PIC32: How to use DSP Library?

Question

I am using a Microstick II Board with the included PIC32MX250F128B and I would like to perform real-time audio processing. I use an analog input to get the input sound and a PWM output, in combination with a low-pass filter followed by an amplifier, as my audio output (connected to headphones).

I would like to do is to perform more advanced signal processing, like filtering, etc. Microchip website says there is a DSP Library dedicated to that. The lib manual can be found here, but there isn't a lot of information in it, just some function prototypes...

In a first time, I would like to perform low-pass and bandpass filtering. Until now, I managed to perform fft using the following code:

// include header files
#include <plib.h>
#include <p32xxxx.h>
#include <dsplib_dsp.h>
#include <fftc.h>

// Config Bits
#pragma config FNOSC = FRCPLL       // Internal Fast RC oscillator (8 MHz) w/ PLL
#pragma config FPLLIDIV = DIV_2     // Divide FRC before PLL (now 4 MHz)
#pragma config FPLLMUL = MUL_20     // PLL Multiply (now 80 MHz)
#pragma config FPLLODIV = DIV_2     // Divide After PLL (now 40 MHz)
#pragma config FWDTEN = OFF         // Watchdog Timer Disabled
#pragma config ICESEL = ICS_PGx1    // ICE/ICD Comm Channel Select (pins 4,5)
#pragma config JTAGEN = OFF         // Disable JTAG
#pragma config FSOSCEN = OFF        // Disable Secondary Oscillator

// Defines

#define fftc fft16c64 // from fftc.h, for N = 64

#define SYSCLK (40000000L)

#define SAMPLES 64          

#define PWM_FREQ   48000            // Output PWM frequency
#define DUTY_CYCLE  1               

int tab[SAMPLES];
int i = 0;

int analogRead(char analogPIN)
{
    AD1CHS = analogPIN << 16;       // AD1CHS<16:19> controls which analog pin goes to the ADC

    AD1CON1bits.SAMP = 1;           // Sampling
    while(AD1CON1bits.SAMP);        // wait until acquisition is done
    while(!AD1CON1bits.DONE);       // wait until conversion done

    return ADC1BUF0;                
}

void adcConfigureManual()
{
    AD1CON1CLR = 0x8000;    // disable ADC before configuration

    AD1CON1 = 0x00E0;       // internal counter ends sampling and starts conversion (manual sample)
    AD1CON2 = 0;            // AD1CON2<15:13> set voltage reference to pins AVSS/AVDD

    // Found on the web (todo: check the datasheet)
    AD1CON3 = 0x0f01;       // TAD = 4*TPB, acquisition time = 15*TAD
} 

int main( void)
{
    SYSTEMConfigPerformance(SYSCLK);

    // Set OC1 to pin 2 with peripheral pin select
    RPA0Rbits.RPA0R = 0x0005;

    // Configure standard PWM mode for output compare module 1 
    OC1CON = 0x0006;

    for(i = 0; i<SAMPLES; i++)
        tab[i] = 0;

    // From datasheet: 
    // PR = [FPB / (PWM Frequency * TMR Prescale Value)] – 1
    PR2 = (SYSCLK / PWM_FREQ) - 1;

    // Initial duty cycle value
    OC1RS = (PR2 + 1) * ((float)DUTY_CYCLE / 100);

    T2CONSET = 0x8000;      // Enable Timer2, prescaler 1:1
    OC1CONSET = 0x8000;     // Enable Output Compare Module 1

        // Configure pins as analog inputs
        ANSELBbits.ANSB3 = 1;   // set RB3 (AN5) to analog
        TRISBbits.TRISB3 = 1;   // set RB3 as an input
        TRISBbits.TRISB5 = 0;   // set RB5 as an output (note RB5 is a digital only pin)

        adcConfigureManual();   // Configure ADC
        AD1CON1SET = 0x8000;    // Enable ADC

        int pos=0, dat = 0;

        int log2N = 6; // log2(64) = 6
        int N = 1 << log2N; // N = 2^6 = 64
        int din[N];
        int dout[N];
        int dout2[N];
        int scratch[N];

    while(1)
    {
            //foo = analogRead 5); // note that we call pin AN5 (RB3) by it's analog number
            dat = analogRead(5);

            //dat += din[pos]*3/10;
            //if(dat > 1023) dat -= 512;

            din[pos] =  dat;

            mips_fft16(dout, din, fftc, scratch, 1);
            mips_fft16(dout2, dout, fftc, scratch, 1);


            if(++pos >= SAMPLES) pos = 0;

            OC1RS = (PR2 + 1) * ( ((float)dout2[pos])/1023); // Write new duty cycle
    }

    return 0;
}

First of all, I would like to know if what I am doing is correct. I mean, there is no ifft function, does the second fft perform an ifft ? Secondly, by doing this, I can hear a high frequency sound, which may result from latency that delays the PWM duty cycle change OC1RS. How can I fix this ? I saw people usually use Timer2 to change the duty cycle, but each time I tried, I get nothing at the output (no sound). How can I implement this in my case ?

Answer 1

I find this best learned using the DCMI tool and simulation, although mipsfir doesn't sim for some reason.

mips_fft16/32 does fft and ifft. Its all the same, really. Transfers input from one domain to the other.

Your implementation has flaws. The output of the mipsfft is a complex number, I am surprised that even compiled. Do a study on complex numbers. I don't see your fftc define, but a size of 1 doesn't make sense in the mips_fft16 call. Its really pretty simple. If you want to create a sine wave in a sample buffer of 32, you put the (amplitute*32)value in din[1].re. din[2].re for a double sine, and so on. So for a sample rate of 16000, 16000/32 = 500 * 1 = 500hz. Note that your output is in dout[x].re. So a call to fft/ifft will look more like this for a 32 sized fft. mips_fft16(dout, din, fftc16c32, scratch, 6);

Also, remember that the output of an fft is going to be real and imaginary, so if you just want peaks don't forget that A squared + B squared = C squared.

Real and imaginary = phase of output, greatly simplified.

OC1RS = (PR2 + 1) * ( ((float)dout2[pos])/1023); // Write new duty cycle

This isn't the proper way to do this at all. The OC1RS update can be anywhere, but I suggest putting in inside a timer at some chosen sample rate, say 16000. Then feed OC1RS with an incrementing data pointer of some sort to walk through your output.