diff --git a/src/algo.c b/src/algo.c
index 1c744cb..fa52137 100644
--- a/src/algo.c
+++ b/src/algo.c
@@ -452,7 +452,7 @@ static int compute_period(struct processing_buffers *b, int bph)
 	if(count > 1)
 		b->sigma = sqrt((sq_sum - count * estimate * estimate)/ (count-1));
 	else
-		b->sigma = b->period;
+		b->sigma = 0;	// No std. dev. estimate possible with just 1 sample
 	return 0;
 }
 
diff --git a/src/audio.c b/src/audio.c
index 05574fe..d8a86db 100644
--- a/src/audio.c
+++ b/src/audio.c
@@ -164,7 +164,7 @@ uint64_t get_timestamp(int light)
 	return ts;
 }
 
-static void fill_buffers(struct processing_buffers *ps, int light)
+void fill_buffers(struct processing_buffers *ps, int light)
 {
 	pthread_mutex_lock(&audio_mutex);
 	uint64_t ts = timestamp;
@@ -187,26 +187,17 @@ static void fill_buffers(struct processing_buffers *ps, int light)
 	}
 }
 
-int analyze_pa_data(struct processing_data *pd, int bph, double la, uint64_t events_from)
+/* Returns if buffer was processed ok */
+bool analyze_pa_data(struct processing_data *pd, int step, int bph, double la, uint64_t events_from)
 {
-	struct processing_buffers *p = pd->buffers;
-	fill_buffers(p, pd->is_light);
+	struct processing_buffers *p = &pd->buffers[step];
 
-	int i;
-	debug("\nSTART OF COMPUTATION CYCLE\n\n");
-	for(i=0; i<NSTEPS; i++) {
-		p[i].last_tic = pd->last_tic;
-		p[i].events_from = events_from;
-		process(&p[i], bph, la, pd->is_light);
-		if( !p[i].ready ) break;
-		debug("step %d : %f +- %f\n",i,p[i].period/p[i].sample_rate,p[i].sigma/p[i].sample_rate);
-	}
-	if(i) {
-		pd->last_tic = p[i-1].last_tic;
-		debug("%f +- %f\n",p[i-1].period/p[i-1].sample_rate,p[i-1].sigma/p[i-1].sample_rate);
-	} else
-		debug("---\n");
-	return i;
+	p->last_tic = pd->last_tic;
+	p->events_from = events_from;
+	process(p, bph, la, pd->is_light);
+	debug("step %d : %f +- %f\n", step, p->period/p->sample_rate, p->sigma/p->sample_rate);
+
+	return p->ready;
 }
 
 int analyze_pa_data_cal(struct processing_data *pd, struct calibration_data *cd)
diff --git a/src/computer.c b/src/computer.c
index 03942d3..3f86ea3 100644
--- a/src/computer.c
+++ b/src/computer.c
@@ -91,19 +91,45 @@ static void compute_update_cal(struct computer *c)
 
 static void compute_update(struct computer *c)
 {
-	int signal = analyze_pa_data(c->pdata, c->actv->bph, c->actv->la, c->actv->events_from);
-	struct processing_buffers *p = c->pdata->buffers;
-	int i;
-	for(i=0; i<NSTEPS && p[i].ready; i++);
-	for(i--; i>=0 && p[i].sigma > p[i].period / 10000; i--);
-	if(i>=0) {
+	struct processing_data *pd = c->pdata;
+	struct processing_buffers *ps = pd->buffers;
+	int step = pd->last_step;
+
+	pd->last_step = 0;
+	/* Do all buffers at once so that all computation interval(s) use the
+	 * same data.  Buffers for some intervals will probably not be used, but
+	 * it's not expensive to fill them.  Processing is the slow part.  */
+	fill_buffers(ps, pd->is_light);
+
+	debug("\nSTART OF COMPUTATION CYCLE\n\n");
+	unsigned int stepmask = BITMASK(NSTEPS); // Mask of available steps
+	do {
+		stepmask &= ~BIT(step);
+		analyze_pa_data(c->pdata, step, c->actv->bph, c->actv->la, c->actv->events_from);
+
+		if (ps[step].ready && ps[step].sigma < ps[step].period / 10000) {
+			// Try next step if it's available
+			if (stepmask & BIT(step+1)) step++;
+		} else {
+			// This step didn't pass, try a lesser step
+			step--;
+		}
+	} while(step >= 0 && stepmask & BIT(step));
+
+	if (step >= 0) {
+		debug("%f +- %f\n", ps[step].period/ps[step].sample_rate, ps[step].sigma/ps[step].sample_rate);
+		pd->last_tic = ps[step].last_tic;
+		pd->last_step = step;
+
 		if(c->actv->pb) pb_destroy_clone(c->actv->pb);
-		c->actv->pb = pb_clone(&p[i]);
+		c->actv->pb = pb_clone(&ps[step]);
 		c->actv->is_old = 0;
-		c->actv->signal = i == NSTEPS-1 && p[i].amp < 0 ? signal-1 : signal;
+		/* Signal's range is 0 to NSTEPS, while step is -1 to NSTEPS-1, i.e. signal = step+1 */
+		c->actv->signal = step+1;
 	} else {
+		debug("---\n");
 		c->actv->is_old = 1;
-		c->actv->signal = -signal;
+		c->actv->signal = 0;
 	}
 }
 
