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mbed Audio Phasing

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Extras:
mbed Microcontroller
Audio Phasing

Non-Exam Extras

This hardware and code implements an audio phasing sound effect.

There are some spurious sounds.

These might be removed by avoiding the use of breadboard and using techniques like single point earthing.

This sound effect works by adding the input signal to a very slightly delayed copy of the input. Depending on the delay time, certain frequencies are enhanced while others subtract and disappear from the output. Adjusting the delay, sweeps these frequencies up or down creating the phasing effect. Music studios almost certainly combine this effect with swept band-pass filtering to make the effect more dramatic.

#include "mbed.h"
 
#define MAX_QUEUE 512          // Indexed from 0 to 511
 
AnalogOut speaker(p18);        // DAC for the speaker/output
AnalogIn  mic(p19);            // ADC for the microphone/input
AnalogIn  pot(p20);            // ADC for the pot

int main()
{
    int queue[MAX_QUEUE];      // Will be a circular buffer

    int ii = 0;                // Queue write position
    int jj = 0;                // Q read position
    int offset = 0;            // Delay (number of samples)
    int oo;                    // The analogue data read or written
    
    while(1)                   // For ever
    {
        oo = mic.read_u16() - 32767;    // Read ADC and remove DC offset
      
        queue[ii] = oo;                 // Add sample to queue
        
        jj = ii - offset;               // Calculate playback position
        
        if (jj < 0) jj = jj + MAX_QUEUE;// If queue pointer is negative, wrap it around.
        
        ii = (ii + 1) % MAX_QUEUE;      // Next position in the circular queue
        
        if (ii == 0)                    // Read the potentiometer
        {
            offset = MAX_QUEUE * pot.read_u16() / 65535;  // Scale the reading to fit the queue
            
            if (offset >= MAX_QUEUE) offset = MAX_QUEUE - 1;  // Limit the upper value
        }

        oo = oo + queue[jj];            // Add zero referenced signals: input + delayed_input

        oo = oo + 32767;                // Restore the DC Offset.
        
        speaker.write_u16(oo);          // DAC output
        
        // FOR DEBUGGING if (!(ii % 16)) printf("oo = %5d | ii = %5d | jj = %5d | offset = %5d \n\r", oo, ii, jj, offset);
    }
}