# Analog Devices Wiki

This version (28 Feb 2019 18:05) was approved by Chad Wentworth.The Previously approved version (14 Nov 2018 22:30) is available. ## Tutorial: Implementing a Ring Modulator Effect

### Bare Metal Project Wizard Setup

Using the bare metal project wizard:

1. Give the project a meaningful name, click Next
2. Choose the Audio Project Fin on the Expansion Fin Selection Page because it is required for part of the tutorial, click Finish

No other options need to be changed.

### Tutorial Overview

A ring modulator is a wild audio effect that generates some very interesting sounds. In this effect, we multiply the incoming audio signal using an oscillator. Here's a nice explanation of how the ring modular effect works and its various parameters.

In this tutorial, we're going to start by building a basic ring modulator effect. We'll then add a few additional bells and whistles. Before you begin this tutorial, it's recommended to quickly go through Tutorial: Tremolo Effect.

Just like last time, we'll be working in the `callback_audio_processing.cpp` file. We can implement this code either on SHARC core 1 or SHARC core 2.

### Basic Ring Modulator with Fixed Parameters

Just like in the tremelo tutorial, we'll start with some fixed parameters. `ringmod_rate` will set the frequency of the sine wave we're using to modulate the signal. `ringmod_blend` will set the mix of clean and modulated audio.

```void processaudio_callback( ) {

static float ringmod_rate = 0.5;  // the rate of our ring modulator (0.0 to 1.0)
static float ringmod_blend = 0.5; // the blend or mix of our tremelo (0.0 to 1.0)
static float t = 0.0;     // current value of t for sine calculations

// Otherwise, perform our C-based block processing here!
for (int i=0;i<AUDIO_BLOCK_SIZE;i++) {

// Calculate our modulation factor for this sample
float ring_factor = sinf(t);

// Update t based on rate and a scalar to map into a nice range
t += (ringmod_rate * 0.02);

// Wrap t if necessary
if (t > 6.28318531) t -= 6.28318531;

// Multiply each incoming sample by our amplitude modulation value for this sample
audiochannel_0_left_out[i]  = (1.0-ringmod_blend)*audiochannel_0_left_in[i] + ringmod_blend*ring_factor*audiochannel_0_left_in[i];
audiochannel_0_right_out[i] = (1.0-ringmod_blend)*audiochannel_0_right_in[i] + ringmod_blend*ring_factor*audiochannel_0_right_in[i];
}
}```

### Controlling the Ring Modulator Parameters with Pots on the Audio Project Fin

Now we will map the HADC0 pot to our rate parameter and the HADC1 pot to our blend / mix parameter. In the case, we're mapping HADC0 to a set of values between 2.0 and 6.0 which gives us a nice wide range of frequencies.

```void processaudio_callback( ) {

float ringmod_rate  = 2.0 + 4.0*multicore_data->audioproj_fin_pot_hadc0; // frequency scaler of our ring modulator (2 to 6.0)
float ringmod_blend = multicore_data->audioproj_fin_pot_hadc1; // the blend or mix of our tremelo (0.0 to 1.0)
static float t = 0.0;     // current value of t for sine calculations

// Perform our C-based block processing here!
for (int i=0;i<AUDIO_BLOCK_SIZE;i++) {

// Calculate our modulation factor for this sample
float ring_factor = sinf(t);

// Update t based on rate and a scalar to map into a nice range
t += (ringmod_rate * 0.02);

// Wrap t if necessary
if (t > 6.28318531) t -= 6.28318531;

// Multiply each incoming sample by our amplitude modulation value for this sample
audiochannel_0_left_out[i]  = (1.0-ringmod_blend)*audiochannel_0_left_in[i] + ringmod_blend*ring_factor*audiochannel_0_left_in[i];
audiochannel_0_right_out[i] = (1.0-ringmod_blend)*audiochannel_0_right_in[i] + ringmod_blend*ring_factor*audiochannel_0_right_in[i];
}
}```

