Why use multiple filters output thousands value, use one or two filter can output normal value?

Question

Problem

If the loop runs only one simulated pin and uses a filter, it will output normally (analog 1600~2200, filter 0~500), but creating an array to store multiple filtered simulated pin values will cause problems, and the output will be abnormal (10000~80000).

If analog read six sensors, and their value is 1600~2200.

It used the code below.

#include "EMGFilters.h"
#define SERIAL_TX_BUFFER_SIZE 4096 //修改串口接收缓冲区大小为2048
const int SensorInputPin[] = {A0, A1, A2, A3, A4, A5};
EMGFilters myFilter[6];
SAMPLE_FREQUENCY sampleRate = SAMPLE_FREQ_500HZ;
NOTCH_FREQUENCY humFreq = NOTCH_FREQ_50HZ;

void setup()
{

    for (int i = 0; i < 6; i++)
    {
        myFilter[i].init(sampleRate, humFreq, true, true, true);
    }
    Serial.begin(115200);
    Serial1.begin(250000);
    analogReadResolution(12);
}
void loop()
{
    long time = millis();
    int x[6] = {0,0,0,0,0,0};
    int analog[6] = {0,0,0,0,0,0};

    for (int index = 0; index < 6; index++)
    {
        //x[index] = myFilter[index].update(analogRead(SensorInputPin[index]));
        analog[index] = analogRead(SensorInputPin[index]);
    }
    while ((millis() - time) < 2)
    {
        delayMicroseconds(100);
    }
    Serial.printf("filter:%d %d %d %d %d %d analog:%d %d %d %d %d %d\n", x[0], x[1], x[2], x[3], x[4], x[5],analog[0],analog[1],analog[2],analog[3],analog[4],analog[5]);
    Serial1.printf("<%d,%d,%d,%d,%d,%d>", x[0], x[1], x[2], x[3], x[4], x[5]);
}

Change index<6 to index<2, and uncomment \\x[index] = myFilter[index].update(analogRead(SensorInputPin[index]));

It will be a normal output using two inputs and filters.

But changing index=0;index<2 to index=0;index<6 will be abnormal output.

If changing to index=3;index<4 or index=2;index<3...etc, it will be normal output:

Sketch:

XIAO MCU schematic in the HTML: https://files.seeedstudio.com/wiki/Seeeduino-XIAO/res/Seeeduino-XIAO-v1.0-SCH-191112.pdf

I don't have a sensor schematic.

Filter code: https://github.com/oymotion/EMGFilters/blob/master/EMGFilters.cpp

I hope each simulation output to look like the first Gif(normal output), what should I do to achieve this?

Answer 1

This is a bug in the filter library you are using. It has already been reported (by you, maybe?).

As Majenko and chrisl suggested in comments, the crux of the problem is that you need one filter per input channel. Using multiple instances of EMGFilters seems like the obvious solution. Alas, the library keeps the filter state in global variables, which essentially means your six filter objects all reference the same actual filter.