Mbed Guitar Tuner Code Problems? - c++

Question

I'm trying to tune a guitar using the mbed LPC microcontrolller.
There is a link below for an example of the project.
https://developer.mbed.org/users/adurand/notebook/guitar-tuner/
However, I am having some problems with it. Firstly, I am simulating the guitar input using an arbitrary waveform generator at the required frequency of each string. The amplifier circuit is constructed the same. I have also changed the code to utilise the Mbed LCD instead of the Nokia. I have used an oscilloscope to validate that the output going into AnalogueIn p20 of the Mbed is amplified and at the correct frequency. This is where I have problems.
Here is the code (with credit to Andrew Durand):
#include "mbed.h"
#include "adc.h"
#include "C12832.h"
#include <math.h>
#define PI 3.1415
#define SAMPLE_RATE 24000
InterruptIn button1(p12);
C12832 lcd(p5, p7, p6, p8, p11);
DigitalOut myled1(LED1);
DigitalOut myled2(LED2);
DigitalOut myled3(LED3);
//LCD and Other Random Variables
int string_select = 0;
float high, high1, in_tune, in_tune1, in_tune2, in_tune3,
low, low1, note, low_mod, high_mod;
char* key;
int Counter = 0;
int Buffer[6000];
float goertzelFilter(int samples[], float freq, int N) {
float s_prev = 0.0;
float s_prev2 = 0.0;
float coeff,normalizedfreq,power,s;
int i;
normalizedfreq = freq / SAMPLE_RATE;
coeff = 2*cos(2*PI*normalizedfreq);
for (i=0; i<N; i++) {
s = samples[i] + coeff * s_prev - s_prev2;
s_prev2 = s_prev;
s_prev = s;
}
power = s_prev2*s_prev2+s_prev*s_prev-coeff*s_prev*s_prev2;
return power;
}
ADC adc(SAMPLE_RATE, 1);
void sample_audio(int chan, uint32_t value) {
Buffer[Counter] = adc.read(p20);
Counter += 1;
}
void button1_pressed() {
string_select++;
if (string_select > 5) string_select = 0;
}
int main() {
//Interupt for Switching Strings
button1.mode(PullDown);
button1.rise(&button1_pressed);
while (1) {
switch (string_select) {
case 0:
note = 82;
key= "E2";
break;
case 1:
note = 110;
key= "A2";
break;
case 2:
note = 147;
key= "D3";
break;
case 3:
note = 196;
key= "G3";
break;
case 4:
note = 247;
key= "B3";
break;
case 5:
note = 330;
key= "E4";
break;
}
//Prepare for burst mode on all ADC pins and set up interrupt handler (using ADC library from Simon Blandford
adc.append(sample_audio);
adc.startmode(0,0);
adc.burst(1);
adc.setup(p20,1);
//start the interrupt and wait for about 4096 samples
adc.interrupt_state(p20,1);
wait(.2);
//Finsh up - Unset pin 20
adc.interrupt_state(p20,0);
adc.setup(p20,0);
int actual_rate = adc.actual_sample_rate();
//for debugging tell the terminal sample rate and how many samples we took
printf("Requested max sample rate is %u, actual max sample rate is %u.\n",
SAMPLE_RATE, actual_rate);
printf("We did %i samples\n",Counter);
high = 0;
low = 0;
for (int i=3; i<46; i+=3) {
high1 = goertzelFilter(Buffer, (note + i ), Counter);
if (high1 > high) high=high1;
}
for (int i=3; i<46; i+=3) {
low1 = goertzelFilter(Buffer, (note - i ), Counter);
if (low1 > low) low=low1;
}
in_tune1 = goertzelFilter(Buffer, (note+1), Counter);
in_tune2 = goertzelFilter(Buffer, note, Counter);
in_tune3 = goertzelFilter(Buffer, (note-1), Counter);
if ((in_tune1 > in_tune2) && (in_tune1 > in_tune3)) in_tune = in_tune1;
else if ((in_tune2 > in_tune1) && (in_tune2 > in_tune3)) in_tune = in_tune2;
else in_tune = in_tune3;
printf("high = %.2f, low = %.2f, in_tune = %.2f", high, low, in_tune);
high_mod = high/in_tune;
low_mod = low/in_tune;
if ((high_mod > .8)&&(low_mod > .8)) {
myled1 = 0;
myled2 = 0;
myled3 = 0;
}
if ((high_mod < .8)&&(low_mod > .8)) {
myled1 = 0;
myled2 = 0;
myled3 = 0;
}
if ((high > in_tune) && (low < in_tune)) { //Guitar string is at correct frequency
myled1 = 0;
myled2 = 1;
myled3 = 0;
} else if (high > in_tune) { // String is higher than the desired frequency
myled1 = 1;
myled2 = 0;
myled3 = 0;
} else if (low < in_tune){ // String is below that of the desired frequency
myled1 = 0;
myled2 = 0;
myled3 = 1;
} else { // Else no input, display squiggles
myled1 = 0;
myled2 = 0;
myled3 = 0;
}
// Display on the LCD
lcd.cls();
lcd.locate(0,0);
lcd.printf("Tuning String: %i", (6-string_select));
lcd.locate(0,11);
lcd.printf("%s at %i Hz",key, (int) note);
lcd.locate(0,23);
if (myled2) lcd.printf("In Tune!"); // if myled2 is on, guitar is in tune
else if (myled3) lcd.printf("Too Low "); // if myled3 is on, guitar is lower than desired tone
else if (myled1) lcd.printf("Too High"); // if myled1 is on, guitar is higher than desired tone
else lcd.printf("No Input Detected"); // corresponds to led case 4 - no guitar input present
Counter = 0;
}
}
Now, when I compile the program, the messages print. However, the LEDs alternate between "Too high", "Too low", and to the squiggles. I'll be the first to admit I'm not the best coder, something I am really going to work on over the Summer. There could be a very significant problem with the code, or it may be something relatively simple. All inputs are appreciated.
Note: I have not connected the debounce circuit on the breadboard - it is connected to the Nokia LCD which I did not use. I am able to change the desired string, between 1-6, using the joystick on the mbed application board instead. Is that okay? Or is the debounce circuit of pivotal importance?
Thanks again.

Answer 1

The loop which finds low is (almost) the same as the loop which finds high in this code:
high = 0;
low = 0;
for (int i=3; i<46; i+=3) {
high1 = goertzelFilter(Buffer, (note + i ), Counter);
if (high1 > high) high=high1;
}
for (int i=3; i<46; i+=3) {
low1 = goertzelFilter(Buffer, (note - i ), Counter);
if (low1 > low) low=low1;
}
I suggest the low part should be like this
low = FLT_MAX;
for (int i=3; i<46; i+=3) {
low1 = goertzelFilter(Buffer, (note - i ), Counter);
if (low1 < low) low=low1;
}
...although I could be mistaken at the intended use of low.