fix(dub): preserve original sound outside dialogue intervals

CHANGELOG.md

@@ -27,6 +27,8 @@ the frozen-backend fallback mirror it for their toolchains.
 
 ### Fixed
 
+- Dubbing preserves original sound outside dialogue and mixes separated background only beneath replacement speech (#2129)
+
 - Dubbing repairs missing speech caches, rejects incomplete output, avoids oversized speaker references, and fits full speech without early clipping (#2129)
 - Workspace sidebars have a working right-edge resize handle, allow 40% more width, remember their size, and keep video controls inside the preview (#2129)
 - Pressing Play while a video is loading starts playback when it is ready instead of reporting playback unavailable (#2129)

backend/api/routers/dub_export.py

@@ -38,6 +38,31 @@ router = APIRouter()
 logger = logging.getLogger("omnivoice.api")
 
 
+async def _preserved_background(job: dict, job_id: str, lang: str, *, prepare: bool = True) -> str:
+    """All mixed preview/download paths share the same dialogue-only bed."""
+    from services.dub_background import surgical_background
+
+    bed = _optional_dub_artifact(job.get("no_vocals_path"), job_id)
+    source = _optional_dub_artifact(job.get("video_path"), job_id) or _optional_dub_artifact(job.get("audio_path"), job_id)
+    if not bed or not source:
+        raise HTTPException(status_code=409, detail={"code": "dub_background_unavailable", "message": "Original audio and background separation are required"})
+    track = (job.get("dubbed_tracks") or {}).get(lang) or {}
+    segments = track.get("source_segments") or job.get("segments") or []
+    if not segments:
+        raise HTTPException(status_code=409, detail={"code": "dub_background_unavailable", "message": "Dialogue timing is required"})
+    if not prepare:
+        return bed
+    strategy = track.get("timing_strategy") or job.get("timing_strategy")
+    plans = job.get("fit_plans" if strategy == "smart_fit" else "video_stretch_plans") or {}
+    entry = (plans.get(lang) or {}) if strategy in {"smart_fit", "stretch_video"} else {}
+    directory = os.path.join(_existing_job_dir_or_404(job_id), "exports")
+    os.makedirs(directory, exist_ok=True)
+    try:
+        return await surgical_background(source, bed, directory, segments, entry.get("plan") or [], float(entry.get("orig_duration") or job.get("duration") or 0))
+    except (ValueError, RuntimeError) as exc:
+        raise HTTPException(status_code=409, detail={"code": "dub_background_unavailable", "message": str(exc)}) from exc
+
+
 def _unique_stamp() -> str:
     """Return a short unique suffix like '20260415T142301-ab12cd34' for export files."""
     return f"{time.strftime('%Y%m%dT%H%M%S')}-{uuid.uuid4().hex[:8]}"
@@ -596,7 +621,7 @@ def _build_audio_export_cmd(
         # Mix the dubbed voice over the original background bed (same weights
         # as the video mux path) so ambience/music is preserved.
         cmd += ["-i", bg_path, "-filter_complex",
-                bed_mix_filter("1:a", "0:a"),
+                bed_mix_filter("1:a", "0:a", bed_gain=1.0),
                 "-map", "[aout]"]
     cmd += codec
     cmd.append(out_path)
@@ -691,7 +716,7 @@ async def dub_download(
         else:
             output_name = f"dubbed_audio_{stamp}.m4a"
         out_path = os.path.join(exports_dir, output_name)
-        bg = _optional_dub_artifact(job.get("no_vocals_path"), job_id) if preserve_bg else None
+        bg = await _preserved_background(job, job_id, lang_code) if preserve_bg else None
         cmd = _build_audio_export_cmd(ffmpeg, track_info["path"], bg, out_path, fmt)
         try:
             rc, _, stderr = await run_ffmpeg(cmd, timeout=1800.0)
@@ -839,17 +864,16 @@ async def dub_download(
         retimed_idx = input_idx
         input_idx += 1
 
-    bg_audio = _optional_dub_artifact(job.get("no_vocals_path"), job_id) if preserve_bg else None
     bg_idx = None
-    if bg_audio and filtered_tracks:
-        cmd += ["-i", bg_audio]
-        bg_idx = input_idx
-        input_idx += 1
-
     tracks_to_process = []
     for lang_code, track_info in filtered_tracks.items():
+        if preserve_bg:
+            bg_audio = await _preserved_background(job, job_id, lang_code)
+            cmd += ["-i", bg_audio]
+            bg_idx = input_idx
+            input_idx += 1
         cmd += ["-i", track_info["path"]]
-        tracks_to_process.append({"lang_code": lang_code, "idx": input_idx, "info": track_info})
+        tracks_to_process.append({"lang_code": lang_code, "idx": input_idx, "bg_idx": bg_idx, "info": track_info})
         input_idx += 1
 
     filter_parts: list[str] = []
@@ -918,7 +942,7 @@ async def dub_download(
         for i, t in enumerate(tracks_to_process):
             tail = f",apad=whole_dur={apad_dur:.4f}" if apad_dur else ""
             filter_parts.append(bed_mix_filter(
-                f"{bg_idx}:a", f"{t['idx']}:a", out=f"aout{i}", tail=tail, uniq=str(i),
+                f"{t['bg_idx']}:a", f"{t['idx']}:a", out=f"aout{i}", tail=tail, uniq=str(i), bed_gain=1.0,
             ))
             t["out_label"] = f"[aout{i}]"
         for t in tracks_to_process:
@@ -1119,7 +1143,7 @@ async def dub_preview_video(
 
     video_path = _dub_artifact(job.get("video_path"), job_id, missing_detail="Source video missing")
 
-    bg_audio = _optional_dub_artifact(job.get("no_vocals_path"), job_id) if preserve_bg else None
+    bg_audio = await _preserved_background(job, job_id, lang, prepare=request.method != "HEAD") if preserve_bg else None
     has_bg = bool(bg_audio)
 
     # realpath-normalised + containment-checked inline BEFORE any filesystem
@@ -1131,7 +1155,7 @@ async def dub_preview_video(
     if not exports_dir.startswith(_base + os.sep):
         raise HTTPException(status_code=400, detail="Invalid job id")
     os.makedirs(exports_dir, exist_ok=True)
-    bg_suffix = "bg" if (preserve_bg and has_bg) else "nobg"
+    bg_suffix = "surgical_v2_" + Path(bg_audio).stem if (preserve_bg and has_bg) else "nobg"
     preview_path = os.path.realpath(
         os.path.join(exports_dir, f"preview_v2_{lang}_{bg_suffix}.mp4")
     )
@@ -1270,7 +1294,7 @@ async def dub_preview_video(
         audio_map = f"{track_idx}:a:0"
         if bg_idx is not None:
             tail = f",apad=whole_dur={apad_dur:.4f}" if apad_dur else ""
-            filter_parts.append(bed_mix_filter(f"{bg_idx}:a", f"{track_idx}:a", tail=tail))
+            filter_parts.append(bed_mix_filter(f"{bg_idx}:a", f"{track_idx}:a", tail=tail, bed_gain=1.0))
             audio_map = "[aout]"
         elif apad_dur:
             filter_parts.append(f"[{track_idx}:a]apad=whole_dur={apad_dur:.4f}[aout]")
@@ -1661,13 +1685,13 @@ async def dub_download_audio(
     exports_dir = os.path.join(job_dir, "exports")
     os.makedirs(exports_dir, exist_ok=True)
 
