diff -urN midi_prog-/buf.c midi_prog/buf.c
--- midi_prog-/buf.c	Wed Sep  9 00:00:00 2015
+++ midi_prog/buf.c	Fri Sep 11 00:00:00 2015
@@ -242,6 +242,17 @@
 	rbuf_set_dbl(&tb->rb, (int)sridx, v);
 }
 
+int
+tbuf_from_to_num(struct tbuf *tb, double from_sec, double to_sec, int *from_sridx, int *to_sridx)
+{
+	int from_i = (int) tbuf_sec_to_sridx(tb, from_sec);
+	int to_i = (int) tbuf_sec_to_sridx(tb, to_sec);
+	int n = to_i - from_i;
+	if(from_sridx) *from_sridx = from_i;
+	if(to_sridx) *to_sridx = to_i;
+	return MAX(n, 0);
+}
+
 
 /* sbuf */
 
@@ -292,6 +303,7 @@
 	tbuf_init(&ab->tb, smp_freq, wsec, is_complex);
 	ab->max_v = 0;
 	ab->sum = 0;
+	ab->type = 0;
 }
 
 void
@@ -307,17 +319,30 @@
 }
 
 double
-abuf_mean(struct abuf *ab, double from_sec, double to_sec)
+abuf_get(struct abuf *ab, double sec)
+{
+	return tbuf_get_dbl(&ab->tb, sec);
+}
+
+double
+abuf_sum(struct abuf *ab, double from_sec, double to_sec, int *ret_n)
 {
 	struct tbuf *tb = &ab->tb;
 	struct rbuf *rb = &tb->rb;
-	int from_i = (int) tbuf_sec_to_sridx(tb, from_sec);
-	int to_i = (int) tbuf_sec_to_sridx(tb, to_sec);
-	int i;
+	int i, from_i, to_i, n = tbuf_from_to_num(tb, from_sec, to_sec, &from_i, &to_i);
 	double sum = 0;
-	if(from_i >= to_i) return 0;
+	if(ret_n) *ret_n = n;
 	for(i=from_i; i<to_i; i++) sum += rbuf_get_dbl(rb, i);
-	return sum / (to_i - from_i);
+	return sum;
+}
+
+double
+abuf_mean(struct abuf *ab, double from_sec, double to_sec, int *ret_n)
+{
+	int n;
+	double sum = abuf_sum(ab, from_sec, to_sec, &n);
+	if(ret_n) *ret_n = n;
+	return n > 0 ? sum / n : 0;
 }
 
 double
@@ -325,11 +350,12 @@
 {
 	struct tbuf *tb = &ab->tb;
 	struct rbuf *rb = &tb->rb;
-	int from_i = (int) tbuf_sec_to_sridx(tb, from_sec);
-	int to_i = (int) tbuf_sec_to_sridx(tb, to_sec);
-	int i = from_i, t = i;
-	double v, tv = rbuf_get_dbl(rb, i);
+	int from_i, to_i, i, t;
+	double v, tv;
 
