Audio input (PDM) Portenta Vision Shield

[sonata00]

I am still new in Arduino sketching and hardware. I have the portenta h7 and vision shield board. I am using this code to try its microphone. However the PDM plotter is not changing. Anyone know what is the problem?

The code:

#include <PDM.h>

// default number of output channels
static const char channels = 1;

// default PCM output frequency
static const int frequency = 16000;

// Buffer to read samples into, each sample is 16-bits
short sampleBuffer[512];

// Number of audio samples read
volatile int samplesRead;

void setup() {
  Serial.begin(9600);
  while (!Serial);

  // Configure the data receive callback
  PDM.onReceive(onPDMdata);

  // Optionally set the gain
  // Defaults to 20 on the BLE Sense and -10 on the Portenta Vision Shield
  // PDM.setGain(30);

  // Initialize PDM with:
  // - one channel (mono mode)
  // - a 16 kHz sample rate for the Arduino Nano 33 BLE Sense
  // - a 32 kHz or 64 kHz sample rate for the Arduino Portenta Vision Shield
  if (!PDM.begin(channels, frequency)) {
    Serial.println("Failed to start PDM!");
    while (1);
  }
}

void loop() {
  // Wait for samples to be read
  if (samplesRead) {

    // Print samples to the serial monitor or plotter
    for (int i = 0; i < samplesRead; i++) {
      if(channels == 2) {
        Serial.print("L:");
        Serial.print(sampleBuffer[i]);
        Serial.print(" R:");
        i++;
      }
      Serial.println(sampleBuffer[i]);
    }

    // Clear the read count
    samplesRead = 0;
  }
}

/**
 * Callback function to process the data from the PDM microphone.
 * NOTE: This callback is executed as part of an ISR.
 * Therefore using `Serial` to print messages inside this function isn't supported.
 * */
void onPDMdata() {
  // Query the number of available bytes
  int bytesAvailable = PDM.available();

  // Read into the sample buffer
  PDM.read(sampleBuffer, bytesAvailable);

  // 16-bit, 2 bytes per sample
  samplesRead = bytesAvailable / 2;
}

Source code: PDM problem with NANO 33 BLE SENSE

The PDM plotter:

[Railroader]

Pmease read and use the topic "How to get the best out of this forum."

[tjaekel]

I have the PDM MICs working (but not on an Arduino sketch, instead as native STM32 project, with HAL drivers).

Important to know is:
PDM is not PCM!
The PDM signal from MICs are Pulse Dense Modulated: it means: the gap (their length) in between the pulses are the audio signals.
PCM is Pulse Code Modulation: so, you get the audio signal values (amplitude sampled).

A PDM signal must be filtered and converted into a PCM signal. There are library function to do so. When you take the PDM signal and you try to display the "values": they are just 0 or 1. A PDM signal printed as amplitudes samples does not show you any reasonable audio waveform.

If you need help - I can share my STM32 IDE code for PDM MICs.
It needs a LIB which will do the PDM filter and conversion to PCM values. The filter is a real "analog" filter, done in SW. I guess, you have to enable also to use the HW Floating Point Unit (FPU).

[sonata00]

Thank you for the explanation ! If you don’t mind, can you share the code? I would appreciate it.

[tjaekel]

Here my PDM MIC for Portenta H7 VisionShield:
VisionShield.zip (17.0 KB)

The source code file should be "self contained" (all what you need), except:
The clock config is done in my system_clock.c (but see the comment in audio.c).

I enable it via:

PDM_MIC_Init(dBAmplifier);

0 stops it, otherwise 1..51 as dB for amplification factor.

I do not know if there is a dependency in code for USB Audio, e.g. a buffer: I send this MIC Audio as USB Audio device to a host PC (via USB-A on breakout board).

See the LIB file in ZIP: you have to add it as LIB to the project (it is for the PDM filter).

[tjaekel]

I am sorry - I send the PDM MIC audio via network (ETH).
I use "VB-Audio Voicemeeter":
VB-Audio, Voicemeeter

I send as UDP, with the right protocol (VBAN). I can receive on PC with a "virtual network" sound card driver. I am not using USB, even I have it up and running in my project (not part of this shared code, but you see the UDP transmission as VBAN).

[tjaekel]

Here is my entire project:
PortentaH7_VFR - GitHub