-    bg_audio = _optional_dub_artifact(job.get("no_vocals_path"), job_id) if preserve_bg else None
+    bg_audio = await _preserved_background(job, job_id, lang_label) if preserve_bg else None
     if bg_audio:
         ffmpeg = find_ffmpeg()
         final_audio_path = os.path.join(exports_dir, f"mixed_dub_{stamp}.wav")
         cmd = [
             ffmpeg, "-i", bg_audio, "-i", wav_path,
-            "-filter_complex", bed_mix_filter("0:a", "1:a"),
+            "-filter_complex", bed_mix_filter("0:a", "1:a", bed_gain=1.0),
             "-map", "[aout]", "-c:a", "pcm_s16le", "-y", final_audio_path
         ]
         try:
@@ -1678,8 +1702,9 @@ async def dub_download_audio(
                 raise Exception("ffmpeg mix produced no output file")
             wav_path = final_audio_path
             logger.info("Dub audio mix completed")
-        except Exception:
+        except Exception as exc:
             logger.exception("Failed to mix audio")
+            raise HTTPException(status_code=500, detail={"code": "dub_background_unavailable", "message": "Could not preserve background audio"}) from exc
 
     base_name = os.path.splitext(job.get('filename', 'audio'))[0]
     safe_name = ''.join(c for c in base_name if c.isalnum() or c in '-_ ').strip() or 'audio'
@@ -1947,20 +1972,23 @@ async def dub_download_mp3(
     os.makedirs(exports_dir, exist_ok=True)
 
     source_path = wav_path
-    bg_audio = _optional_dub_artifact(job.get("no_vocals_path"), job_id) if preserve_bg else None
+    bg_audio = await _preserved_background(job, job_id, lang_label) if preserve_bg else None
     if bg_audio:
         mixed_path = os.path.join(exports_dir, f"mixed_mp3_{stamp}.wav")
         cmd_mix = [
             ffmpeg, "-i", bg_audio, "-i", wav_path,
-            "-filter_complex", bed_mix_filter("0:a", "1:a"),
+            "-filter_complex", bed_mix_filter("0:a", "1:a", bed_gain=1.0),
             "-map", "[aout]", "-c:a", "pcm_s16le", "-y", mixed_path
         ]
         try:
             rc, _, _ = await run_ffmpeg(cmd_mix, timeout=900.0)
             if rc == 0 and os.path.exists(mixed_path) and os.path.getsize(mixed_path) > 0:
                 source_path = mixed_path
-        except Exception:
+            else:
+                raise RuntimeError("Background mixing failed")
+        except Exception as exc:
             logger.exception("Failed to mix audio for MP3")
+            raise HTTPException(status_code=500, detail={"code": "dub_background_unavailable", "message": "Could not preserve background audio"}) from exc
 
     mp3_path = os.path.join(exports_dir, f"dubbed_{stamp}.mp3")
     # Accept '128', '192k' etc. — normalize to ffmpeg's 'Nk' form and clamp

backend/api/routers/dub_generate.py

@@ -1961,6 +1961,7 @@ async def dub_generate(job_id: str, req: DubRequest):
             "language_code": lang_code,
             "duration": round(track_dur, 4),
             "timing_strategy": strategy,
+            "source_segments": [{"start": seg.start, "end": seg.end} for seg in req.segments],
         }
 
         # Persist the timing strategy + (for Mode B) the per-segment stretch

backend/services/dub_background.py

@@ -0,0 +1,130 @@
+"""Dialogue-only replacement beds: original outside speech, separated bed inside."""
+from __future__ import annotations
+
+import asyncio
+import hashlib
+import json
+import math
+import os
+import tempfile
+from pathlib import Path
+
+from services.ffmpeg_utils import find_ffmpeg, run_ffmpeg
+from services.video_retime import expand_retime_chunks
+
+RATE = 48000
+FADE_S = .01
+_locks: dict[str, asyncio.Lock] = {}
+
+
+def dialogue_intervals(segments: list[dict]) -> list[tuple[float, float]]:
+    intervals = []
+    for row in segments:
+        a, b = float(row['start']), float(row['end'])
+        if not math.isfinite(a) or not math.isfinite(b) or a < 0 or b <= a:
+            raise ValueError('Invalid dialogue interval')
+        intervals.append((a, b))
+    merged: list[tuple[float, float]] = []
+    for a, b in sorted(intervals):
+        if merged and a <= merged[-1][1]:
+            merged[-1] = (merged[-1][0], max(b, merged[-1][1]))
+        else:
+            merged.append((a, b))
+    return merged
+
+
+def splice_background(original: str, separated: str, output: str, intervals: list[tuple[float, float]]) -> None:
+    """Stream in bounded memory; crossfades lie INSIDE dialogue intervals."""
+    import numpy as np
+    import soundfile as sf
+
+    with sf.SoundFile(original) as src, sf.SoundFile(separated) as bed:
+        if src.samplerate != bed.samplerate or src.channels != bed.channels:
+            raise ValueError('Background inputs must have matching sample format')
+        if bed.frames < src.frames - int(.1 * src.samplerate):
+            raise ValueError('Separated background is incomplete')
+        with sf.SoundFile(output, 'w', samplerate=src.samplerate, channels=src.channels, subtype='FLOAT') as out:
+            offset = 0
+            active = 0
+            while True:
+                wave = src.read(65536, dtype='float32', always_2d=True)
+                if not len(wave):
+                    break
+                background = bed.read(len(wave), dtype='float32', always_2d=True)
+                if len(background) < len(wave):
+                    background = np.pad(background, ((0, len(wave)-len(background)), (0, 0)))
+                times = np.arange(offset, offset + len(wave)) / src.samplerate
+                mask = np.zeros(len(wave), dtype='float32')
+                while active < len(intervals) and intervals[active][1] < times[0]:
+                    active += 1
+                for a, b in intervals[active:]:
+                    if a > times[-1]:
+                        break
+                    fade = min(FADE_S, (b-a)/2)
+                    envelope = np.clip(np.minimum((times-a)/fade, (b-times)/fade), 0, 1)
+                    mask = np.maximum(mask, envelope)
+                out.write(wave * (1-mask[:, None]) + background * mask[:, None])
+                offset += len(wave)
+
+
+async def _checked(cmd: list[str]) -> None:
+    rc, _, error = await run_ffmpeg(cmd, timeout=1800.0)
+    if rc:
+        raise RuntimeError('Could not preserve original background audio: ' + str(error)[-500:])
+
+
+async def surgical_background(source: str, separated: str, cache_dir: str, segments: list[dict], plan: list[dict], duration: float) -> str:
+    for chunk in plan:
+        ratio = float(chunk["stretch_ratio"])
+        if not math.isfinite(ratio) or ratio <= 0:
+            raise ValueError("Invalid background retiming ratio")
+    intervals = dialogue_intervals(segments)
+    if not intervals:
+        raise ValueError('Dialogue timing is required to preserve original background audio')
+    identity = [(p, os.stat(p).st_size, os.stat(p).st_mtime_ns) for p in (source, separated)]
+    key = hashlib.sha256(json.dumps([1, identity, intervals, plan, duration], sort_keys=True).encode()).hexdigest()[:24]
+    target = str(Path(cache_dir) / f'surgical_{key}.wav')
+    async with _locks.setdefault(target, asyncio.Lock()):
+        if os.path.isfile(target):
+            return target
+        ffmpeg = find_ffmpeg()
+        with tempfile.TemporaryDirectory(prefix='.surgical-', dir=cache_dir) as tmp:
+            original, bed, spliced = [str(Path(tmp)/name) for name in ('source.wav', 'bed.wav', 'spliced.wav')]
+            for inp, out in ((source, original), (separated, bed)):
+                await _checked([ffmpeg, '-y', '-i', inp, '-map', '0:a:0', '-vn', '-ar', str(RATE), '-ac', '2', '-c:a', 'pcm_f32le', out])
+            await asyncio.to_thread(splice_background, original, bed, spliced, intervals)
+            if plan and any(abs(float(p['stretch_ratio'])-1) > 1e-6 for p in plan):
+                chunks = expand_retime_chunks(plan, duration)
+                # Bound filter buffering for long projects; trim each batch's
+                # input before splitting it among the chunk filters.
+                batches = []
+                for batch_index in range(0, len(chunks), 16):
+                    batch = chunks[batch_index:batch_index+16]
+                    origin = batch[0][0]
+                    filters = []
+                    for i, (a, b, ratio) in enumerate(batch):
+                        rate = 1 / ratio
+                        tempos = []
+                        while rate < .5:
+                            tempos.append('atempo=0.5')
+                            rate /= .5
+                        while rate > 2:
+                            tempos.append('atempo=2')
+                            rate /= 2
+                        tempos.append(f'atempo={rate:.9f}')
+                        length = (b-a)*ratio
+                        filters.append(f'[0:a]atrim=start={a-origin:.9f}:end={b-origin:.9f},asetpts=PTS-STARTPTS,' + ','.join(tempos) + f',apad,atrim=duration={length:.9f}[c{i}]')
+                    filters.append(''.join(f'[c{i}]' for i in range(len(batch))) + f'concat=n={len(batch)}:v=0:a=1[out]')
+                    script = Path(tmp)/'retime.txt'
+                    script.write_text(';'.join(filters))
+                    batch_name = f'batch{batch_index}.wav'
+                    output = str(Path(tmp)/batch_name)
+                    await _checked([ffmpeg, '-y', '-ss', str(origin), '-t', str(batch[-1][1]-origin), '-i', spliced, '-filter_complex_script', str(script), '-map', '[out]', '-c:a', 'pcm_f32le', output])
+                    batches.append(batch_name)
+                listing = Path(tmp)/'concat.txt'
+                listing.write_text(''.join(f"file '{name}'\n" for name in batches))
+                retimed = str(Path(tmp)/'retimed.wav')
+                await _checked([ffmpeg, '-y', '-f', 'concat', '-safe', '1', '-i', str(listing), '-c:a', 'copy', retimed])
+                spliced = retimed
+            os.replace(spliced, target)
+    return target

