之前基于 FireBeetle ESP32 和全向MEMS麦克风模块（SEN0487）制作过一个在OLED 屏幕上显示当前环境声音频谱的装置【参考1】。这次制作的是能够输出 VGA 信号的频谱装置，这样，用户能够在显示器或者电视机上看到实时频谱输出。
具体的VGA 显示原理，可以在之前的介绍中看到【参考2】，这次的设计硬件部分与之类似。电路图如下：
其中主控和VGA 部分如下：VGA本质上还是模拟信号，这里使用电阻能够输出不同电平的模拟信号，三根GPIO能够实现2^3=16种组合，因此也意味着能够实现16种颜色.
下面是用于连接全向MEMS麦克风模块的接口：
板子上带有一个 USB 公头用于取电，另外还有一个 USB母头，如果你的显示设备没有 VGA接口只有HDMI接口，那么需要一个VGA转HDMI线，而这种线通常使用USB公头取电，这种情况可以直接将它连接到这个 USB母头取电。
同样的，为了便于从充电宝取电，还设计了一个负载消耗电路。
PCB设计如下：
3D预览如下：
焊接后的实物如下：
接下来就可以进行软件的设计了,。基本原理是：首先通过ADC进行采样，然后将采样结果进行 FFT ，最终得到的是采样期间每个频率的能量。我们将这个数值显示在 VGA上就得到了期望的结果：
#include <arduinoFFT.h>
#include "fabgl.h"
// VGA 显示
fabgl::VGA16Controller DisplayController;
Canvas cv(&DisplayController);
//ZivDebug #define SAMPLES 1024 // Must be a power of 2
#define SAMPLES 256 // Must be a power of 2
#define SAMPLING_FREQ 40000 // Hz, must be 40000 or less due to ADC conversion time. Determines maximum frequency that can be analysed by the FFT Fmax=sampleF/2.
#define AMPLITUDE 1000 // Depending on your audio source level, you may need to alter this value. Can be used as a 'sensitivity' control.
#define AUDIO_IN_PIN A0 // Signal in on this pin
#define NOISE 500 // Used as a crude noise filter, values below this are ignored
const uint8_t kMatrixWidth = 16; // Matrix width
const uint8_t kMatrixHeight = 16; // Matrix height
#define NUM_BANDS 16 // To change this, you will need to change the bunch of if statements describing the mapping from bins to bands
#define BAR_WIDTH (kMatrixWidth / (NUM_BANDS - 1)) // If width >= 8 light 1 LED width per bar, >= 16 light 2 LEDs width bar etc
#define TOP (kMatrixHeight - 0) // Don't allow the bars to go offscreen
// Sampling and FFT stuff
unsigned int sampling_period_us;
byte peak[] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}; // The length of these arrays must be >= NUM_BANDS
int oldBarHeights[] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
int bandValues[] = {0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0};
double vReal[SAMPLES];
double vImag[SAMPLES];
unsigned long newTime;
arduinoFFT FFT = arduinoFFT(vReal, vImag, SAMPLES, SAMPLING_FREQ);
unsigned long int Elsp1;
int h[16];
int Height,Width;
void setup() {
Serial.begin(115200);
sampling_period_us = round(1000000 * (1.0 / SAMPLING_FREQ));
DisplayController.begin();
// 设定分辨率
DisplayController.setResolution(VGA_640x400_60Hz);
Height=cv.getHeight();
Width=cv.getWidth();
cv.setBrushColor(Color::Red );
// get a font for about 40x14 text screen
cv.selectFont(&fabgl::FONT_8x8);
cv.setGlyphOptions(GlyphOptions().FillBackground(true));
}
void loop() {
static int64_t stime = esp_timer_get_time();
static int FPS = 0;
static int FPSCounter = 0;
// Reset bandValues[]
for (int i = 0; i < NUM_BANDS; i++) {
bandValues[i] = 0;
}
// Sample the audio pin
for (int i = 0; i < SAMPLES; i++) {
newTime = micros();
vReal[i] = analogRead(AUDIO_IN_PIN); // A conversion takes about 9.7uS on an ESP32
vImag[i] = 0;
while ((micros() - newTime) < sampling_period_us) {
/* chill */
}
}
// Compute FFT
FFT.DCRemoval();
FFT.Windowing(FFT_WIN_TYP_HAMMING, FFT_FORWARD);
FFT.Compute(FFT_FORWARD);
FFT.ComplexToMagnitude();
// Analyse FFT results
for (int i = 2; i < (SAMPLES / 2); i++) { // Don't use sample 0 and only first SAMPLES/2 are usable. Each array element represents a frequency bin and its value the amplitude.
