fix: 속도 조절을 length_scale에서 사후 타임스트레치로 분리(발음 뭉개짐 개선)

- 자연 속도(1.0)로 합성 후 Rubberband(R3)로 배속 → 발음/피치 보존
- 폴백: ffmpeg atempo → librosa. rubberband-cli/pyrubberband 추가
- 원인: MeloTTS speed는 length_scale 축소라 고배속에서 자음이 뭉개짐

app/engines/melo_engine.py

@@ -7,7 +7,11 @@
 from __future__ import annotations
 
 import io
+import os
 import re
+import shutil
+import subprocess
+import tempfile
 import threading
 
 import numpy as np
@@ -15,10 +19,15 @@ import soundfile as sf
 import torch
 
 try:
-    import librosa  # 피치 조절용
+    import librosa  # 피치 조절 / 폴백 타임스트레치
 except Exception:  # pragma: no cover
     librosa = None
 
+try:
+    import pyrubberband as prb  # 고품질 타임스트레치(발음 보존)
+except Exception:  # pragma: no cover
+    prb = None
+
 from melo.api import TTS
 
 from .base import BaseEngine
@@ -175,17 +184,79 @@ class MeloEngine(BaseEngine):
         return list(spk2id.values())[0]
 
     def _synth_segment(
-        self, text: str, lang: str, speaker: str | None, speed: float
+        self, text: str, lang: str, speaker: str | None
     ) -> tuple[np.ndarray, int]:
+        # 항상 자연 속도(1.0)로 합성 — 속도는 사후 타임스트레치로 처리해 발음 유지
         model = self._get_model(lang)
         speaker_id = self._resolve_speaker(model, lang, speaker)
         with self._global_lock:
             audio = model.tts_to_file(
-                text, speaker_id, output_path=None, speed=speed, quiet=True
+                text, speaker_id, output_path=None, speed=1.0, quiet=True
             )
         sr = model.hps.data.sampling_rate
         return np.asarray(audio, dtype=np.float32), sr
 
+    def _time_stretch(self, audio: np.ndarray, sr: int, rate: float) -> np.ndarray:
+        """오디오를 rate 배속으로 변경(피치 보존). rate>1 = 빠르게.
+
+        MeloTTS 의 length_scale 방식(speed 파라미터)은 배속을 올리면 발음이
+        뭉개지므로, 자연 속도로 합성한 뒤 여기서 고품질 타임스트레치를 적용한다.
+        우선순위: Rubberband(R3) → ffmpeg atempo → librosa(phase vocoder).
+        """
+        if abs(rate - 1.0) < 1e-3:
+            return audio
+        audio = np.asarray(audio, dtype=np.float32)
+
+        # 1) Rubberband R3 (발음/포먼트 보존이 가장 좋음)
+        if prb is not None and shutil.which("rubberband"):
+            try:
+                out = prb.time_stretch(
+                    audio, sr, rate, rbargs={"--engine": "finer"}
+                )
+                return np.asarray(out, dtype=np.float32)
+            except Exception:
+                try:
+                    out = prb.time_stretch(audio, sr, rate)
+                    return np.asarray(out, dtype=np.float32)
+                except Exception:
+                    pass
+
+        # 2) ffmpeg atempo (추가 의존성 없음, 무난)
+        if shutil.which("ffmpeg") and 0.5 <= rate <= 2.0:
+            try:
+                return self._atempo(audio, sr, rate)
+            except Exception:
+                pass
+
+        # 3) librosa phase vocoder (최후 폴백)
+        if librosa is not None:
+            try:
+                return librosa.effects.time_stretch(audio, rate).astype(np.float32)
+            except Exception:
+                try:
+                    return librosa.effects.time_stretch(
+                        audio, rate=rate
+                    ).astype(np.float32)
+                except Exception:
+                    pass
+        return audio
+
+    @staticmethod
+    def _atempo(audio: np.ndarray, sr: int, rate: float) -> np.ndarray:
+        with tempfile.TemporaryDirectory() as d:
+            src = os.path.join(d, "in.wav")
+            dst = os.path.join(d, "out.wav")
+            sf.write(src, audio, sr, format="WAV", subtype="PCM_16")
+            subprocess.run(
+                [
+                    "ffmpeg", "-hide_banner", "-loglevel", "error", "-y",
+                    "-i", src, "-filter:a", f"atempo={rate:.4f}", dst,
+                ],
+                check=True,
+            )
+            out, _ = sf.read(dst, dtype="float32")
+            return np.asarray(out, dtype=np.float32)
+
     # ---- 합성 -----------------------------------------------------
     def synth_wav(
         self,
@@ -215,7 +286,7 @@ class MeloEngine(BaseEngine):
             gap = None
             for seg_lang, seg_text in parts:
                 seg_spk = spk_by_lang.get(seg_lang, en_speaker)
-                audio, seg_sr = self._synth_segment(seg_text, seg_lang, seg_spk, speed)
+                audio, seg_sr = self._synth_segment(seg_text, seg_lang, seg_spk)
                 if sr is None:
                     sr = seg_sr
                     gap = np.zeros(int(sr * 0.06), dtype=np.float32)
@@ -229,7 +300,11 @@ class MeloEngine(BaseEngine):
                 audios.append(audio)
             audio = np.concatenate(audios) if audios else np.zeros(1, dtype=np.float32)
         else:
-            audio, sr = self._synth_segment(text, language, speaker, speed)
+            audio, sr = self._synth_segment(text, language, speaker)
+
+        # 속도: 자연 속도 합성 결과에 고품질 타임스트레치 적용(발음 유지)
+        if abs(speed - 1.0) > 1e-3:
+            audio = self._time_stretch(audio, sr, speed)
 
         if abs(pitch) > 1e-3 and librosa is not None:
             audio = librosa.effects.pitch_shift(audio, sr, n_steps=pitch)