backend/services/ffmpeg_utils.py

@@ -89,6 +89,7 @@ def bed_mix_filter(
     duration: str = "longest",
     tail: str = "",
     uniq: str = "",
+    bed_gain: float = BED_GAIN,
 ) -> str:
     """One ffmpeg filter chain mixing `voice_in` over `bed_in` at original level.
 
@@ -110,22 +111,22 @@ def bed_mix_filter(
         # Gains applied per input, amix reduced to a plain sum: levels are
         # exact for the whole timeline, including after either stream ends.
         return (
-            f"[{bed_in}]aresample={BED_MIX_SAMPLE_RATE},{stereo},volume={BED_GAIN:g}[{b}];"
+            f"[{bed_in}]aresample={BED_MIX_SAMPLE_RATE},{stereo},volume={bed_gain:g}[{b}];"
             f"[{voice_in}]aresample={BED_MIX_SAMPLE_RATE},{stereo},volume={VOICE_GAIN:g}[{v}];"
             f"[{b}][{v}]amix=inputs=2:duration={duration}:dropout_transition=2:"
-            f"normalize=0,alimiter=level=false:limit=0.98{tail}[{out}]"
+            f"normalize=0,alimiter=level=false:limit=0.98:latency=1{tail}[{out}]"
         )
     # Legacy ffmpeg (<5, no `normalize`): cancel amix's normalization with a
     # compensating multiply. Exact while both streams run; if one ends early
     # the tail is over-boosted into the limiter until the graph ends — a known
     # quirk accepted only on old ffmpeg, where the alternative is no export.
-    total = BED_GAIN + VOICE_GAIN
+    total = bed_gain + VOICE_GAIN
     return (
         f"[{bed_in}]aresample={BED_MIX_SAMPLE_RATE},{stereo}[{b}];"
         f"[{voice_in}]aresample={BED_MIX_SAMPLE_RATE},{stereo}[{v}];"
         f"[{b}][{v}]amix=inputs=2:duration={duration}:dropout_transition=2:"
-        f"weights={BED_GAIN:g} {VOICE_GAIN:g},volume={total:g},"
-        f"alimiter=level=false:limit=0.98{tail}[{out}]"
+        f"weights={bed_gain:g} {VOICE_GAIN:g},volume={total:g},"
+        f"alimiter=level=false:limit=0.98:latency=1{tail}[{out}]"
     )
 
 

docs/electron-dubbing.md

@@ -223,3 +223,20 @@ retrying. Camera-cut segmentation uses nearby timed word boundaries when availab
 and skips cuts inside speech that cannot be assigned safely. This improves phrase
 timing; it does not promise phoneme-level lip sync or correct inaccurate source
 transcripts automatically.
+
+### Preserve sound outside dialogue
+
+Background-preserving previews and audio/video exports keep the original stereo
+sound outside dialogue intervals, including audience reactions, music and ambience.
+Inside those intervals, they mix dubbed speech over the separated background, with
+10 ms transitions contained within the dialogue boundaries. Each generated language
+stores its source intervals; older tracks use their saved project intervals.
+Retimed modes also retime this background to follow the video. Ordinary Strict Slot
+and Concise modes keep the original background timeline.
+
+Original media and a complete separated background are required. A missing or failed
+background mix stops export rather than silently exporting speech alone. Explicit
+speech-only export remains available. The preserved bed is cached locally and rebuilt
+when source files, dialogue intervals or the language's retiming plan change.
+Separation can still affect sounds overlapping dialogue; exact isolation from a
+single mixed recording is not guaranteed. Correct dialogue boundaries matter.

electron/src/renderer/src/features/dub/dub-page.tsx

@@ -452,7 +452,7 @@ export function DubPage() {
             const revision = fingerprintRevision(session.fingerprintsByLang?.[track]);
             return {
               track,
-              path: `/dub/preview-video/${job}?lang=${encodeURIComponent(track)}${revision ? `&v=${revision}` : ''}`,
+              path: `/dub/preview-video/${job}?mix=surgical2&lang=${encodeURIComponent(track)}${revision ? `&v=${revision}` : ''}`,
             };
           })
         : [],
@@ -469,7 +469,7 @@ export function DubPage() {
         : apiPath(
             preview === 'original'
               ? `/dub/media/${job}`
-              : `/dub/preview-video/${job}?lang=${encodeURIComponent(preview)}${previewRevision ? `&v=${previewRevision}` : ''}`,
+              : `/dub/preview-video/${job}?mix=surgical2&lang=${encodeURIComponent(preview)}${previewRevision ? `&v=${previewRevision}` : ''}`,
           ),
     [job, preview, previewRevision, session.inputType],
   );

electron/src/renderer/src/i18n/locales/ar.json

@@ -2581,6 +2581,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "الكلام مفقود أو غير قابل للقراءة. أعد توليد المقطع المتأثر قبل التصدير.",
-    "timingOverflow": "يتجاوز الكلام الوقت المخصص له. اختصر الترجمة أو اختر التوقيت الصارم أو تمديد الفيديو."
+    "timingOverflow": "يتجاوز الكلام الوقت المخصص له. اختصر الترجمة أو اختر التوقيت الصارم أو تمديد الفيديو.",
+    "backgroundUnavailable": "تعذّر الحفاظ على الصوت الأصلي. تحقّق من فصل الخلفية وتوقيت الحوار ثم أعد المحاولة، أو صدّر الكلام فقط بشكل صريح."
   }
 }

