ESP32 Forum

Posted: **Thu Aug 21, 2025 3:36 pm**

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:

Code: Select all

Posted: **Sat Aug 23, 2025 6:47 pm**

Try

Code: Select all

int sample = I2S.read();

Posted: **Sun Aug 24, 2025 1:32 pm**

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 )

Posted: **Sun Aug 24, 2025 2:31 pm**

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.

Posted: **Wed Aug 27, 2025 4:23 pm**

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:

Code: Select all

Posted: **Wed Aug 27, 2025 9:29 pm**

Any further things I've missed?

My last post maybe?

Posted: **Thu Aug 28, 2025 12:13 am**

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.

Posted: **Thu Aug 28, 2025 6:28 pm**

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

Posted: **Thu Aug 28, 2025 7:59 pm**

I'm giving up.

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