Beep as logging begins to use as a synchronisation point for video

docs/Blackbox.md

@@ -143,6 +143,10 @@ power-cycled, it begins a fresh new log file. If you arm and disarm several time
 several flights), those logs will be combined together into one file. The command line tools will ask you to pick which
 one of these flights you want to display/decode.
 
+If your craft has a buzzer attached, a short beep will be played when you arm. You can later use this beep to
+synchronize your recorded flight video with the rendered flight data log (the beep is shown as a blue line in the flight
+data log, which you can sync against the beep in your recorded audio track).
+
 The OpenLog requires a couple of seconds of delay after powerup before it's ready to record, so don't arm your craft
 immediately after connecting the battery (you'll probably be waiting for the flight controller to become ready during
 that time anyway!)

src/main/blackbox/blackbox.c

@@ -35,6 +35,7 @@
 #include "drivers/pwm_rx.h"
 #include "drivers/accgyro.h"
 #include "drivers/light_led.h"
+#include "drivers/sound_beeper.h"
 
 #include "common/printf.h"
 
@@ -228,6 +229,7 @@ typedef enum BlackboxState {
     BLACKBOX_STATE_SEND_GPS_H_HEADERS,
     BLACKBOX_STATE_SEND_GPS_G_HEADERS,
     BLACKBOX_STATE_SEND_SYSINFO,
+    BLACKBOX_STATE_PRERUN,
     BLACKBOX_STATE_RUNNING
 } BlackboxState;
 
@@ -1045,6 +1047,10 @@ static bool sendFieldDefinition(const char * const *headerNames, unsigned int he
     return headerXmitIndex < headerCount;
 }
 
+/**
+ * Transmit a portion of the system information headers. Begin with a xmitIndex of 0. Returns the next xmitIndex to
+ * call with, or -1 if transmission is complete.
+ */
 static int blackboxWriteSysinfo(int xmitIndex)
 {
     union floatConvert_t {
@@ -1104,12 +1110,32 @@ static int blackboxWriteSysinfo(int xmitIndex)
             // One more pause for good luck
         break;
         default:
-            blackboxSetState(BLACKBOX_STATE_RUNNING);
+            return -1;
     }
 
     return xmitIndex + 1;
 }
 
+// Beep the buzzer and write the current time to the log as a synchronization point
+static void blackboxPlaySyncBeep()
+{
+    uint32_t now = micros();
+
+    /*
+     * The regular beep routines aren't going to work for us, because they queue up the beep to be executed later.
+     * Our beep is timing sensitive, so start beeping now without setting the beeperIsOn flag.
+     */
+    BEEP_ON;
+
+    // Have the regular beeper code turn off the beep for us eventually, since that's not timing-sensitive
+    queueConfirmationBeep(1);
+
+    blackboxWrite('E');
+    blackboxWrite(FLIGHT_LOG_EVENT_SYNC_BEEP);
+
+    writeUnsignedVB(now);
+}
+
 void handleBlackbox(void)
 {
     int i;
@@ -1157,6 +1183,13 @@ void handleBlackbox(void)
         case BLACKBOX_STATE_SEND_SYSINFO:
             //On entry of this state, headerXmitIndex is 0
             headerXmitIndex = blackboxWriteSysinfo(headerXmitIndex);
+
+            blackboxSetState(BLACKBOX_STATE_PRERUN);
+        break;
+        case BLACKBOX_STATE_PRERUN:
+            blackboxPlaySyncBeep();
+
+            blackboxSetState(BLACKBOX_STATE_RUNNING);
         break;
         case BLACKBOX_STATE_RUNNING:
             // On entry to this state, blackboxIteration, blackboxPFrameIndex and blackboxIFrameIndex are reset to 0

src/main/blackbox/blackbox_fielddefs.h

@@ -94,3 +94,7 @@ typedef enum FlightLogFieldSign {
     FLIGHT_LOG_FIELD_SIGNED   = 1
 } FlightLogFieldSign;
 
+typedef enum FlightLogEvent {
+    FLIGHT_LOG_EVENT_SYNC_BEEP = 0,
+    FLIGHT_LOG_EVENT_LOG_END = 255
+} FlightLogEvent;