diff --git a/firmware/zoom.c b/firmware/zoom.c
index 4ddd1f3..5199309 100644
--- a/firmware/zoom.c
+++ b/firmware/zoom.c
@@ -17,35 +17,30 @@ uint16_t send_adc, send_dac;
 /* A "step" in the DAC output at the resolution we want. */
 #define DAC_STEP 0x0004
 
-/* 1-bit change at DAC changes the ADC count by this much */
-#define ADC_DAC_SCALING (21.234)
+/* 1 bit at the ADC is this much change at the DAC */
+#define ADC_DAC_SCALING (1.0/21.234)
 
 uint16_t dac_cmd = ((uint32_t)DAC_MAX + DAC_MIN) / 2;
 
 /* 128 KHz, after each ADC sample */
 void fast_callback(void)
 {
-}
+	/* Run at 64 KHz */
+	static int count = 0;
+	if (count++ < 1) return;
+	count = 0;
 
-/* 8 KHz, after DMA buffer filled */
-void slow_callback(void)
-{
 	/* Get most recent sample. */
 	uint16_t v = adc_dmabuf[adc_offset];
 
-	/* Send most recent sample to PC */
-	send_adc = v;
-	send_dac = dac_cmd;
-	send_data = 1;
-
-#define RECENTER
-#ifdef RECENTER
+#if 1
 	/* Update DAC.  Add whatever is necessary to cause the ADC
-	   to recenter at 0x0800 */
+	   to change by 150% towards the center of 0x0800
+       (intentional overshoot to increase slew rate capability) */
 	{
-		int32_t adjustment = ((int32_t) v - 0x0800) / ADC_DAC_SCALING;
+		int32_t adjustment = ((int32_t) v - 0x0800) * 1.50 * ADC_DAC_SCALING;
 		int32_t newdac = dac_cmd + adjustment;
-		if (adjustment) {
+		//if (adjustment) {
 			if (newdac < DAC_MIN)
 				dac_cmd = DAC_MIN;
 			else if (newdac > DAC_MAX)
@@ -53,9 +48,10 @@ void slow_callback(void)
 			else
 				dac_cmd = newdac;
 			dac_write(dac_cmd);
-		}
+		//}
 	}
-#else
+#endif
+#if 0
 	/* Update DAC if necessary */
 	if (v < ADC_BIGMIN && dac_cmd < (DAC_MAX - DAC_BIGSTEP))
 		dac_cmd += DAC_BIGSTEP;
@@ -71,6 +67,18 @@ void slow_callback(void)
 #endif
 }
 
+/* 8 KHz, after DMA buffer filled */
+void slow_callback(void)
+{
+	/* Get most recent sample. */
+	uint16_t v = adc_dmabuf[adc_offset];
+
+	/* Send most recent sample to PC */
+	send_adc = v;
+	send_dac = dac_cmd;
+	send_data = 1;
+}
+
 void debug_callback(void)
 {
 	send_adc = adc_dmabuf[adc_offset];
@@ -140,7 +148,7 @@ int main(void)
 	} else {
 		uart1_init(500000);
 
-		//adc_adc_callback = fast_callback;
+		adc_adc_callback = fast_callback;
 		adc_dma_callback = slow_callback;
 
 		while(1) {