Replace beeps with a cathedral gong

Additive gong synthesis: seven inharmonic partials with per-partial
decay (fundamental 165Hz rings 2.6s, highs die in 0.2s), a detuned
pair for shimmer, 45ms distant attack, and three feedback-comb
reflections (95/210/370ms, net gain .93) for a dense stone-space tail.
Normalized to -12dBFS at codec volume 58: quiet but voluminous.
Synthesized into PSRAM at boot (~4s), plays 5s, blocking is fine for
bring-up — audio moves to its own task in phase 2.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

firmware/main/desklock_main.c

@@ -20,38 +20,106 @@
 
 static const char *TAG = "desklock";
 
-#define CHIME_RATE 16000
-#define CHIME_SAMPLES (CHIME_RATE / 2) /* 0.5 s */
+#define GONG_RATE 16000
+#define GONG_SECONDS 5
+#define GONG_SAMPLES (GONG_RATE * GONG_SECONDS)
+#define GONG_VOLUME 58     /* codec volume: discreet, not loud */
+#define GONG_PEAK 8000.0f  /* ~-12 dBFS headroom */
 
-/* Bring-up proof for the audio path: C5 -> E5 with a fade-out. */
-static void play_boot_chime(void)
+/* A distant temple gong in a stone space.
+ *
+ * Inharmonic partials with per-partial decay (lows ring for seconds, highs
+ * die fast), a detuned fundamental pair for slow shimmer, a soft 45 ms
+ * attack so the strike reads as far away, and three early reflections that
+ * stand in for cathedral walls. Fundamental sits at 165 Hz — low enough to
+ * feel voluminous, high enough for the bare 2 W speaker to reproduce.
+ */
+static const struct {
+    float ratio;
+    float amp;
+    float tau; /* seconds to 1/e */
+} gong_partials[] = {
+    { 1.000f, 0.34f, 2.6f },
+    { 1.004f, 0.22f, 2.2f }, /* beating partner -> shimmer */
+    { 1.590f, 0.16f, 1.4f },
+    { 2.140f, 0.11f, 0.9f },
+    { 2.760f, 0.08f, 0.55f },
+    { 3.570f, 0.05f, 0.35f },
+    { 4.800f, 0.03f, 0.22f },
+};
+
+static const struct {
+    int delay_ms;
+    float gain;
+} gong_echoes[] = { { 95, 0.45f }, { 210, 0.30f }, { 370, 0.18f } };
+
+static void play_boot_gong(void)
 {
     esp_codec_dev_handle_t spk = bsp_audio_codec_speaker_init();
     if (spk == NULL) {
-        ESP_LOGW(TAG, "speaker init failed — no chime");
+        ESP_LOGW(TAG, "speaker init failed — no gong");
         return;
     }
     esp_codec_dev_sample_info_t fs = {
-        .sample_rate = CHIME_RATE,
+        .sample_rate = GONG_RATE,
         .channel = 1,
         .bits_per_sample = 16,
     };
-    esp_codec_dev_set_out_vol(spk, 70);
+    esp_codec_dev_set_out_vol(spk, GONG_VOLUME);
     esp_codec_dev_open(spk, &fs);
 
-    int16_t *buf = malloc(CHIME_SAMPLES * sizeof(int16_t));
-    if (buf != NULL) {
-        float phase = 0.0f;
-        for (int i = 0; i < CHIME_SAMPLES; i++) {
-            float freq = (i < CHIME_SAMPLES / 2) ? 523.25f : 659.25f;
-            phase += 2.0f * (float)M_PI * freq / CHIME_RATE;
-            float envelope = 1.0f - (float)i / CHIME_SAMPLES;
-            buf[i] = (int16_t)(sinf(phase) * 12000.0f * envelope);
-        }
-        esp_codec_dev_write(spk, buf, CHIME_SAMPLES * sizeof(int16_t));
-        free(buf);
-        ESP_LOGI(TAG, "boot chime played");
+    float *dry = heap_caps_calloc(GONG_SAMPLES, sizeof(float), MALLOC_CAP_SPIRAM);
+    int16_t *pcm = heap_caps_malloc(GONG_SAMPLES * sizeof(int16_t), MALLOC_CAP_SPIRAM);
+    if (dry == NULL || pcm == NULL) {
+        ESP_LOGW(TAG, "no memory for gong");
+        goto out;
     }
+
+    for (size_t p = 0; p < sizeof(gong_partials) / sizeof(gong_partials[0]); p++) {
+        float step = 2.0f * (float)M_PI * 165.0f * gong_partials[p].ratio / GONG_RATE;
+        float decay = expf(-1.0f / (gong_partials[p].tau * GONG_RATE));
+        float env = gong_partials[p].amp;
+        float phase = 0.0f;
+        for (int i = 0; i < GONG_SAMPLES; i++) {
+            phase += step;
+            env *= decay;
+            dry[i] += sinf(phase) * env;
+        }
+    }
+
+    const int attack = GONG_RATE * 45 / 1000; /* distant, not percussive */
+    for (int i = 0; i < attack; i++) {
+        dry[i] *= (float)i / attack;
+    }
+
+    float peak = 1e-6f;
+    for (int i = 0; i < GONG_SAMPLES; i++) {
+        float s = dry[i];
+        for (size_t e = 0; e < sizeof(gong_echoes) / sizeof(gong_echoes[0]); e++) {
+            int j = i - GONG_RATE * gong_echoes[e].delay_ms / 1000;
+            if (j >= 0) {
+                s += dry[j] * gong_echoes[e].gain;
+            }
+        }
+        const int fade = GONG_RATE * 2 / 5; /* last 400 ms to silence */
+        if (i > GONG_SAMPLES - fade) {
+            s *= (float)(GONG_SAMPLES - i) / fade;
+        }
+        dry[i] = s;
+        if (fabsf(s) > peak) {
+            peak = fabsf(s);
+        }
+    }
+    for (int i = 0; i < GONG_SAMPLES; i++) {
+        pcm[i] = (int16_t)(dry[i] * (GONG_PEAK / peak));
+    }
+
+    esp_codec_dev_write(spk, pcm, GONG_SAMPLES * sizeof(int16_t));
+    ESP_LOGI(TAG, "boot gong played");
+
+out:
+    free(dry);
+    free(pcm);
     esp_codec_dev_close(spk);
 }
 
@@ -89,7 +157,7 @@ void app_main(void)
     bsp_display_start();
     bsp_display_backlight_on();
     show_placeholder_face();
-    play_boot_chime();
+    play_boot_gong();
 
     ESP_LOGI(TAG, "DeskLock up (placeholder face)");