ESP32-S3 PDM Microphone

[geoffr]

I'm trying to use a PDM microphone with the ESP32-S3 and arduino-esp32 library. I know I am reading microphone data (as it reacts to me blowing into the microphone), however I'm consistently getting negative values interspersed into the I2S output data. Has anyone got PDM working that can see where my error is..?
Why am I seeing negative values at all (should be gated by the I2S.available() check)? And why am I seeing more than -1 (it returns -1 if nothing..?)

Code:

Code: Select all

void setup() {

  // setup 42 PDM clock and 41 PDM data pins
  I2S.setPinsPdmRx(42, 41);

  // start I2S at 16 kHz with 16-bits per sample
  if (!I2S.begin(I2S_MODE_PDM_RX, 16000, I2S_DATA_BIT_WIDTH_16BIT, I2S_SLOT_MODE_MONO)) {
    Serial.println("Failed to initialize I2S!");
    while (1); // do nothing
  }

}

void loop() {

  if (I2S.available() > 0) {
  // Read data from microphone
  int8_t sample = I2S.read(); // -1 if no data is available or an error occurred.
  Serial.println(sample);
  }

}

Sample Output:

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-17
-14
-11
-6
-3
4
9
14
20
23
28
29
30

[MicroController]

Try

Code: Select all

int sample = I2S.read();

[geoffr]

Sure, changing to int means no negative numbers but I get samples in the ~6553x range which is not a valid sample (It's not that noisy!).

If you look at the definition of read() it's "Read the next available byte from the I2S interface." So, it's only 8 bits which ~65553x is definitely not.
( https://docs.espressif.com/projects/ard ... i/i2s.html )

[MicroController]

The documentation is wrong.
read() doesn't return a byte but one sample worth of data. Your samples are 16 bits (I2S_DATA_BIT_WIDTH_16BIT), so you have to interpret the sample values as signed 16-bit (int16_t).
Note that audio samples are usually represented and handled as signed values where the sound waves oscillate around 0.

[geoffr]

Looks like the documentation doesn't line up
read() - Read the next available byte from the I2S interface.
https://docs.espressif.com/projects/ard ... i/i2s.html

But that would change things!

Although as I noted above it still does not work correctly - I get samples in the ~6553x range which is not a valid sample (It's not that noisy!). Any further things I've missed?

Sample Output:

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65535
65534
65534
65534
65535
65535
65535
0
1
0
0
1
1
2

[MicroController]

Any further things I've missed?

My last post maybe?

[lbernstone]

To be explicit about what MicroController is saying, cast it as an int16_t:
int16_t sample = (int16_t)I2S.read();
That should make those 65535 results show up as -1.

[geoffr]

yes, that does change 65535 to -1. I do however have other samples which are crazy large as seen by 65534 above. With the change to int16_t I get the output below. Why am I seeing -2 and -3 (or even much more negative!)

Code: Select all

0
-1
-1
1
0
0
-1
-2
-1
2
2
-1
-3
-2
-2
0
1

Code: Select all

=====================================
PDM test
=====================================
0
-6595
-32768
-29199
-32156
-30203
-31348
-30704
-31063
-30865
-30973

[MicroController]

I'm giving up.