+	tbuf_from_to_num(tb, from_sec, to_sec, &from_i, &to_i);
+	t = i = from_i;
+	tv = rbuf_get_dbl(rb, i);
 	for(i++; i<to_i; i++){
 		if((v = rbuf_get_dbl(rb, i)) > tv){
 			tv = v;
@@ -338,4 +364,123 @@
 	}
 	if(ret_sec) *ret_sec = tbuf_sridx_to_sec(tb, t);
 	return tv;
+}
+
+
+/* a2buf */
+
+void
+a2buf_init(struct a2buf *a2b, struct abuf *a, struct abuf *b,
+	   double A2, double B2, double AB, double A, double B, double C)
+{
+	struct abuf *c = a ? a : b;
+
+	a2b->a = a;
+	a2b->b = b;
+	a2b->A2 = A2;
+	a2b->B2 = B2;
+	a2b->AB = AB;
+	a2b->A = A;
+	a2b->B = B;
+	a2b->C = C;
+
+	a2b->ab.type = 1;
+
+	a2b->tb = c->type == 0 ? &c->tb : ((struct a2buf *)c)->tb;
+}
+
+static double
+a2buf_abuf_get(struct abuf *ab, double sec)
+{
+	return ab->type == 0 ? abuf_get(ab, sec) : a2buf_get( (struct a2buf *)ab, sec);
+}
+
+double
+a2buf_get(struct a2buf *a2b, double sec)
+{
+	double v_a = a2b->a ? a2buf_abuf_get(a2b->a, sec) : 0;
+	double v_b = a2b->b ? a2buf_abuf_get(a2b->b, sec) : 0;
+	double sum = a2b->C;
+
+	if(a2b->A != 0 && v_a != 0) sum += a2b->A * v_a;
+	if(a2b->B != 0 && v_b != 0) sum += a2b->B * v_b;
+
+	if(a2b->AB != 0 && v_a != 0 && v_b != 0) sum += a2b->AB * v_a * v_b;
+
+	if(a2b->A2 != 0 && v_a != 0) sum += a2b->A2 * v_a * v_a;
+	if(a2b->B2 != 0 && v_b != 0) sum += a2b->B2 * v_b * v_b;
+
+	return sum;
+}
+
+double
+a2buf_sum(struct a2buf *a2b, double from_sec, double to_sec, int *ret_n)
+{
+	struct tbuf *tb = a2b->tb;
+	int i, from_i, to_i, n = tbuf_from_to_num(tb, from_sec, to_sec, &from_i, &to_i);
+	double sum = 0;
+	if(ret_n) *ret_n = n;
+	for(i=from_i; i<to_i; i++){
+		double sec = tbuf_sridx_to_sec(tb, i);
+		sum += a2buf_get(a2b, sec);
+	}
+	return sum;
+}
+
+double
+a2buf_mean(struct a2buf *a2b, double from_sec, double to_sec, int *ret_n)
+{
+	int n;
+	double sum = a2buf_sum(a2b, from_sec, to_sec, &n);
+	if(ret_n) *ret_n = n;
+	return n > 0 ? sum / n : 0;
+}
+
+double
+a2buf_stdev(struct a2buf *a2b, double from_sec, double to_sec, int *ret_n, double *ret_mean)
+{
+	double mean = a2buf_mean(a2b, from_sec, to_sec, ret_n);
+	struct a2buf t, t2;
+
+	if(ret_mean) *ret_mean = mean;
+	a2buf_init(&t, &a2b->ab, NULL, 0,0,0,1,0,mean);
+	a2buf_init(&t2, &t.ab, NULL, 1,0,0,0,0,0);
+	return sqrt(a2buf_mean(&t2, from_sec, to_sec, NULL));
+}
+
+double
+a2buf_correlation(struct a2buf *a2b_a, struct a2buf *a2b_b, double from_sec, double to_sec, int *ret_n)
+{
+	double mean_a, stdev_a = a2buf_stdev(a2b_a, from_sec, to_sec, ret_n, &mean_a);
+	double mean_b, stdev_b = a2buf_stdev(a2b_b, from_sec, to_sec, NULL, &mean_b);
+	double div = stdev_a * stdev_b;
+	struct a2buf ta, tb, t;
+
+	if(div == 0) return 0;
+
+	a2buf_init(&ta, &a2b_a->ab, NULL, 0,0,0,1,0,mean_a);
+	a2buf_init(&tb, &a2b_b->ab, NULL, 0,0,0,1,0,mean_b);
+	a2buf_init(&t, &ta.ab, &tb.ab, 0,0,1,0,0,0);
+	return a2buf_mean(&t, from_sec, to_sec, NULL) / div;
+}
+
+double
+a2buf_corr_k(struct a2buf *a2b_a, struct a2buf *a2b_b, double from_sec, double to_sec, int *ret_n)
+{
+	double stdev_a = a2buf_stdev(a2b_a, from_sec, to_sec, ret_n, NULL);
+	double stdev_b = a2buf_stdev(a2b_b, from_sec, to_sec, NULL, NULL);
+	double k_r = 0;
+	double k_l = stdev_b / stdev_a * 1.2; // !!!
+	double k, v;
+	struct a2buf t;
+
+	do{
+		k = (k_l + k_r) * 0.5;
+		a2buf_init(&t, &a2b_a->ab, &a2b_b->ab, 0,0,0,-k,1,0);
+		v = a2buf_correlation(a2b_a, &t, from_sec, to_sec, NULL);
+
+		if(v < 0) k_l = k;
+		else k_r = k;
+	}while(ABS(v) > 0.001); // !!!
+	return k;
 }
diff -urN midi_prog-/buf.h midi_prog/buf.h
--- midi_prog-/buf.h	Wed Sep  9 00:00:00 2015
+++ midi_prog/buf.h	Fri Sep 11 00:00:00 2015
@@ -63,6 +63,7 @@
 double complex tbuf_get_cpx(struct tbuf *tb, double sec);
 
 void tbuf_set_dbl(struct tbuf *tb, double sec, double v);
+int tbuf_from_to_num(struct tbuf *tb, double from_sec, double to_sec, int *from_sridx, int *to_sridx);
 
 
 struct sbuf{
@@ -81,11 +82,33 @@
 	struct tbuf tb;
 	double max_v;
 	double sum;
+	int type;
 };
 
