diff --git a/apps/worker/video_processing/bgm_mixer_pure.py b/apps/worker/video_processing/bgm_mixer_pure.py
new file mode 100755
index 000000000..cd3872f48
--- /dev/null
+++ b/apps/worker/video_processing/bgm_mixer_pure.py
@@ -0,0 +1,351 @@
+"""BGM 混音纯逻辑模块.
+
+所有函数均为纯函数，不调用 FFmpeg、不操作文件。
+便于单元测试，也方便被其他模块复用。
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Optional
+
+
+@dataclass
+class BGMPureConfig:
+    """BGM 混音配置（纯数据类）."""
+
+    volume: float = 0.3
+    fade_in: float = 0.0
+    fade_out: float = 0.0
+    loop_enabled: bool = True
+    sidechain_enabled: bool = False
+    sidechain_ratio: float = 0.3
+    sidechain_attack: float = 0.02
+    sidechain_release: float = 0.5
+    sidechain_threshold: float = -25.0
+
+
+# ── 循环判断与计算 ─────────────────────────────────────────────────────────────
+
+
+def should_loop_bgm(
+    bgm_duration: float,
+    target_duration: float,
+    loop_enabled: bool = True,
+) -> bool:
+    """判断是否需要循环 BGM.
+
+    当 BGM 时长小于目标时长的 90% 时才循环，
+    避免 BGM 只差一点点就铺满还要循环一次的情况。
+
+    Args:
+        bgm_duration: BGM 原始时长（秒）
+        target_duration: 目标时长（秒）
+        loop_enabled: 是否允许循环
+
+    Returns:
+        是否需要循环
+    """
+    if not loop_enabled:
+        return False
+    if bgm_duration <= 0:
+        return False
+    if target_duration <= 0:
+        return False
+    return bgm_duration < target_duration * 0.9
+
+
+def calculate_loop_count(bgm_duration: float, target_duration: float) -> int:
+    """计算需要循环的次数.
+
+    多算 2 次作为余量，避免末尾因为精度问题不够长。
+
+    Args:
+        bgm_duration: BGM 原始时长（秒）
+        target_duration: 目标时长（秒）
+
+    Returns:
+        循环次数，至少 1
+    """
+    if bgm_duration <= 0:
+        return 1
+    if target_duration <= 0:
+        return 1
+    if bgm_duration >= target_duration:
+        return 1
+    return max(1, int(target_duration / bgm_duration) + 2)
+
+
+# ── BGM 预处理滤镜链构建 ──────────────────────────────────────────────────────
+
+
+def build_bgm_filter_chain(
+    bgm_duration: float,
+    target_duration: float,
+    volume: float = 0.3,
+    fade_in: float = 0.0,
+    fade_out: float = 0.0,
+    loop_enabled: bool = True,
+) -> str:
+    """构建 BGM 预处理滤镜链.
+
+    处理顺序：循环 → 音量 → 淡入 → 淡出 → 截断 → 重置时间戳
+
+    Args:
+        bgm_duration: BGM 原始时长（秒）
+        target_duration: 目标时长（秒）
+        volume: 音量 0.0~1.0
+        fade_in: 淡入时长（秒）
+        fade_out: 淡出时长（秒）
+        loop_enabled: 是否允许循环
+
+    Returns:
+        FFmpeg filter_complex 字符串（逗号分隔）
+    """
+    # 兜底：目标时长不能为 0 或负数
+    safe_target = max(5.0, target_duration) if target_duration <= 0 else target_duration
+
+    filter_parts: list[str] = []
+
+    # 1. 循环
+    needs_loop = should_loop_bgm(bgm_duration, safe_target, loop_enabled)
+    if needs_loop:
+        loop_count = calculate_loop_count(bgm_duration, safe_target)
+        filter_parts.append(f"aloop=loop={loop_count}:size=0")
+
+    # 2. 音量调节（钳制到 0~1）
+    safe_volume = max(0.0, min(1.0, volume))
+    if abs(safe_volume - 1.0) > 0.001:
+        filter_parts.append(f"volume={safe_volume:.3f}")
+
+    # 3. 淡入
+    if fade_in > 0:
+        filter_parts.append(f"afade=t=in:st=0:d={fade_in:.3f}")
+
+    # 4. 淡出（从 target_duration - fade_out 开始）
+    if fade_out > 0 and safe_target > fade_out:
+        fade_start = safe_target - fade_out
+        filter_parts.append(f"afade=t=out:st={fade_start:.3f}:d={fade_out:.3f}")
+
+    # 5. 截断到目标时长
+    filter_parts.append(f"atrim=0:{safe_target:.3f}")
+
+    # 6. 重置时间戳
+    filter_parts.append("asetpts=N/SR/TB")
+
+    return ",".join(filter_parts)
+
+
+# ── 混音滤镜构建 ──────────────────────────────────────────────────────────────
+
+
+def calculate_sidechain_ratio(sidechain_ratio: float) -> float:
+    """计算 sidechain 压缩比.
+
+    sidechain_ratio 表示闪避时 BGM 音量降低比例（0~1），
+    映射到 FFmpeg sidechaincompress 的 ratio 参数（2:1 ~ 10:1）。
+
+    ratio = 1 / (1 - sidechain_ratio)
+
+    Args:
+        sidechain_ratio: 闪避比例 0.0~1.0
+
+    Returns:
+        FFmpeg ratio 值（2.0 ~ 10.0）
+    """
+    if sidechain_ratio <= 0:
+        return 2.0
+    if sidechain_ratio >= 1.0:
+        return 10.0
+    raw_ratio = 1.0 / (1.0 - sidechain_ratio)
+    return max(2.0, min(10.0, raw_ratio))
+
+
+def build_simple_mix_filter() -> str:
+    """构建普通 amix 混音滤镜.
