Audio Input isnt working correctly

Mbed OS
1

hi,

I’m trying to capture a sin wave from a microphone and output it on an oscilloscope. I have to do a lot more with the sin wave but I’m facing an issue in the first step itself.

This is the code i’m using

#include <mbed.h>

#define BUFFER_SIZE 256

AnalogIn microphone(PA_3); 
AnalogOut speaker(PA_4); 

float buffer[BUFFER_SIZE];
int bufferIndex = 0;

int main() {
    while (true) {
        // Record
        buffer[bufferIndex] = microphone.read();
        
        // Playback
        speaker.write(buffer[bufferIndex]);

        bufferIndex = (bufferIndex + 1) % BUFFER_SIZE;

        wait_us(1000); // Wait for 1ms
    }
}

The output is absolutely nonsense. When I printf the values of input all I get is 0.5 continuously. Thanks.

2

Maybe the amplitude of the incoming microphone signal is just too low for the MCU to be able to read it? If you hook up an O-scope to the input, what’s the peak to peak voltage of the signal, and also what’s the center voltage? Also, what Mbed board are you using?

3

I’m using a STM32F4 discovery board (F429I). When I connect it to the board I get a Pk - Pk of 50mV and Max of 2.9 V. But when I probe it directly to the mic I get the correct display. So my channel 1 is the output from the board and channel 2 is the output from the mic. Ch 2 and ch 1 are not matching. Ch 2 displays a sin wave where as ch 1 is garbled noise.

4

Hmm… if you connect the ADC pin to 3.3V, does the output match? And if not, what is the value of the floats in buffer? That’s where I’d start debugging,