 void abuf_init(struct abuf *ab, double smp_freq, double wsec);
 void abuf_add(struct abuf *ab, double v);
-double abuf_mean(struct abuf *ab, double from_sec, double to_sec);
+double abuf_get(struct abuf *ab, double sec);
+double abuf_sum(struct abuf *ab, double from_sec, double to_sec, int *ret_n);
+double abuf_mean(struct abuf *ab, double from_sec, double to_sec, int *ret_n);
 double abuf_max(struct abuf *ab, double from_sec, double to_sec, double *ret_sec);
+
+
+struct a2buf{
+	struct abuf ab, *a, *b;
+	struct tbuf *tb;
+
+	/* A2 * a^2 + B2 * b^2 + AB * a*b + A * a + B * b + C */
+	double A2, B2, AB, A, B, C;
+};
+
+void a2buf_init(struct a2buf *a2b, struct abuf *a, struct abuf *b,
+		double A2, double B2, double AB, double A, double B, double C);
+double a2buf_get(struct a2buf *a2b, double sec);
+
+double a2buf_sum(struct a2buf *a2b, double from_sec, double to_sec, int *ret_n);
+double a2buf_mean(struct a2buf *a2b, double from_sec, double to_sec, int *ret_n);
+double a2buf_stdev(struct a2buf *a2b, double from_sec, double to_sec, int *ret_n, double *ret_mean);
+double a2buf_correlation(struct a2buf *a2b_a, struct a2buf *a2b_b, double from_sec, double to_sec, int *ret_n);
+double a2buf_corr_k(struct a2buf *a2b_a, struct a2buf *a2b_b, double from_sec, double to_sec, int *ret_n);
 
 #endif
diff -urN midi_prog-/pitch.c midi_prog/pitch.c
--- midi_prog-/pitch.c	Thu Sep 10 00:00:00 2015
+++ midi_prog/pitch.c	Fri Sep 11 00:00:00 2015
@@ -82,7 +82,8 @@
 
 	ost->thres = opt_double("-thres", ac, av, 0.03);
 	ost->mean_thres = opt_double("-mean_thres", ac, av, 0.1);
-	ost->mean = -1;
+	ost->sum = -1;
+	ost->n = 0;
 }
 
 int
@@ -104,20 +105,24 @@
 	int onoff = v >= max_v * ost->thres;
 	int chg = chatt_add(&ost->ct, sec, onoff), chg2, chg_v = -1;
 
-	if(ost->mean >= 0){
-		ost->mean = abuf_mean(ost->ab, ost->on_sec, sec);
-		chg2 = chatt_add(&ost->ct2, sec, v >= ost->mean * ost->mean_thres);
+	if(ost->sum >= 0){
+		int onoff2;
+
+		ost->sum += v;
+		ost->n++;
+		onoff2 = v * ost->n >= ost->sum * ost->mean_thres;
+		chg2 = chatt_add(&ost->ct2, sec, onoff2);
 	}
 
 	if(!ost->v){
-		if(ost->mean >= 0 && chg2 && ost->ct2.onoff){
+		if(ost->sum >= 0 && chg2 && ost->ct2.onoff){
 			ost->on_sec = ost->ct2.sec;
 			chg_v = 1;
 		}else if(chg && ost->ct.onoff){
 			ost->on_sec = ost->ct.sec;
 			chg_v = 1;
 
-			ost->mean = abuf_mean(ost->ab, ost->on_sec, sec);
+			ost->sum = abuf_sum(ost->ab, ost->on_sec, sec, &ost->n);
 			chatt_set(&ost->ct2, 1);
 		}
 	}else{
@@ -125,7 +130,7 @@
 			ost->off_sec = ost->ct.sec;
 			chg_v = 0;
 
-			ost->mean = -1;
+			ost->sum = -1;
 		}else if(chg2 && !ost->ct2.onoff){
 			ost->off_sec = ost->ct2.sec;
 			chg_v = 0;
diff -urN midi_prog-/pitch.h midi_prog/pitch.h
--- midi_prog-/pitch.h	Thu Sep 10 00:00:00 2015
+++ midi_prog/pitch.h	Fri Sep 11 00:00:00 2015
@@ -27,7 +27,8 @@
 
 	double thres;
 	double mean_thres;
-	double mean;
+	double sum;
+	int n;
 };
 
 void onoff_stat_init(struct onoff_stat *ost, struct abuf *ab, int ac, char **av);
diff -urN midi_prog-/tool.c midi_prog/tool.c
--- midi_prog-/tool.c	Thu Sep 10 00:00:00 2015
+++ midi_prog/tool.c	Fri Sep 11 00:00:00 2015
@@ -124,7 +124,7 @@
 double
 note_onoff_v(struct note_onoff *nto, double on_sec, double off_sec)
 {
-	return abuf_mean(&nto->ab, on_sec, off_sec);
+	return abuf_mean(&nto->ab, on_sec, off_sec, NULL);
 }
 
 int