Hz). Since the ICS43432 doesn’t tollerate such low frequencies, I had to up it to something above 7.19 kHz and I picked 12500 Hz together with DATASIZE=1024 in order to get similarly spaced FFT bins. Could there be an issue with so much data being buffered?

Well. Really appreciating you to look into this with me. DIscussing gives new insights and helps to get my thoughts clear in order to phrase things (do I am not sure this post is the best example of that). I feel we made some progress already by reproducing this without having LMIC involved … BTW, I have reduced the code in github somewhat further and it still stalls.

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I fear my Feather M0 RFM95 is currently in a box on Andreas’ desk – left from the last conference. ;-)

I compiled the sketch with a MKR1000 as board – just for testing and got this

Build-Optionen wurden verändert, alles wird neu kompiliert
Der Sketch verwendet 220912 Bytes (84%) des Programmspeicherplatzes. Das > Maximum sind 262144 Bytes.


Thanks @Clemens, that is I think very similar to the number I got on the Feather M0. The sketch that lives in hardware in my beehive (I2S, FFT and LMIC) does the following

Sketch uses 72072 bytes (27%) of program storage space. Maximum is 262144 bytes.

But … if it doesn’t work reliably with slightly different hardware, it is of no use, of course.


Hi Wouter, to be honest I never checked the memory consumption of the M0 because I did not come to any limitation till now. But I was surprised to see such a high load. I produced a lot of bins so I thought this is the reason for the high consumption. But I changed some variables, see First Steps (digital) Sound Recording like sampleRate, fftSize, bitsPerSample but I got still the same 84%.

Seems the Arduino Uno has some other compiling options, for the default blink sketch you get

Der Sketch verwendet 930 Bytes (2%) des Programmspeicherplatzes. Das Maximum sind 32256 Bytes.
Globale Variablen verwenden 9 Bytes (0%) des dynamischen Speichers, 2039 Bytes für lokale Variablen verbleiben. Das Maximum sind 2048 Bytes.

So there is a differentiation between program memory and dynamically allocated memory.

Nevertheless I had a look at the spec of the ATSAMD21G18, the MC on the Feather M0:

  • 256KB in-system self-programmable Flash
  • 32KB SRAM Memory

I thought – compared with the “normal Uno” – this is much more then we ever would need. But it seems I’m wrong in case it is a memory problem you are running in now.

Compared with this, the LoPy is outstanding:

  • RAM: 4MB
  • External flash: 8MB

So I’m asking myself if this could be a reason also to move to the ESP32? The only drawback I see is that the Arduino FFT lib is not available for the ESP as far as I know, but there are other FFT libs! And in case we have WiFi nearby we could record and transmit with a higher bin rate also.

made this topic public #47

So far I haven’t run into a memory problem: all my sketches with I2S, FFT and LMIC are below 30% program space usage. And it seems that it should port pretty well to an M4 in the future if need be.

The Arduino Sound library just takes a lot of space, which is why I avoided it and used the Arduino I2S in conjunction with Adafruit Zero FFT.

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slightly OT as this building block is a commercial product, but they also feature a Cortex M0 and a ICS43432.:

  • Measures sound pressure level in dB(A/B/C/D/Z) and ITU-R 468
  • Measures spectrum with up to 512 bins and up to 80 samples per second
  • Frequency range 40Hz to 40960Hz
  • Noise floor 30dB, maximum 120dB

documentation and examples for many programming languages:

firmware, design files, code:

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New informatiom for me, good to know! I wondered how you got such a low memory footprint!


Announced frequency range down to 40 Hz is interesting while the ICS43432 is specified from 50 Hz on.


Yes and No: 50 Hz here is only the -3 dB point on frequency response, the unit actually was measured with down to 10 Hz (@ 20 dB attenuation over response @1kHz) :


But: bins over, say, 22 kHz are unusable with this mic. So the 40960 Hz are likely only zipper noise! ;)

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wow, there’s a lot going on here :) cool!

I am afraid it would take some time for me to get started with the code.

I am not sure if this helps, but maybe we could decide for one or several audio test-snippets and both run our code on it? Then we could compare the FFT outcomes?


I can only imagine to play the file with a PC / mobile phone and to record it with the mic of the hardware so we would have a strong bias (speaker, background noise, .mic). Or we would have to connect the input channel with a I2S audio player to get comparable outputs.


Hi @weef, thanks for this! I hadn’t come across this one. It will be good to have some more source code to look at and get inspired by!

split this topic #56

3 posts were merged into an existing topic: Audio acquisition and analysis with ESP32

Audio acquisition and analysis with ESP32

Reading from I2S using Adafruit_ZeroI2S

Dear @wjmb,

searching for enableRx throughout the repository (and also on the Web) effectively just yields the results Search · enableRx · GitHub, pointing to the dma_passthrough.ino example file.

This effectively will also pull in the ZeroDMA library, which actually does not sound bad either.

If you want to actually follow this route: My plan would be to produce a minimum working example by stripping all tx-related things from dma_passthrough.ino and then try to mix in some details from zerodma_memcpy.ino to actually synchronize your code with the DMA transfer and being able to actually do something with the data after the transfer completed.

By working on that level, you see that you are more directly working with the hardware there: I believe the ArduinoSound library already implements things like double buffering on top of that (which is where things are currently going wrong with your setup re. the observed starvation/stalling behavior).

On the other hand, you might detour to a completely different route. We will be happy to hear about your next findings and observations.


P.S.: Saying that, reading about DMA I2S callback not triggering at high frequency · Issue #294 · arduino/ArduinoCore-samd · GitHub and having a look at GitHub - fablabbcn/smartcitizen-kit-audio: Audio analysis library for the digital I2S microphone on the smartcitizen-kit 2.0 again, it looks like others are also struggling. So, I’m glad you already opened I2S.end() hangs · Issue #386 · arduino/ArduinoCore-samd · GitHub - they might even find something what we are missing ;].


Hi Andreas,

Thanks for the suggestion … though I am not quite sure which parts of dma_passthrough.ino would be applicable to the M0 (an #ifndef captures the situation here the target is not an M4).

Do you think that DMA is needed in the situation where we can serialize getting a sample and analysing it later?


So true, it’s even in the header comments. And to make matters worse, they are even using a different ADC, right?

/*  This example shows how to pass data through using the
 *  ZeroI2S and ZeroDMA libraries.
 *  This example is for M4 devices only
 *  Try this with the AK4556 I2S ADC/DAC

So, maybe let’s consider this departure as abandoned, right?

Looking at how the sound-pressure-level-bricklet people are doing it on the M0 might yield better rewards (thanks again, @weef!) while watching for updates on the issue you created at I2S.end() hangs · Issue #386 · arduino/ArduinoCore-samd · GitHub.


I expected already they would not use the Arduino HAL at all and that proved right. The result is concise C code which looks just perfect. Enjoy browsing sound-pressure-level-bricklet/software/src.