### Using Different Waveforms to Modulate the Signal (Selectable with SW2)

Some ring modulator pedals allow the user to select between different waveforms to use to modulate the original signal. In this version, we'll use SW2 to select between a sine wave, a triangle wave and a square wave. And SW1 will be used to toggle the effect on and off.

```float square_wave( float t ) {

float x = sinf(t);
if (x >= 0.0) return 1.0;
else return -1.0;
}

float triangle_wave( float t ) {

return asinf(cosf(t))/1.57079633;

}

#pragma optimize_for_speed
void processaudio_callback( ) {

float ringmod_rate  = 2.0 + 4.0*multicore_data->audioproj_fin_pot_hadc0; // frequency scaler of our ring modulator (2 to 6.0)
float ringmod_blend = multicore_data->audioproj_fin_pot_hadc1; // the blend or mix of our tremelo (0.0 to 1.0)
static float t = 0.0;     // current value of t for sine calculations

static uint8_t current_modulator = 0;
static bool ringmod_enabled = false;

// Perform our C-based block processing here!
for (int i=0;i<AUDIO_BLOCK_SIZE;i++) {

// Calculate our modulation factor for this sample depending on which modulator is selected
float ring_factor;
if (current_modulator == 0) {
ring_factor = sinf(t);
}
else if (current_modulator == 1) {
ring_factor = triangle_wave(t);
}
else if (current_modulator == 2) {
ring_factor = square_wave(t);
}
// Update t based on rate and a scalar to map into a nice range
t += (ringmod_rate * 0.02);

// Wrap t if necessary
if (t > 6.28318531) t -= 6.28318531;

// If ringmod enabled, apply the effect
if (ringmod_enabled) {
// Multiply each incoming sample by our amplitude modulation value for this sample
audiochannel_0_left_out[i]  = (1.0-ringmod_blend)*audiochannel_0_left_in[i] + ringmod_blend*ring_factor*audiochannel_0_left_in[i];
audiochannel_0_right_out[i] = (1.0-ringmod_blend)*audiochannel_0_right_in[i] + ringmod_blend*ring_factor*audiochannel_0_right_in[i];

// Otherwise, just pass the audio through
} else {
audiochannel_0_left_out[i]  = audiochannel_0_left_in[i];
audiochannel_0_right_out[i] = audiochannel_0_right_in[i];
}
}

// Use SW1 / PB_1 to toggle the effect
if (multicore_data->audioproj_fin_sw_1_core1_pressed) {
multicore_data->audioproj_fin_sw_1_core1_pressed = false;
ringmod_enabled = !ringmod_enabled;
}

// Use SW2 / PB_2 to cycle through the modulating waveforms
if (multicore_data->audioproj_fin_sw_2_core1_pressed) {
multicore_data->audioproj_fin_sw_2_core1_pressed = false;
current_modulator += 1;

if (current_modulator >= 3) {
current_modulator = 0;
}
}
}```

When we press SW2, the LED next to the push button toggles. To disable this behavior, open `Callback_Pushbuttons.cpp` in the `<PROJECT_NAME>_core0` project. Locate the callback for PB2 (SW2) on the Audio Project Fin and comment out the code to toggle the LED:

```// Call back for PB2 on SHARC Audio Module Audio Project Fin
void pushbutton_callback_external_2( void  * data_object ) {

/*
static bool sw2_state = false;

// Toggle the LED below the button to indicate state

sw2_state = !sw2_state;
if (sw2_state) {
gpio_write(GPIO_AUDIOPROJ_FIN_LED_SW2, GPIO_HIGH);
} else {
gpio_write(GPIO_AUDIOPROJ_FIN_LED_SW2, GPIO_LOW);
}
*/
// Update our multicore structure to let the SHARCs know that a PB has been pressed
multicore_data->audioproj_fin_sw_2_core1_pressed = true;
multicore_data->audioproj_fin_sw_2_core2_pressed = true; 