if (vReal[i] > NOISE) { // Add a crude noise filter
//16 bands, 12kHz top band
if (i <= 2 ) bandValues[0] += (int)vReal[i];
if (i > 2 && i <= 3 ) bandValues[1] += (int)vReal[i];
if (i > 3 && i <= 5 ) bandValues[2] += (int)vReal[i];
if (i > 5 && i <= 7 ) bandValues[3] += (int)vReal[i];
if (i > 7 && i <= 9 ) bandValues[4] += (int)vReal[i];
if (i > 9 && i <= 13 ) bandValues[5] += (int)vReal[i];
if (i > 13 && i <= 18 ) bandValues[6] += (int)vReal[i];
if (i > 18 && i <= 25 ) bandValues[7] += (int)vReal[i];
if (i > 25 && i <= 36 ) bandValues[8] += (int)vReal[i];
if (i > 36 && i <= 50 ) bandValues[9] += (int)vReal[i];
if (i > 50 && i <= 69 ) bandValues[10] += (int)vReal[i];
if (i > 69 && i <= 97 ) bandValues[11] += (int)vReal[i];
if (i > 97 && i <= 135) bandValues[12] += (int)vReal[i];
if (i > 135 && i <= 189) bandValues[13] += (int)vReal[i];
if (i > 189 && i <= 264) bandValues[14] += (int)vReal[i];
if (i > 264 ) bandValues[15] += (int)vReal[i];
}
}
// Process the FFT data into bar heights
for (byte band = 0; band < NUM_BANDS; band++) {
// Scale the bars for the display
int barHeight = bandValues[band] / AMPLITUDE;
if (barHeight > TOP) barHeight = TOP;
// Small amount of averaging between frames
barHeight = ((oldBarHeights[band] * 1) + barHeight) / 2;
// Move peak up
if (barHeight > peak[band]) {
peak[band] = min(TOP, barHeight);
}
h[band] = barHeight;
// Save oldBarHeights for averaging later
oldBarHeights[band] = barHeight;
}
if (millis() - Elsp1 > 10) {
for (byte band = 0; band < NUM_BANDS; band++)
if (peak[band] > 0) peak[band] -= 1;
cv.setBrushColor(Color::Black );
cv.clear();
cv.setBrushColor(Color::Red );
for (int i = 0; i < 16; i++) {
if (h[i] != 0) {
//cv.fillRectangle(cv.getWidth()*i / 16, 0, cv.getWidth() * (i + 1) / 16, cv.getHeight() *h[i] / 16);
cv.fillRectangle(Width*i / 16, Height -1 , Width * (i + 1) / 16-1, (Height-1) *(16-h[i]) / 16);
}
Serial.print(h[i], HEX);
Serial.print("");
}
Serial.println("");
Elsp1 = millis();
}
if (esp_timer_get_time() - stime > 1000000) {
// calculate FPS
FPS = FPSCounter;
stime = esp_timer_get_time();
FPSCounter = 0;
}
++FPSCounter;
// display test state and FPS
cv.setPenColor(Color::Blue);
cv.setBrushColor(Color::Yellow);
cv.drawTextFmt(80, 5, "%d FPS ", FPS);
}
参考：
- https://mc.dfrobot.com.cn/thread-314320-1-1.html
- https://mc.dfrobot.com.cn/thread-311156-1-1.html?fromuid=70205
本文提到的电路图下载：
本文提到的完整代码：