// simple project using Arduino UNO and 128x64 SSD1306 IIC OLED Display to display animated icon from www.icons8.com

// https://www.youtube.com/watch?v=MJt9kSNlsU4
// OLED Oscilloscope

#include <Wire.h>
#include <SPI.h>
#include <Wire.h>                                     // I2C library for display
#include "fix_fft.h"                                  // fixed point Fast Fourier Transform library
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

#define SCREEN_WIDTH 128 // OLED display width, in pixels
#define SCREEN_HEIGHT 64 // OLED display height, in pixels

#define OLED_ADDR 0x3C                                // OLED I2C address
#define audioIn A0                                    // audio input port

// Declaration for an SSD1306 display connected to I2C (SDA, SCL pins)
#define OLED_RESET    -1 // Reset pin # (or -1 if sharing Arduino reset pin)
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, OLED_RESET);

char re[128], im[128];                                // real and imaginary FFT result arrays
byte ylim = 60;                                       // OLED y-axis drawing boundary limit

void setup() {
  display.begin(SSD1306_SWITCHCAPVCC, OLED_ADDR);     // init OLED display
  display.clearDisplay();                             // blank the display
  display.setTextSize(1);                             // configure text properties
  display.setTextColor(WHITE);
  analogReference(DEFAULT);                           // use the default analog reference of 5 volts (on 5V Arduino boards)
                                                      // or 3.3 volts (on 3.3V Arduino boards)
};

void loop() {

  // The FFT real/imaginary data are stored in a char data type as a signed -128 to 127 number
  // This allows a waveform to swing centered around a 0 reference data point
  // The ADC returns data between 0-1023 so it is scaled to fit within a char by dividing by 4 and subtracting 128.
  // eg (0 / 4) - 128 = -128 and (1023 / 4) - 128 = 127

  for (byte i = 0; i < 128; i++) {                    // read 128 analog input samples from ADC
    int sample = analogRead(audioIn);
    re[i] = sample / 4 - 128;                         // scale the samples to fit within a char variable
    im[i] = 0;                                        // there are no imaginary samples associated with the time domain so set to 0
  };

  fix_fft(re, im, 7, 0);                              // send the samples for FFT conversion, returning the real/imaginary results in the same arrays

  display.clearDisplay();                             // clear display
  display.setCursor(0, 0);                            // set cursor to top left corner of the display
  display.print("Audio Spectrum");                    // print a header on the top row of the display

  // The data array will contain frequency bin data in locations 0..127 for samples up to the sampling frequency of approx. 9 KHz
  // Each frequency bin will represent a center frequency of approximately (9 KHz / 128 samples) = 70 Hz
  // Due to Nyquist sampling requirements, we can only consider sampled frequency data up to (sampling rate / 2) or (9 KHz / 2) = 4.5 KHz
  // Therefore we only acknowledge the first 64 frequency bins [0..63] = [0..4.5KHz]

  for (byte i = 0; i < 64; i++) {
    int dat = sqrt(re[i] * re[i] + im[i] * im[i]);     // frequency magnitude is the square root of the sum of the squares of the real and imaginary parts of a vector
    display.drawLine(i * 2, ylim, i * 2, ylim - dat, WHITE); // draw bars for each frequency bin from 0 Hz to 4.5 KHz
  };

  display.display();                                   // update the display
};