electron/src/renderer/src/i18n/locales/de.json

@@ -2573,6 +2573,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Sprache fehlt oder ist nicht lesbar. Erzeuge das betroffene Segment vor dem Export erneut.",
-    "timingOverflow": "Die Sprache überschreitet ihr Zeitfenster. Kürze die Übersetzung oder wähle ein festes Zeitfenster oder die Videostreckung."
+    "timingOverflow": "Die Sprache überschreitet ihr Zeitfenster. Kürze die Übersetzung oder wähle ein festes Zeitfenster oder die Videostreckung.",
+    "backgroundUnavailable": "Der Originalton konnte nicht erhalten werden. Prüfe Hintergrundtrennung und Dialogzeiten und versuche es erneut, oder exportiere ausdrücklich nur Sprache."
   }
 }

electron/src/renderer/src/i18n/locales/en.json

@@ -2573,6 +2573,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Speech is missing or unreadable. Regenerate the affected segment before exporting.",
-    "timingOverflow": "Speech exceeds its time slot. Shorten the translation or choose Strict Slot or Stretch Video."
+    "timingOverflow": "Speech exceeds its time slot. Shorten the translation or choose Strict Slot or Stretch Video.",
+    "backgroundUnavailable": "Original sound could not be preserved. Check background separation and dialogue timing, then retry, or explicitly export speech only."
   }
 }

electron/src/renderer/src/i18n/locales/es.json

@@ -2575,6 +2575,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Falta audio de voz o no se puede leer. Regenera el segmento afectado antes de exportar.",
-    "timingOverflow": "La voz supera su intervalo de tiempo. Acorta la traducción o elige un intervalo estricto o alargar el vídeo."
+    "timingOverflow": "La voz supera su intervalo de tiempo. Acorta la traducción o elige un intervalo estricto o alargar el vídeo.",
+    "backgroundUnavailable": "No se pudo conservar el sonido original. Revisa la separación del fondo y los tiempos del diálogo e inténtalo de nuevo, o exporta solo la voz explícitamente."
   }
 }

electron/src/renderer/src/i18n/locales/fr.json

@@ -2575,6 +2575,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "La parole est absente ou illisible. Régénérez le segment concerné avant l’exportation.",
-    "timingOverflow": "La parole dépasse son créneau. Raccourcissez la traduction ou choisissez un créneau strict ou l’étirement vidéo."
+    "timingOverflow": "La parole dépasse son créneau. Raccourcissez la traduction ou choisissez un créneau strict ou l’étirement vidéo.",
+    "backgroundUnavailable": "Le son original n’a pas pu être préservé. Vérifiez la séparation du fond et les temps du dialogue, puis réessayez, ou exportez explicitement la voix seule."
   }
 }

electron/src/renderer/src/i18n/locales/hi.json

@@ -2573,6 +2573,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "बोली गायब है या पढ़ी नहीं जा सकती। निर्यात से पहले प्रभावित खंड दोबारा बनाएँ।",
-    "timingOverflow": "बोली अपने समय खंड से लंबी है। अनुवाद छोटा करें या सख्त समय खंड अथवा वीडियो खिंचाव चुनें।"
+    "timingOverflow": "बोली अपने समय खंड से लंबी है। अनुवाद छोटा करें या सख्त समय खंड अथवा वीडियो खिंचाव चुनें।",
+    "backgroundUnavailable": "मूल ध्वनि सुरक्षित नहीं रखी जा सकी। पृष्ठभूमि पृथक्करण और संवाद का समय जाँचकर फिर कोशिश करें, या केवल वाणी निर्यात करने का विकल्प चुनें।"
   }
 }

electron/src/renderer/src/i18n/locales/id.json

@@ -2573,6 +2573,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Audio ucapan hilang atau tidak dapat dibaca. Buat ulang segmen terkait sebelum mengekspor.",
-    "timingOverflow": "Ucapan melebihi jatah waktunya. Persingkat terjemahan atau pilih slot ketat atau rentangkan video."
+    "timingOverflow": "Ucapan melebihi jatah waktunya. Persingkat terjemahan atau pilih slot ketat atau rentangkan video.",
+    "backgroundUnavailable": "Suara asli tidak dapat dipertahankan. Periksa pemisahan latar dan waktu dialog, lalu coba lagi, atau pilih ekspor suara saja."
   }
 }

electron/src/renderer/src/i18n/locales/it.json

@@ -2575,6 +2575,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Il parlato manca o non è leggibile. Rigenera il segmento interessato prima di esportare.",
-    "timingOverflow": "Il parlato supera il tempo disponibile. Accorcia la traduzione oppure scegli uno slot rigoroso o estendi il video."
+    "timingOverflow": "Il parlato supera il tempo disponibile. Accorcia la traduzione oppure scegli uno slot rigoroso o estendi il video.",
+    "backgroundUnavailable": "Impossibile preservare il suono originale. Controlla la separazione del sottofondo e i tempi del dialogo, poi riprova, oppure esporta esplicitamente solo la voce."
   }
 }

electron/src/renderer/src/i18n/locales/ja.json

@@ -2573,6 +2573,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "音声がないか読み取れません。書き出す前に該当セグメントを再生成してください。",
-    "timingOverflow": "音声が割り当て時間を超えています。翻訳を短くするか、厳密な時間枠または動画の引き伸ばしを選んでください。"
+    "timingOverflow": "音声が割り当て時間を超えています。翻訳を短くするか、厳密な時間枠または動画の引き伸ばしを選んでください。",
+    "backgroundUnavailable": "元の音声を保持できませんでした。背景音の分離と台詞のタイミングを確認して再試行するか、音声のみの書き出しを選択してください。"
   }
 }

electron/src/renderer/src/i18n/locales/ko.json

@@ -2573,6 +2573,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "음성이 없거나 읽을 수 없습니다. 내보내기 전에 해당 구간을 다시 생성하세요.",
-    "timingOverflow": "음성이 할당된 시간을 초과합니다. 번역을 줄이거나 엄격한 시간 구간 또는 비디오 늘이기를 선택하세요."
+    "timingOverflow": "음성이 할당된 시간을 초과합니다. 번역을 줄이거나 엄격한 시간 구간 또는 비디오 늘이기를 선택하세요.",
+    "backgroundUnavailable": "원본 소리를 보존할 수 없습니다. 배경음 분리와 대사 타이밍을 확인한 후 다시 시도하거나 음성만 내보내기를 선택하세요."
   }
 }

electron/src/renderer/src/i18n/locales/nl.json

@@ -2573,6 +2573,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Spraak ontbreekt of is onleesbaar. Genereer het betreffende segment opnieuw voordat je exporteert.",
-    "timingOverflow": "De spraak overschrijdt het tijdvak. Verkort de vertaling of kies een strikt tijdvak of het uitrekken van de video."
+    "timingOverflow": "De spraak overschrijdt het tijdvak. Verkort de vertaling of kies een strikt tijdvak of het uitrekken van de video.",
+    "backgroundUnavailable": "Het oorspronkelijke geluid kon niet behouden blijven. Controleer de achtergrondscheiding en dialoogtijden en probeer opnieuw, of kies expliciet voor alleen spraak exporteren."
   }
 }

electron/src/renderer/src/i18n/locales/pl.json

@@ -2577,6 +2577,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Brak mowy lub nie można jej odczytać. Przed eksportem wygeneruj ponownie dany segment.",
-    "timingOverflow": "Mowa przekracza przydzielony czas. Skróć tłumaczenie albo wybierz ścisły przedział czasu lub wydłużenie filmu."
+    "timingOverflow": "Mowa przekracza przydzielony czas. Skróć tłumaczenie albo wybierz ścisły przedział czasu lub wydłużenie filmu.",
+    "backgroundUnavailable": "Nie udało się zachować oryginalnego dźwięku. Sprawdź oddzielenie tła i czasy dialogów, a następnie spróbuj ponownie lub wybierz eksport samej mowy."
   }
 }

