diff --git a/main.cpp b/main.cpp
index bbda937..0e82aa5 100644
--- a/main.cpp
+++ b/main.cpp
@@ -79,6 +79,7 @@ static void dsp_thread() {
             frames_mag[i + len/2] = abs(frames_ft_rev[i]);
 
         // stabilize laser
+        bool tick = false;
         if((std::chrono::steady_clock::now() - last_tec) >= 100ms) {
             float freq_peak_local;
             freq_peak_local = 40.0f*float(distance(frames_mag.begin(), max_element(frames_mag.begin(), frames_mag.end())))/float(len);
@@ -88,6 +89,7 @@ static void dsp_thread() {
             tec_socket.write_some(net::buffer(std::format(":SOUR2:VOLT {:.3f}\n",
                 std::max(tec_bias+tec_p*freq_error, 0.0f))));
             last_tec += 100ms;
+            tick = true;
         }
 
         // update waterfall
@@ -99,6 +101,9 @@ static void dsp_thread() {
                 waterfall_data[i] = 0xff000000 | 0x010101*std::min(int(frames_mag[j]/900.0f), 255);
             }
             waterfall_data[int(freq_setpoint*waterfall_width)/40] = 0xff0000ff;
+            if(tick)
+                for(int i=0;i<100;i++)
+                    waterfall_data[i] = 0xffff0000;
         }
 
         // FPS counter