/* compute autocorrelations */
	for (i=0;i<=order;i++) {
		sum = 0;
		for (k=0;k<(n-i);k++)
			sum += x[k] * x[k+i];
		r[i] = sum;
	}

	/* compute predictor coefficients */
	if (r[0] == 0) 
		/* no signal ! */
		retcode = 1;
	else {
		*pe = r[0];
		pc[0] = 1.0;
		for (k=1;k<=order;k++) {
			sum = 0;
			for (i=1;i<=k;i++)
				sum -= pc[k-i] * r[i];
			akk = sum/(*pe);
			/* new predictor coefficients */
			pc[k] = akk;
			for (i=1;i<=(k/2);i++) {
				ai = pc[i];
				aj = pc[k-i];
				pc[i] = ai + akk * aj;
				pc[k-i] = aj + akk * ai;
			}
			/* new prediction error */
			*pe = *pe * (1.0 - akk*akk);
			if (*pe <= 0)
				/* negative/zero error ! */
				retcode = 2;
		}
	}