electron/src/renderer/src/i18n/locales/pt.json

@@ -2575,6 +2575,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "A fala está ausente ou ilegível. Gere novamente o segmento afetado antes de exportar.",
-    "timingOverflow": "A fala excede o intervalo de tempo. Encurte a tradução ou escolha um intervalo estrito ou estique o vídeo."
+    "timingOverflow": "A fala excede o intervalo de tempo. Encurte a tradução ou escolha um intervalo estrito ou estique o vídeo.",
+    "backgroundUnavailable": "Não foi possível preservar o som original. Verifique a separação do fundo e os tempos do diálogo e tente novamente, ou escolha exportar apenas a fala."
   }
 }

electron/src/renderer/src/i18n/locales/ru.json

@@ -2577,6 +2577,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Речь отсутствует или не читается. Перед экспортом заново сгенерируйте этот сегмент.",
-    "timingOverflow": "Речь выходит за отведённое время. Сократите перевод или выберите строгий интервал либо растяжение видео."
+    "timingOverflow": "Речь выходит за отведённое время. Сократите перевод или выберите строгий интервал либо растяжение видео.",
+    "backgroundUnavailable": "Не удалось сохранить исходный звук. Проверьте отделение фона и время реплик, затем повторите попытку или выберите экспорт только речи."
   }
 }

electron/src/renderer/src/i18n/locales/sv.json

@@ -2573,6 +2573,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Tal saknas eller kan inte läsas. Generera det berörda segmentet på nytt före export.",
-    "timingOverflow": "Talet överskrider sin tidslucka. Korta översättningen eller välj strikt tidslucka eller sträck ut videon."
+    "timingOverflow": "Talet överskrider sin tidslucka. Korta översättningen eller välj strikt tidslucka eller sträck ut videon.",
+    "backgroundUnavailable": "Originalljudet kunde inte bevaras. Kontrollera bakgrundssepareringen och dialogens tider och försök igen, eller välj att endast exportera tal."
   }
 }

electron/src/renderer/src/i18n/locales/th.json

@@ -2573,6 +2573,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "เสียงพูดหายไปหรืออ่านไม่ได้ สร้างช่วงที่มีปัญหาใหม่ก่อนส่งออก",
-    "timingOverflow": "เสียงพูดยาวเกินช่วงเวลาที่กำหนด ย่อคำแปลหรือเลือกช่วงเวลาแบบเคร่งครัดหรือยืดวิดีโอ"
+    "timingOverflow": "เสียงพูดยาวเกินช่วงเวลาที่กำหนด ย่อคำแปลหรือเลือกช่วงเวลาแบบเคร่งครัดหรือยืดวิดีโอ",
+    "backgroundUnavailable": "ไม่สามารถรักษาเสียงต้นฉบับได้ ตรวจสอบการแยกเสียงพื้นหลังและเวลาบทสนทนาแล้วลองอีกครั้ง หรือเลือกส่งออกเฉพาะเสียงพูด"
   }
 }

electron/src/renderer/src/i18n/locales/tr.json

@@ -2573,6 +2573,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Konuşma eksik veya okunamıyor. Dışa aktarmadan önce ilgili bölümü yeniden oluşturun.",
-    "timingOverflow": "Konuşma ayrılan süreyi aşıyor. Çeviriyi kısaltın veya kesin zaman aralığını ya da videoyu uzatmayı seçin."
+    "timingOverflow": "Konuşma ayrılan süreyi aşıyor. Çeviriyi kısaltın veya kesin zaman aralığını ya da videoyu uzatmayı seçin.",
+    "backgroundUnavailable": "Özgün ses korunamadı. Arka plan ayrımını ve diyalog zamanlarını kontrol edip yeniden deneyin veya yalnızca konuşmayı dışa aktarmayı seçin."
   }
 }

electron/src/renderer/src/i18n/locales/uk.json

@@ -2577,6 +2577,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Мовлення відсутнє або не читається. Перед експортом повторно згенеруйте цей сегмент.",
-    "timingOverflow": "Мовлення перевищує відведений час. Скоротіть переклад або виберіть строгий інтервал чи розтягнення відео."
+    "timingOverflow": "Мовлення перевищує відведений час. Скоротіть переклад або виберіть строгий інтервал чи розтягнення відео.",
+    "backgroundUnavailable": "Не вдалося зберегти оригінальний звук. Перевірте відокремлення фону й час реплік, потім повторіть спробу або виберіть експорт лише мовлення."
   }
 }

electron/src/renderer/src/i18n/locales/vi.json

@@ -2573,6 +2573,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Lời nói bị thiếu hoặc không đọc được. Hãy tạo lại đoạn bị ảnh hưởng trước khi xuất.",
-    "timingOverflow": "Lời nói vượt quá thời gian được phân bổ. Hãy rút ngắn bản dịch hoặc chọn khung thời gian nghiêm ngặt hay kéo dài video."
+    "timingOverflow": "Lời nói vượt quá thời gian được phân bổ. Hãy rút ngắn bản dịch hoặc chọn khung thời gian nghiêm ngặt hay kéo dài video.",
+    "backgroundUnavailable": "Không thể giữ âm thanh gốc. Hãy kiểm tra việc tách âm nền và thời gian hội thoại rồi thử lại, hoặc chọn chỉ xuất giọng nói."
   }
 }

electron/src/renderer/src/i18n/locales/zh-CN.json

@@ -2577,6 +2577,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "语音缺失或无法读取。请在导出前重新生成受影响的片段。",
-    "timingOverflow": "语音超出分配的时长。请缩短译文，或选择严格时间段或拉伸视频。"
+    "timingOverflow": "语音超出分配的时长。请缩短译文，或选择严格时间段或拉伸视频。",
+    "backgroundUnavailable": "无法保留原始声音。请检查背景音分离和对白时间后重试，或明确选择仅导出语音。"
   }
 }

electron/src/renderer/src/i18n/locales/zh-TW.json

@@ -2573,6 +2573,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "語音缺失或無法讀取。請在匯出前重新產生受影響的片段。",
-    "timingOverflow": "語音超出分配的時長。請縮短譯文，或選擇嚴格時間區段或延展影片。"
+    "timingOverflow": "語音超出分配的時長。請縮短譯文，或選擇嚴格時間區段或延展影片。",
+    "backgroundUnavailable": "無法保留原始聲音。請檢查背景音分離和對白時間後重試，或明確選擇僅匯出語音。"
   }
 }

electron/src/renderer/src/lib/api/client.test.ts

@@ -23,6 +23,13 @@ afterEach(() => {
 });
 
 describe('errorFromResponse', () => {
+  it('localizes background preservation errors and retains diagnostics', async () => {
+    const detail = { code: 'dub_background_unavailable', message: 'Raw diagnostic' };
+    const err = await errorFromResponse(new Response(JSON.stringify({ detail }), { status: 409 }));
+    expect(err.detail).not.toBe('Raw diagnostic');
+    expect(err.detail).not.toContain('Raw diagnostic');
+    expect(err.payload?.detail).toEqual(detail);
+  });
   it('uses a string detail verbatim and keeps the payload', async () => {
     const res = new Response(JSON.stringify({ detail: 'Unsupported instruct items' }), {
       status: 400,

electron/src/renderer/src/lib/api/client.ts

@@ -44,6 +44,8 @@ export function describeError(err: unknown): string {
 }
 
 function detailToString(detail: unknown): string {
+  if (detail && typeof detail === 'object' && 'code' in detail && detail.code === 'dub_background_unavailable')
+    return tr('dubIntegrity.backgroundUnavailable');
   if (typeof detail === 'string') return detail;
   if (detail == null) return '';
   if (

