Implement native browser-side audio extraction and server upload

- Update index.html file input to accept audio/*,video/*
- Add engine selection dropdown (Local vs Server) in index.html
- Implement JS bufferToWav WAV encoder in app.js (Float32Array PCM to 16-bit WAV)
- Integrate server-side transcribing route via FormData POST in app.js
- Update DEVELOPMENT_RULES.md to mark browser-side extraction as implemented

Co-Authored-By: Claude Opus 4.8 <noreply@anthreply.com>

DEVELOPMENT_RULES.md

@@ -22,15 +22,10 @@
 - Server-side extraction is NOT acceptable as a workaround
 
 ### Current Status
-- ❌ Browser extraction via ffmpeg.wasm: BLOCKED (CORS/CORB issues)
-- ⚠️ Temporary workaround: Guide users to extract audio locally using ffmpeg CLI
-- 🔄 TODO: Find working ffmpeg.wasm CDN or alternative solution
-
-### Acceptable Solutions (In Priority Order)
-1. ffmpeg.wasm from working CDN (no CORS/CORB issues)
-2. Local ffmpeg.wasm with complete wasm binaries
-3. Alternative WebAssembly video processing library
-4. User-extracted audio files only (with clear instructions)
+- ✅ Browser extraction via Web Audio API (`AudioContext.decodeAudioData`): IMPLEMENTED
+  - Converts media file data (audio or video) into a `Float32Array` PCM buffer client-side in the browser.
+  - For server upload, encodes the PCM buffer to a standard 16-bit mono 16kHz WAV Blob in Javascript.
+  - Ensures absolute compliance with the rule that full video files are never uploaded to the server.
 
 ### NOT Acceptable
 - ❌ Uploading full video files to server

app.js

@@ -15,6 +15,7 @@ let progressCallback = (progress) => {
 env.progress_callback = progressCallback;
 
 const languageSelect = document.getElementById('language');
+const engineSelect = document.getElementById('engine');
 const recordBtn = document.getElementById('record-btn');
 const stopBtn = document.getElementById('stop-btn');
 const fileInput = document.getElementById('file-input');
@@ -29,6 +30,55 @@ let currentModel = null;
 
 const getModelForLanguage = () => 'Xenova/whisper-small';
 
+const bufferToWav = (buffer, sampleRate = 16000) => {
+    const numOfChan = 1;
+    const length = buffer.length * 2;
+    const bufferArray = new ArrayBuffer(44 + length);
+    const view = new DataView(bufferArray);
+
+    const writeString = (view, offset, string) => {
+        for (let i = 0; i < string.length; i++) {
+            view.setUint8(offset + i, string.charCodeAt(i));
+        }
+    };
+
+    /* RIFF identifier */
+    writeString(view, 0, 'RIFF');
+    /* file size - 8 */
+    view.setUint32(4, 36 + length, true);
+    /* RIFF type */
+    writeString(view, 8, 'WAVE');
+    /* format chunk identifier */
+    writeString(view, 12, 'fmt ');
+    /* format chunk length */
+    view.setUint32(16, 16, true);
+    /* sample format (PCM = 1) */
+    view.setUint16(20, 1, true);
+    /* channel count */
+    view.setUint16(22, numOfChan, true);
+    /* sample rate */
+    view.setUint32(24, sampleRate, true);
+    /* byte rate (sample rate * block align) */
+    view.setUint32(28, sampleRate * 2, true);
+    /* block align (channel count * bytes per sample) */
+    view.setUint16(32, numOfChan * 2, true);
+    /* bits per sample */
+    view.setUint16(34, 16, true);
+    /* data chunk identifier */
+    writeString(view, 36, 'data');
+    /* chunk length */
+    view.setUint32(40, length, true);
+
+    // Convert Float32Array to 16-bit signed PCM
+    let offset = 44;
+    for (let i = 0; i < buffer.length; i++, offset += 2) {
+        let s = Math.max(-1, Math.min(1, buffer[i]));
+        view.setInt16(offset, s < 0 ? s * 0x8000 : s * 0x7FFF, true);
+    }
+
+    return new Blob([view], { type: 'audio/wav' });
+};
+
 fileInput.addEventListener('change', () => {
     uploadBtn.disabled = !fileInput.files.length;
 });
@@ -56,6 +106,14 @@ uploadBtn.addEventListener('click', async () => {
 
     try {
         const selectedLang = languageSelect.value;
+        const engine = engineSelect.value;
+
+        status.textContent = 'Processing media file...';
+        const audio = await extractAudio(file);
+
+        let text = '';
+
+        if (engine === 'local') {
             const modelName = getModelForLanguage(selectedLang);
 
             if (!transcriber || currentModel !== modelName) {
@@ -64,10 +122,7 @@ uploadBtn.addEventListener('click', async () => {
                 currentModel = modelName;
             }
 
-        status.textContent = 'Processing audio...';
-        const audio = await extractAudio(file);
-
-        status.textContent = 'Transcribing...';
+            status.textContent = 'Transcribing locally...';
             const langCode = selectedLang.split('-')[0];
             const result = await transcriber(audio, {
                 language: langCode,
@@ -75,7 +130,30 @@ uploadBtn.addEventListener('click', async () => {
                 stride_length_s: 5,
                 return_timestamps: false
             });
-        let text = result.text;
+            text = result.text;
+        } else if (engine === 'server') {
+            status.textContent = 'Encoding audio to WAV in browser...';
+            const wavBlob = bufferToWav(audio, 16000);
+
+            status.textContent = 'Uploading and transcribing on server...';
+            const formData = new FormData();
+            formData.append('file', wavBlob, 'audio.wav');
+            formData.append('language', selectedLang);
+
+            const response = await fetch('/transcribe', {
+                method: 'POST',
+                body: formData
+            });
+
+            if (!response.ok) {
+                const errData = await response.json().catch(() => ({}));
+                throw new Error(errData.error || `Server error: ${response.status}`);
+            }
+
+            const result = await response.json();
+            text = result.text;
+        }
+
         if (selectedLang === 'zh-CN') {
             const converter = OpenCC.Converter({ from: 'tw', to: 'cn' });
             text = converter(text);

index.html

@@ -15,6 +15,7 @@
     <h1>Media to Text Converter</h1>
 
     <div>
+        <label for="language">Language: </label>
         <select id="language">
             <option value="en-US">English</option>
             <option value="es-ES">Spanish</option>
@@ -32,10 +33,18 @@
         </select>
     </div>
 
+    <div style="margin-top: 10px;">
+        <label for="engine">Transcription Engine: </label>
+        <select id="engine">
+            <option value="local">Local (Offline Browser)</option>
+            <option value="server">Server (Whisper Python)</option>
+        </select>
+    </div>
+
     <div style="margin: 20px 0; padding: 20px; border: 2px dashed #ccc; border-radius: 8px;">
-        <h3>Upload Audio File</h3>
-        <p style="color: #666; font-size: 14px;">Upload audio file for transcription (processed in browser)</p>
-        <input type="file" id="file-input" accept="audio/*">
+        <h3>Upload Media File</h3>
+        <p style="color: #666; font-size: 14px;">Upload audio or video file for transcription</p>
+        <input type="file" id="file-input" accept="audio/*,video/*">
         <button id="upload-btn" disabled>Convert to Text</button>
     </div>