Encoder: try INT8 first, auto-fallback to INT4 if ConvInteger unsupported in browser

mega-asr.js

@@ -234,14 +234,24 @@ async function loadAll() {
   setProgress(30);
 
   // 3. ONNX sessions
-  // INT8 audio encoder + INT4 decoders gives the best size/quality tradeoff
-  // (91.9% vs 87.8% INT4-only on VITW). Encoder is single-file (no .data sidecar).
-  setLoaderStatus("audio encoder INT8 (~320 MB)...");
-  state.encoder = await createSessionSimple(
-    `${HF_ROOT}/onnx/audio_encoder_int8.onnx`,
-    "audio_encoder INT8",
-    p => setProgress(30 + p * 10),
-  );
+  // Try INT8 encoder first (highest quality in browser); fall back to INT4
+  // encoder if the browser ORT lacks the ConvInteger op (which it does in
+  // both WebGPU and WASM backends as of 1.20).
+  setLoaderStatus("audio encoder (INT8 → INT4 fallback) ...");
+  let encLabel = "audio_encoder INT8";
+  let encUrl = `${HF_ROOT}/onnx/audio_encoder_int8.onnx`;
+  try {
+    state.encoder = await createSessionSimple(encUrl, encLabel, p => setProgress(30 + p * 10));
+  } catch (e) {
+    log(`INT8 encoder unsupported in browser (${e.message}); falling back to INT4 + sidecar`);
+    encLabel = "audio_encoder INT4";
+    state.encoder = await createSession(
+      `${HF_ROOT}/onnx/audio_encoder_int4.onnx`,
+      `${HF_ROOT}/onnx/audio_encoder_int4.onnx.data`,
+      encLabel,
+      p => setProgress(30 + p * 10),
+    );
+  }
   setProgress(40);
 
   setLoaderStatus("decoder prefill (~970 MB)...");