frontend/src/i18n/locales/ar.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "الكلام مفقود أو غير قابل للقراءة. أعد توليد المقطع المتأثر قبل التصدير.",
-    "timingOverflow": "يتجاوز الكلام الوقت المخصص له. اختصر الترجمة أو اختر التوقيت الصارم أو تمديد الفيديو."
+    "timingOverflow": "يتجاوز الكلام الوقت المخصص له. اختصر الترجمة أو اختر التوقيت الصارم أو تمديد الفيديو.",
+    "backgroundUnavailable": "تعذّر الحفاظ على الصوت الأصلي. تحقّق من فصل الخلفية وتوقيت الحوار ثم أعد المحاولة، أو صدّر الكلام فقط بشكل صريح."
   }
 }

frontend/src/i18n/locales/de.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Sprache fehlt oder ist nicht lesbar. Erzeuge das betroffene Segment vor dem Export erneut.",
-    "timingOverflow": "Die Sprache überschreitet ihr Zeitfenster. Kürze die Übersetzung oder wähle ein festes Zeitfenster oder die Videostreckung."
+    "timingOverflow": "Die Sprache überschreitet ihr Zeitfenster. Kürze die Übersetzung oder wähle ein festes Zeitfenster oder die Videostreckung.",
+    "backgroundUnavailable": "Der Originalton konnte nicht erhalten werden. Prüfe Hintergrundtrennung und Dialogzeiten und versuche es erneut, oder exportiere ausdrücklich nur Sprache."
   }
 }

frontend/src/i18n/locales/en.json

@@ -3316,6 +3316,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Speech is missing or unreadable. Regenerate the affected segment before exporting.",
-    "timingOverflow": "Speech exceeds its time slot. Shorten the translation or choose Strict Slot or Stretch Video."
+    "timingOverflow": "Speech exceeds its time slot. Shorten the translation or choose Strict Slot or Stretch Video.",
+    "backgroundUnavailable": "Original sound could not be preserved. Check background separation and dialogue timing, then retry, or explicitly export speech only."
   }
 }

frontend/src/i18n/locales/es.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Falta audio de voz o no se puede leer. Regenera el segmento afectado antes de exportar.",
-    "timingOverflow": "La voz supera su intervalo de tiempo. Acorta la traducción o elige un intervalo estricto o alargar el vídeo."
+    "timingOverflow": "La voz supera su intervalo de tiempo. Acorta la traducción o elige un intervalo estricto o alargar el vídeo.",
+    "backgroundUnavailable": "No se pudo conservar el sonido original. Revisa la separación del fondo y los tiempos del diálogo e inténtalo de nuevo, o exporta solo la voz explícitamente."
   }
 }

frontend/src/i18n/locales/fr.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "La parole est absente ou illisible. Régénérez le segment concerné avant l’exportation.",
-    "timingOverflow": "La parole dépasse son créneau. Raccourcissez la traduction ou choisissez un créneau strict ou l’étirement vidéo."
+    "timingOverflow": "La parole dépasse son créneau. Raccourcissez la traduction ou choisissez un créneau strict ou l’étirement vidéo.",
+    "backgroundUnavailable": "Le son original n’a pas pu être préservé. Vérifiez la séparation du fond et les temps du dialogue, puis réessayez, ou exportez explicitement la voix seule."
   }
 }

frontend/src/i18n/locales/hi.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "बोली गायब है या पढ़ी नहीं जा सकती। निर्यात से पहले प्रभावित खंड दोबारा बनाएँ।",
-    "timingOverflow": "बोली अपने समय खंड से लंबी है। अनुवाद छोटा करें या सख्त समय खंड अथवा वीडियो खिंचाव चुनें।"
+    "timingOverflow": "बोली अपने समय खंड से लंबी है। अनुवाद छोटा करें या सख्त समय खंड अथवा वीडियो खिंचाव चुनें।",
+    "backgroundUnavailable": "मूल ध्वनि सुरक्षित नहीं रखी जा सकी। पृष्ठभूमि पृथक्करण और संवाद का समय जाँचकर फिर कोशिश करें, या केवल वाणी निर्यात करने का विकल्प चुनें।"
   }
 }

frontend/src/i18n/locales/id.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Audio ucapan hilang atau tidak dapat dibaca. Buat ulang segmen terkait sebelum mengekspor.",
-    "timingOverflow": "Ucapan melebihi jatah waktunya. Persingkat terjemahan atau pilih slot ketat atau rentangkan video."
+    "timingOverflow": "Ucapan melebihi jatah waktunya. Persingkat terjemahan atau pilih slot ketat atau rentangkan video.",
+    "backgroundUnavailable": "Suara asli tidak dapat dipertahankan. Periksa pemisahan latar dan waktu dialog, lalu coba lagi, atau pilih ekspor suara saja."
   }
 }

frontend/src/i18n/locales/it.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Il parlato manca o non è leggibile. Rigenera il segmento interessato prima di esportare.",
-    "timingOverflow": "Il parlato supera il tempo disponibile. Accorcia la traduzione oppure scegli uno slot rigoroso o estendi il video."
+    "timingOverflow": "Il parlato supera il tempo disponibile. Accorcia la traduzione oppure scegli uno slot rigoroso o estendi il video.",
+    "backgroundUnavailable": "Impossibile preservare il suono originale. Controlla la separazione del sottofondo e i tempi del dialogo, poi riprova, oppure esporta esplicitamente solo la voce."
   }
 }

frontend/src/i18n/locales/ja.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "音声がないか読み取れません。書き出す前に該当セグメントを再生成してください。",
-    "timingOverflow": "音声が割り当て時間を超えています。翻訳を短くするか、厳密な時間枠または動画の引き伸ばしを選んでください。"
+    "timingOverflow": "音声が割り当て時間を超えています。翻訳を短くするか、厳密な時間枠または動画の引き伸ばしを選んでください。",
+    "backgroundUnavailable": "元の音声を保持できませんでした。背景音の分離と台詞のタイミングを確認して再試行するか、音声のみの書き出しを選択してください。"
   }
 }

frontend/src/i18n/locales/ko.json

@@ -3316,6 +3316,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "음성이 없거나 읽을 수 없습니다. 내보내기 전에 해당 구간을 다시 생성하세요.",
-    "timingOverflow": "음성이 할당된 시간을 초과합니다. 번역을 줄이거나 엄격한 시간 구간 또는 비디오 늘이기를 선택하세요."
+    "timingOverflow": "음성이 할당된 시간을 초과합니다. 번역을 줄이거나 엄격한 시간 구간 또는 비디오 늘이기를 선택하세요.",
+    "backgroundUnavailable": "원본 소리를 보존할 수 없습니다. 배경음 분리와 대사 타이밍을 확인한 후 다시 시도하거나 음성만 내보내기를 선택하세요."
   }
 }

frontend/src/i18n/locales/nl.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Spraak ontbreekt of is onleesbaar. Genereer het betreffende segment opnieuw voordat je exporteert.",
-    "timingOverflow": "De spraak overschrijdt het tijdvak. Verkort de vertaling of kies een strikt tijdvak of het uitrekken van de video."
+    "timingOverflow": "De spraak overschrijdt het tijdvak. Verkort de vertaling of kies een strikt tijdvak of het uitrekken van de video.",
+    "backgroundUnavailable": "Het oorspronkelijke geluid kon niet behouden blijven. Controleer de achtergrondscheiding en dialoogtijden en probeer opnieuw, of kies expliciet voor alleen spraak exporteren."
   }
 }

