変声機について

[どうか教えてください！]

コード:

/*voice changer*/
#define F_CPU	16000000
/*external ocilater 16MHz*/

#include <avr/io.h>
#include <avr/delay.h>
#include <inttypes.h>
#include <avr/pgmspace.h>
#include <avr/interrupt.h>

#define BUFSIZE 768
//macro

#define cbi(addr,bit) addr &= ~(1<<bit)
#define sbi(addr,bit) addr |=  (1<<bit)

int8_t res; //encoder result(-1 - +1)
ISR(TIMER0_OVF_vect)
{
	const int8_t dir[]  PROGMEM = {
		 0,1,-1,0,-1,0,0,1,1,0,0,-1,0,-1,1,0 }; /* direction table */
	static volatile uint8_t i; //index
	i = (i << 2) + ((PINC >>2) & 3);
    i &= 15;
	res = dir[i];
}
void delay_long(uint8_t x)
{
	while(x--){
		_delay_ms(10);
	}
}

void initio(void)
{
//setting ports
	PORTB = 0b00000001;
	DDRB  = 0b00000101;
	PORTD = 0xff;
	PORTC = 0x1c;
//setting timers
	TCCR0 = 0x03;
	TIMSK = _BV(TOIE0); //intrrupt interval 1.024ms
	TCCR1A = 0b00100001;
	TCCR1B = 0b00001001;
//setting AD-converter
	ADMUX = 0b01100000; //ADC0 AVcc
	ADCSR = 0b11100101;
}

main()
{
	uint8_t rate = 1;
	uint8_t frac_rate = 0; //0-127,fraction of rate
	uint8_t buf[BUFSIZE];
	uint8_t *wp = buf;
	uint8_t *rp = buf;
	uint8_t frac_rp = 0; //0-127,fraction of rate
	uint8_t *tmp_p;
	int16_t tmp;
	
	initio();
	sei();
	delay_long(50);
	
	for(;;){
	//update rate
		tmp = frac_rate + res;
		res = 0;
		if(tmp < 0){
			rate--;
			frac_rate = 128 + tmp;
		}else if(tmp > 127){
			rate++;
			frac_rate = tmp - 128;
		}else{
			frac_rate = tmp;
		}
		
	//regulate rate (0.33<rate<3)
		if(rate >=3){
			rate = 3;
			frac_rate = 0;
		}if(rate == 0 && frac_rate < 43){
			rate = 0;
			frac_rate = 43;
		}
	//output and input analog value
		loop_until_bit_is_set(ADCSR,ADIF);
		sbi(ADCSR,ADIF);
		*wp = ADCH;
		
		tmp_p = rp;
		if(++tmp_p >= buf+BUFSIZE)
			tmp_p = buf;
		OCR1BL = (*tmp_p*frac_rp + *rp*(127 - frac_rp))>>7;
		
	//update pointers
		if(++wp == buf+BUFSIZE)
			wp = buf;
		
		frac_rp += frac_rate;
		if(frac_rp >= 128){
			rp++;
			frac_rp -= 128;
		}
		rp += rate;
		if(rp >= buf+BUFSIZE)
			rp -= BUFSIZE;
	}
}

解読お願いします！
どう動くのかさっぱりわからないです。