@@ -251,6 +277,7 @@ struct computer *start_computer(int nominal_sr, int bph, double la, int cal, int
 	pd->buffers = p;
 	pd->last_tic = 0;
 	pd->is_light = light;
+	pd->last_step = 0;
 
 	struct calibration_data *cd = malloc(sizeof(struct calibration_data));
 	setup_cal_data(cd);
diff --git a/src/output_panel.c b/src/output_panel.c
index 0e7fc8b..835e4f8 100644
--- a/src/output_panel.c
+++ b/src/output_panel.c
@@ -112,11 +112,20 @@ static double amplitude_to_time(double lift_angle, double amp)
 	return asin(lift_angle / (2 * amp)) / M_PI;
 }
 
-static double draw_watch_icon(cairo_t *c, int signal, int happy, int light)
+/** Draw the watch graphic that has status info.
+ *
+ * @param[in,out] c Cairo context to use.
+ * @param signal Signal level, i.e. dots, 0 to NSTEPS inclusive.
+ * @param partial Specified signal level is only partially achieved.
+ * @param happy Green happy face or red frowny face.
+ * @param light Indicate light sampling mode.
+ * @return Y coodinate of top margin.
+ */
+
+static double draw_watch_icon(cairo_t *c, int signal, bool partial, bool happy, bool light)
 {
-	happy = !!happy;
-	cairo_set_line_width(c,3);
-	cairo_set_source(c,happy?green:red);
+	cairo_set_line_width(c, 3);
+	cairo_set_source(c, happy ? green : red);
 	cairo_move_to(c, OUTPUT_WINDOW_HEIGHT * 0.5, OUTPUT_WINDOW_HEIGHT * 0.5);
 	cairo_line_to(c, OUTPUT_WINDOW_HEIGHT * 0.75, OUTPUT_WINDOW_HEIGHT * (0.75 - 0.5*happy));
 	cairo_move_to(c, OUTPUT_WINDOW_HEIGHT * 0.5, OUTPUT_WINDOW_HEIGHT * 0.5);
@@ -126,7 +135,7 @@ static double draw_watch_icon(cairo_t *c, int signal, int happy, int light)
 	cairo_stroke(c);
 	int l = OUTPUT_WINDOW_HEIGHT * 0.8 / (2*NSTEPS - 1);
 	int i;
-	cairo_set_line_width(c,1);
+	cairo_set_line_width(c, 1);
 	for(i = 0; i < signal; i++) {
 		cairo_move_to(c, OUTPUT_WINDOW_HEIGHT + 0.5*l, OUTPUT_WINDOW_HEIGHT * 0.9 - 2*i*l);
 		cairo_line_to(c, OUTPUT_WINDOW_HEIGHT + 1.5*l, OUTPUT_WINDOW_HEIGHT * 0.9 - 2*i*l);
@@ -134,7 +143,7 @@ static double draw_watch_icon(cairo_t *c, int signal, int happy, int light)
 		cairo_line_to(c, OUTPUT_WINDOW_HEIGHT + 0.5*l, OUTPUT_WINDOW_HEIGHT * 0.9 - (2*i+1)*l);
 		cairo_line_to(c, OUTPUT_WINDOW_HEIGHT + 0.5*l, OUTPUT_WINDOW_HEIGHT * 0.9 - 2*i*l);
 		cairo_stroke_preserve(c);
-		cairo_fill(c);
+		if (i < signal-1 || !partial) cairo_fill(c);
 	}
 	if(light) {
 		int l = OUTPUT_WINDOW_HEIGHT * 0.15;
@@ -194,7 +203,8 @@ static gboolean output_draw_event(GtkWidget *widget, cairo_t *c, struct output_p
 	struct processing_buffers *p = snst->pb;
 	int old = snst->is_old;
 
-	double x = draw_watch_icon(c,snst->signal,snst->calibrate ? snst->signal==NSTEPS : snst->signal, snst->is_light);
+	double x = draw_watch_icon(c, snst->signal, snst->amp <= 0,
+				   snst->signal >= (snst->calibrate ? NSTEPS : 1), snst->is_light);
 
 	cairo_text_extents_t extents;
 
diff --git a/src/tg.h b/src/tg.h
index 789f66c..04f3e11 100644
--- a/src/tg.h
+++ b/src/tg.h
@@ -75,6 +75,8 @@
 #endif
 
 #define UNUSED(X) (void)(X)
+#define BIT(n) (1u << (n))
+#define BITMASK(n) ((1u << (n)) - 1u)
 
 /* algo.c */
 struct processing_buffers {
@@ -122,13 +124,15 @@ int process_cal(struct processing_buffers *p, struct calibration_data *cd);
 struct processing_data {
 	struct processing_buffers *buffers;
 	uint64_t last_tic;
+	int last_step;	//!< Guess of step (buffers index) to try first, based on last iteration
 	int is_light;
 };
 
 int start_portaudio(int *nominal_sample_rate, double *real_sample_rate);
 int terminate_portaudio();
 uint64_t get_timestamp(int light);
-int analyze_pa_data(struct processing_data *pd, int bph, double la, uint64_t events_from);
+void fill_buffers(struct processing_buffers *ps, int light);
+bool analyze_pa_data(struct processing_data *pd, int step, int bph, double la, uint64_t events_from);
 int analyze_pa_data_cal(struct processing_data *pd, struct calibration_data *cd);
 void set_audio_light(bool light);