frontend/src/i18n/locales/pl.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Brak mowy lub nie można jej odczytać. Przed eksportem wygeneruj ponownie dany segment.",
-    "timingOverflow": "Mowa przekracza przydzielony czas. Skróć tłumaczenie albo wybierz ścisły przedział czasu lub wydłużenie filmu."
+    "timingOverflow": "Mowa przekracza przydzielony czas. Skróć tłumaczenie albo wybierz ścisły przedział czasu lub wydłużenie filmu.",
+    "backgroundUnavailable": "Nie udało się zachować oryginalnego dźwięku. Sprawdź oddzielenie tła i czasy dialogów, a następnie spróbuj ponownie lub wybierz eksport samej mowy."
   }
 }

frontend/src/i18n/locales/pt.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "A fala está ausente ou ilegível. Gere novamente o segmento afetado antes de exportar.",
-    "timingOverflow": "A fala excede o intervalo de tempo. Encurte a tradução ou escolha um intervalo estrito ou estique o vídeo."
+    "timingOverflow": "A fala excede o intervalo de tempo. Encurte a tradução ou escolha um intervalo estrito ou estique o vídeo.",
+    "backgroundUnavailable": "Não foi possível preservar o som original. Verifique a separação do fundo e os tempos do diálogo e tente novamente, ou escolha exportar apenas a fala."
   }
 }

frontend/src/i18n/locales/ru.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Речь отсутствует или не читается. Перед экспортом заново сгенерируйте этот сегмент.",
-    "timingOverflow": "Речь выходит за отведённое время. Сократите перевод или выберите строгий интервал либо растяжение видео."
+    "timingOverflow": "Речь выходит за отведённое время. Сократите перевод или выберите строгий интервал либо растяжение видео.",
+    "backgroundUnavailable": "Не удалось сохранить исходный звук. Проверьте отделение фона и время реплик, затем повторите попытку или выберите экспорт только речи."
   }
 }

frontend/src/i18n/locales/sv.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Tal saknas eller kan inte läsas. Generera det berörda segmentet på nytt före export.",
-    "timingOverflow": "Talet överskrider sin tidslucka. Korta översättningen eller välj strikt tidslucka eller sträck ut videon."
+    "timingOverflow": "Talet överskrider sin tidslucka. Korta översättningen eller välj strikt tidslucka eller sträck ut videon.",
+    "backgroundUnavailable": "Originalljudet kunde inte bevaras. Kontrollera bakgrundssepareringen och dialogens tider och försök igen, eller välj att endast exportera tal."
   }
 }

frontend/src/i18n/locales/th.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "เสียงพูดหายไปหรืออ่านไม่ได้ สร้างช่วงที่มีปัญหาใหม่ก่อนส่งออก",
-    "timingOverflow": "เสียงพูดยาวเกินช่วงเวลาที่กำหนด ย่อคำแปลหรือเลือกช่วงเวลาแบบเคร่งครัดหรือยืดวิดีโอ"
+    "timingOverflow": "เสียงพูดยาวเกินช่วงเวลาที่กำหนด ย่อคำแปลหรือเลือกช่วงเวลาแบบเคร่งครัดหรือยืดวิดีโอ",
+    "backgroundUnavailable": "ไม่สามารถรักษาเสียงต้นฉบับได้ ตรวจสอบการแยกเสียงพื้นหลังและเวลาบทสนทนาแล้วลองอีกครั้ง หรือเลือกส่งออกเฉพาะเสียงพูด"
   }
 }

frontend/src/i18n/locales/tr.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Konuşma eksik veya okunamıyor. Dışa aktarmadan önce ilgili bölümü yeniden oluşturun.",
-    "timingOverflow": "Konuşma ayrılan süreyi aşıyor. Çeviriyi kısaltın veya kesin zaman aralığını ya da videoyu uzatmayı seçin."
+    "timingOverflow": "Konuşma ayrılan süreyi aşıyor. Çeviriyi kısaltın veya kesin zaman aralığını ya da videoyu uzatmayı seçin.",
+    "backgroundUnavailable": "Özgün ses korunamadı. Arka plan ayrımını ve diyalog zamanlarını kontrol edip yeniden deneyin veya yalnızca konuşmayı dışa aktarmayı seçin."
   }
 }

frontend/src/i18n/locales/uk.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Мовлення відсутнє або не читається. Перед експортом повторно згенеруйте цей сегмент.",
-    "timingOverflow": "Мовлення перевищує відведений час. Скоротіть переклад або виберіть строгий інтервал чи розтягнення відео."
+    "timingOverflow": "Мовлення перевищує відведений час. Скоротіть переклад або виберіть строгий інтервал чи розтягнення відео.",
+    "backgroundUnavailable": "Не вдалося зберегти оригінальний звук. Перевірте відокремлення фону й час реплік, потім повторіть спробу або виберіть експорт лише мовлення."
   }
 }

frontend/src/i18n/locales/vi.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "Lời nói bị thiếu hoặc không đọc được. Hãy tạo lại đoạn bị ảnh hưởng trước khi xuất.",
-    "timingOverflow": "Lời nói vượt quá thời gian được phân bổ. Hãy rút ngắn bản dịch hoặc chọn khung thời gian nghiêm ngặt hay kéo dài video."
+    "timingOverflow": "Lời nói vượt quá thời gian được phân bổ. Hãy rút ngắn bản dịch hoặc chọn khung thời gian nghiêm ngặt hay kéo dài video.",
+    "backgroundUnavailable": "Không thể giữ âm thanh gốc. Hãy kiểm tra việc tách âm nền và thời gian hội thoại rồi thử lại, hoặc chọn chỉ xuất giọng nói."
   }
 }

frontend/src/i18n/locales/zh-CN.json

@@ -3316,6 +3316,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "语音缺失或无法读取。请在导出前重新生成受影响的片段。",
-    "timingOverflow": "语音超出分配的时长。请缩短译文，或选择严格时间段或拉伸视频。"
+    "timingOverflow": "语音超出分配的时长。请缩短译文，或选择严格时间段或拉伸视频。",
+    "backgroundUnavailable": "无法保留原始声音。请检查背景音分离和对白时间后重试，或明确选择仅导出语音。"
   }
 }

frontend/src/i18n/locales/zh-TW.json

@@ -2835,6 +2835,7 @@
   },
   "dubIntegrity": {
     "missingSpeech": "語音缺失或無法讀取。請在匯出前重新產生受影響的片段。",
-    "timingOverflow": "語音超出分配的時長。請縮短譯文，或選擇嚴格時間區段或延展影片。"
+    "timingOverflow": "語音超出分配的時長。請縮短譯文，或選擇嚴格時間區段或延展影片。",
+    "backgroundUnavailable": "無法保留原始聲音。請檢查背景音分離和對白時間後重試，或明確選擇僅匯出語音。"
   }
 }

frontend/src/pages/DubTab.jsx

@@ -552,7 +552,7 @@ export default function DubTab(props) {
   // previewMode is 'original' or a dubbed language code (multi-language switcher).
   const previewIsDub = previewMode !== 'original' && hasDubbedTrack;
   const videoSrc = previewIsDub
-    ? `${API}/dub/preview-video/${dubJobId}?lang=${encodeURIComponent(previewMode)}&preserve_bg=${preserveBg ? 1 : 0}&v=${dubGenNonce}`
+    ? `${API}/dub/preview-video/${dubJobId}?mix=surgical2&lang=${encodeURIComponent(previewMode)}&preserve_bg=${preserveBg ? 1 : 0}&v=${dubGenNonce}`
     : `${API}/dub/media/${dubJobId}`;
   // The video is the normal transport, but WaveSurfer can fall back to a
   // companion audio element when a WebView decodes the picture without its

frontend/src/utils/download.js

@@ -5,6 +5,7 @@
 // native save dialog instead; calling that dialog when no Tauri runtime is
 // present throws "Cannot read properties of undefined (reading 'invoke')"
 // (issue #256), so callers must guard on isTauri and route here otherwise.
+import i18next from 'i18next';
 import { apiFetch } from '../api/client';
 
