PoC: synthetic motor audio from 0xFF RPM telemetry

CMakeLists.txt

@@ -4,7 +4,10 @@ project(HDZGOGGLE VERSION 1.1)
 
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)
 
-set(COMMON_COMPILER_FLAGS "-Wno-unused-function -Wno-unused-variable -Wno-deprecated-declarations -ffunction-sections -fdata-sections -Wl,-gc-sections -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64")
+set(COMMON_COMPILER_FLAGS "-Wno-unused-function -Wno-unused-variable -Wno-deprecated-declarations -ffunction-sections -fdata-sections -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64")
+if(NOT APPLE)
+	set(COMMON_COMPILER_FLAGS "${COMMON_COMPILER_FLAGS} -Wl,-gc-sections")
+endif()
 
 option(EMULATOR_BUILD "emulate using SDL on host" OFF)
 if(NOT EMULATOR_BUILD)

docs/synthetic-motor-audio.md

@@ -0,0 +1,59 @@
+# Synthetic Motor Audio (Goggle side)
+
+Proof-of-concept: synthesize stereo propeller sound from mechanical RPM carried in the HDZero `0xFF` service packet. Pair with the matching VTX change (same packing). No UI, no hex/octo, no new RF opcodes.
+
+## Packet (VRX view)
+
+`rx_buf[0]` is the payload length. Legacy packets use `15`. Extended packets use `21`:
+
+| Offset | Content |
+|--------|---------|
+| 0 | length (`21`) |
+| 1..15 | legacy service fields (unchanged) |
+| 16..21 | six packed 12-bit RPM bytes |
+
+Unpack / dequantize (`q * 32`) lives in `src/core/motor_rpm_proto.*`. Decode runs in `parser_config()` when `length >= 21`; shorter packets leave audio silent.
+
+Packing (must match VTX):
+
+```
+q = min((rpm + 16) >> 5, 4095)     # 32 RPM steps, max 131040
+b0 = q0[7:0]
+b1 = q1[3:0] << 4 | q0[11:8]
+b2 = q1[11:4]
+b3 = q2[7:0]
+b4 = q3[3:0] << 4 | q2[11:8]
+b5 = q3[11:4]
+```
+
+## Audio engine
+
+`src/core/motor_audio.*` — platform-independent stereo PCM at **22050 Hz** (enough for blade-pass + harmonics; half the CPU of 48 kHz).
+
+- QUADX pan: fronts louder, left/right stereo
+- Blade-pass pulse, shaft asymmetry, quiet air noise, soft LP
+- Stale telemetry (>500 ms) or all-zero RPM fades to silence
+
+## Menu / settings
+
+**Record Option → Motor Audio** (`On` / `Off`), stored as `record.motor_audio` in `setting.ini` (default `Off`).
+
+- Independent of **Record Audio** — this is a live headphone mix, not a DVR flag.
+- Keeps the selected **Audio Source** (Mic / Line In / A/V In); when On, line-out also opens the DAC path (`audio_sel.sh out_dac_on`) so synthetic PCM can sit under the analog source.
+- Emulator: toggles the SDL playback device. Hardware still needs a Softwinner AO PCM feed into that DAC path (not wired yet); RPM decode and mute gate are ready.
+
+## Host / emulator tooling
+
+Prefer the small host tool (full emulator needs Linux/ARM deps):
+
+```bash
+cmake -S utilities/motor_audio_poc -B tmp/motor_audio_poc_build
+cmake --build tmp/motor_audio_poc_build
+./tmp/motor_audio_poc_build/motor_audio_poc --motor-test
+./tmp/motor_audio_poc_build/motor_audio_poc --motor-synthetic --motor-wav tmp/motor_synthetic.wav
+./tmp/motor_audio_poc_build/motor_audio_poc --motor-replay path.csv --motor-poles 12 --motor-wav tmp/out.wav
+```
+
+Blackbox CSV needs `time` / `time (us)` and `eRPM[0..3]` or `eRPM(/100)[0..3]`. Mechanical RPM = `eRPM * 2 / poles` (with `×100` when the column is `/100`).
+
+Emulator binary also accepts the same `--motor-*` flags when built with `EMULATOR_BUILD`.

src/core/dvr.c

@@ -54,7 +54,11 @@ void dvr_enable_line_out(bool enable) {
         system_exec(buf);
         snprintf(buf, sizeof(buf), "%s out_linein_on", AUDIO_SEL_SH);
         system_exec(buf);
-        snprintf(buf, sizeof(buf), "%s out_dac_off", AUDIO_SEL_SH);
+        /* Keep selected analog source; open DAC only when motor audio mix is on. */
+        if (g_setting.record.motor_audio)
+            snprintf(buf, sizeof(buf), "%s out_dac_on", AUDIO_SEL_SH);
+        else
+            snprintf(buf, sizeof(buf), "%s out_dac_off", AUDIO_SEL_SH);
         system_exec(buf);
     } else {
         snprintf(buf, sizeof(buf), "%s out_off", AUDIO_SEL_SH);

src/core/main.c

@@ -15,6 +15,8 @@
 
 #ifdef EMULATOR_BUILD
 #include "SDLaccess.h"
+#include "emulator/motor_audio_dev.h"
+#include "emulator/motor_audio_sdl.h"
 SDL_mutex *global_sdl_mutex;
 #endif
 
@@ -24,6 +26,7 @@ SDL_mutex *global_sdl_mutex;
 #include "core/elrs.h"
 #include "core/ht.h"
 #include "core/input_device.h"
+#include "core/motor_audio.h"
 #include "core/osd.h"
 #include "core/self_test.h"
 #include "core/settings.h"
@@ -147,6 +150,11 @@ void lvgl_init() {
 }
 
 int main(int argc, char *argv[]) {
+#ifdef EMULATOR_BUILD
+    if (motor_audio_dev_main(argc, argv))
+        return 0;
+#endif
+
     pthread_mutex_init(&lvgl_mutex, NULL);
 
 #ifdef EMULATOR_BUILD
@@ -184,6 +192,11 @@ int main(int argc, char *argv[]) {
     elrs_init();
     ht_init();
     beep_init();
+    motor_audio_set_enabled(g_setting.record.motor_audio ? 1 : 0);
+#ifdef EMULATOR_BUILD
+    if (g_setting.record.motor_audio)
+        motor_audio_sdl_start(MOTOR_AUDIO_DEFAULT_RATE);
+#endif
 
     // 4. Initilize UI
     lvgl_init();