 /**
@@ -40,7 +41,12 @@ export async function browserDownload(url, fallbackName, deps = {}) {
   const urlApi = deps.url ?? globalThis.URL;
 
   const response = await _fetch(url);
-  if (!response.ok) throw new Error('Download failed');
+  if (!response.ok) {
+    const body = typeof response.json === 'function' ? await response.json().catch(() => null) : null;
+    if (body?.detail?.code === 'dub_background_unavailable')
+      throw new Error(i18next.t('dubIntegrity.backgroundUnavailable'));
+    throw new Error('Download failed');
+  }
 
   const serverName = parseFilenameFromContentDisposition(
     response.headers?.get?.('content-disposition'),

tests/test_dub_audio_export.py

@@ -78,7 +78,7 @@ def test_background_mix_preserves_bed_level_and_bandwidth(monkeypatch):
     s = _flat(cmd)
     assert f"aresample={fu.BED_MIX_SAMPLE_RATE}" in s, "bed bandwidth collapses to the 24kHz voice rate"
     assert "normalize=0" in s, "amix normalization not disabled — bed level depends on stream lifetimes"
-    assert f"volume={fu.BED_GAIN:g}" in s and f"volume={fu.VOICE_GAIN:g}" in s
+    assert "volume=1[" in s and f"volume={fu.VOICE_GAIN:g}" in s
     assert fu.BED_GAIN >= 0.85, "bed gain drifted away from 'almost like the original'"
     assert "alimiter" in s  # full-scale mixing needs the peak guard
 

tests/test_dub_export_unique.py

@@ -42,6 +42,11 @@ def app_client(tmp_path, monkeypatch):
     import main as _main
     importlib.reload(_main)
 
+    import services.dub_background as background
+    async def fake_background(source, separated, *args):
+        return separated
+    monkeypatch.setattr(background, "surgical_background", fake_background)
+
     from fastapi.testclient import TestClient
     with TestClient(_main.app) as client:
         yield client, _dc, _dx, tmp_path
@@ -68,7 +73,7 @@ def _seed_job_with_tracks(dc, tmp_path: Path):
         "no_vocals_path": str(bg_wav),
         "duration": 1.0,
         "filename": "clip.mp4",
-        "segments": [],
+        "segments": [{"start": 0.1, "end": 0.8}],
         "dubbed_tracks": {"es": {"path": str(track_wav), "language": "Spanish", "language_code": "es"}},
         "scene_cuts": [],
     }
@@ -127,7 +132,7 @@ class TestDubExportUniqueness:
         assert response.headers["accept-ranges"] == "bytes"
         assert response.content == b""
         run_ffmpeg.assert_not_called()
-        assert not (job_dir / "exports" / "preview_v2_es_bg.mp4").exists()
+        assert not (job_dir / "exports" / "preview_v2_es_surgical_v2_no_vocals.mp4").exists()
 
     def test_preview_is_faststart_and_reuses_immutable_cache(self, app_client):
         client, dc, dx, tmp = app_client
@@ -152,7 +157,7 @@ class TestDubExportUniqueness:
         assert commands[0][commands[0].index("-movflags") + 1] == "+faststart"
         assert first.headers["cache-control"] == "private, max-age=31536000, immutable"
         assert first.headers["accept-ranges"] == "bytes"
-        assert (job_dir / "exports" / "preview_v2_es_bg.mp4").is_file()
+        assert (job_dir / "exports" / "preview_v2_es_surgical_v2_no_vocals.mp4").is_file()
 
     def test_original_only_resolves_stale_default_before_retime(self, app_client):
         client, dc, dx, tmp = app_client

tests/test_dub_surgical_background.py

@@ -0,0 +1,69 @@
+"""Regression: separation must not erase audience reactions between dialogue."""
+import asyncio
+import numpy as np
+import pytest
+import soundfile as sf
+
+from services.dub_background import dialogue_intervals, splice_background, surgical_background
+
+
+def test_original_stereo_samples_survive_outside_dialogue(tmp_path):
+    sr = 48000
+    rng = np.random.default_rng(42)
+    original = rng.uniform(-.4, .4, (sr*3, 2)).astype('float32')
+    bed = np.full_like(original, .02)
+    src, bg, out = [str(tmp_path/p) for p in ('src.wav', 'bg.wav', 'out.wav')]
+    sf.write(src, original, sr, subtype='FLOAT')
+    sf.write(bg, bed, sr, subtype='FLOAT')
+    splice_background(src, bg, out, [(1, 2)])
+    mixed, _ = sf.read(out, dtype='float32')
+    np.testing.assert_array_equal(mixed[:sr], original[:sr])
+    np.testing.assert_array_equal(mixed[2*sr:], original[2*sr:])
+    np.testing.assert_array_equal(mixed[sr+480:2*sr-480], bed[sr+480:2*sr-480])
+    assert np.isfinite(mixed).all()
+
+
+def test_overlap_is_one_replacement_region():
+    assert dialogue_intervals([{'start':2,'end':3},{'start':1,'end':2.5}]) == [(1,3)]
+
+
+@pytest.mark.parametrize('end', [0, float('nan'), float('inf')])
+def test_invalid_timing_is_rejected(end):
+    with pytest.raises(ValueError):
+        dialogue_intervals([{'start':0,'end':end}])
+
+
+def test_incomplete_background_is_not_silently_padded(tmp_path):
+    src, bg, out = [str(tmp_path/p) for p in ('src.wav', 'bg.wav', 'out.wav')]
+    sf.write(src, np.ones((48000,2))*.1, 48000)
+    sf.write(bg, np.ones((100,2))*.02, 48000)
+    with pytest.raises(ValueError, match='incomplete'):
+        splice_background(src, bg, out, [(0,1)])
+
+
+def test_real_ffmpeg_retime_and_cache_invalidation(tmp_path):
+    sr = 48000
+    src, bg = [str(tmp_path/p) for p in ('src.wav', 'bg.wav')]
+    wave = np.ones((sr*3,2), dtype='float32')*.1
+    sf.write(src, wave, sr, subtype='FLOAT')
+    sf.write(bg, wave*.2, sr, subtype='FLOAT')
+    async def run():
+        segments=[{'start':1,'end':2}]
+        plain=await surgical_background(src,bg,str(tmp_path),segments,[],3)
+        assert await surgical_background(src,bg,str(tmp_path),segments,[],3) == plain
+        changed=await surgical_background(src,bg,str(tmp_path),[{'start':.5,'end':2}],[],3)
+        assert changed != plain
+        retimed=await surgical_background(src,bg,str(tmp_path),segments,[{'orig_start':1,'orig_end':2,'stretch_ratio':2}],3)
+        assert sf.info(retimed).duration == pytest.approx(4, abs=.01)
+        values,_=sf.read(retimed)
+        assert values[int(3.5*sr),0] == pytest.approx(.1,abs=.001)
+    asyncio.run(run())
+
+
+def test_missing_separation_blocks_preserved_export(monkeypatch):
+    import api.routers.dub_export as de
+    from fastapi import HTTPException
+    monkeypatch.setattr(de, '_optional_dub_artifact', lambda *_: None)
+    with pytest.raises(HTTPException) as error:
+        asyncio.run(de._preserved_background({}, 'job', 'bn'))
+    assert error.value.status_code == 409