+
+    两路输入：[0:a] 主音频，[1:a] BGM
+    主音频权重 1.0，BGM 已在预处理阶段调好音量。
+    amix 会自动归一化，用 volume=2 补偿衰减。
+
+    Returns:
+        filter_complex 字符串
+    """
+    return "[0:a][1:a]amix=inputs=2:duration=first:dropout_transition=0[outa];" "[outa]volume=2[final]"
+
+
+def build_sidechain_mix_filter(
+    threshold: float = -25.0,
+    ratio: float = 0.3,
+    attack: float = 0.02,
+    release: float = 0.5,
+) -> str:
+    """构建 sidechain 人声闪避混音滤镜.
+
+    流程：
+    1. BGM[1:a] 经过 sidechaincompress，用主音频[0:a]做触发
+    2. 主音频 + 压缩后的 BGM amix 混音
+    3. volume=1.5 轻微补偿
+
+    Args:
+        threshold: 触发阈值（dB）
+        ratio: 闪避比例 0.0~1.0（会被转换为 FFmpeg ratio）
+        attack: 攻击时间（秒）
+        release: 释放时间（秒）
+
+    Returns:
+        filter_complex 字符串
+    """
+    ffmpeg_ratio = calculate_sidechain_ratio(ratio)
+
+    return (
+        f"[1:a][0:a]sidechaincompress="
+        f"threshold={threshold}dB:"
+        f"ratio={ffmpeg_ratio:.1f}:"
+        f"attack={attack:.3f}:"
+        f"release={release:.3f}:"
+        f"knee=6[bgm_comp];"
+        f"[0:a][bgm_comp]amix=inputs=2:duration=first:dropout_transition=0[outa];"
+        f"[outa]volume=1.5[final]"
+    )
+
+
+# ── 配置验证与规范化 ──────────────────────────────────────────────────────────
+
+
+def normalize_bgm_config(config: dict) -> dict:
+    """规范化 BGM 配置字典.
+
+    将各种类型的输入值转换为正确的类型，
+    并进行边界钳制。
+
+    Args:
+        config: 原始配置字典
+
+    Returns:
+        规范化后的配置字典
+    """
+    result: dict = {}
+
+    # volume: 0.0 ~ 1.0
+    result["volume"] = max(0.0, min(1.0, float(config.get("volume", 0.3))))
+
+    # fade_in: >= 0
+    result["fade_in"] = max(0.0, float(config.get("fade_in", 0.0)))
+
+    # fade_out: >= 0
+    result["fade_out"] = max(0.0, float(config.get("fade_out", 0.0)))
+
+    # loop_enabled: bool
+    result["loop_enabled"] = bool(config.get("loop_enabled", True))
+
+    # sidechain_enabled: bool
+    result["sidechain_enabled"] = bool(config.get("sidechain_enabled", False))
+
+    # sidechain_ratio: 0.0 ~ 1.0
+    result["sidechain_ratio"] = max(0.0, min(1.0, float(config.get("sidechain_ratio", 0.3))))
+
+    # sidechain_attack: > 0
+    result["sidechain_attack"] = max(0.001, float(config.get("sidechain_attack", 0.02)))
+
+    # sidechain_release: > 0
+    result["sidechain_release"] = max(0.01, float(config.get("sidechain_release", 0.5)))
+
+    # sidechain_threshold: dB
+    result["sidechain_threshold"] = float(config.get("sidechain_threshold", -25.0))
+
+    return result
+
+
+def validate_bgm_config(config: dict) -> tuple[bool, list[str]]:
+    """验证 BGM 配置是否合法.
+
+    Args:
+        config: 配置字典
+
+    Returns:
+        (是否合法, 错误信息列表)
+    """
+    errors: list[str] = []
+
+    volume = config.get("volume", 0.3)
+    if not isinstance(volume, (int, float)):
+        errors.append("volume 必须是数字")
+    elif volume < 0 or volume > 1:
+        errors.append("volume 必须在 0~1 之间")
+
+    fade_in = config.get("fade_in", 0)
+    if not isinstance(fade_in, (int, float)):
+        errors.append("fade_in 必须是数字")
+    elif fade_in < 0:
+        errors.append("fade_in 不能为负数")
+
+    fade_out = config.get("fade_out", 0)
+    if not isinstance(fade_out, (int, float)):
+        errors.append("fade_out 必须是数字")
+    elif fade_out < 0:
+        errors.append("fade_out 不能为负数")
+
+    sidechain_ratio = config.get("sidechain_ratio", 0.3)
+    if not isinstance(sidechain_ratio, (int, float)):
+        errors.append("sidechain_ratio 必须是数字")
+    elif sidechain_ratio < 0 or sidechain_ratio > 1:
+        errors.append("sidechain_ratio 必须在 0~1 之间")
+
+    return (len(errors) == 0, errors)
+
+
+# ── 时长相关工具 ──────────────────────────────────────────────────────────────
+
+
+def calculate_fade_out_start(
+    target_duration: float,
+    fade_out: float,
+) -> Optional[float]:
+    """计算淡出开始时间.
+
+    如果淡出时长大于等于目标时长，返回 None（不做淡出）。
+
+    Args:
+        target_duration: 目标时长（秒）
+        fade_out: 淡出时长（秒）
+
+    Returns:
+        淡出开始时间（秒），如果不需要淡出返回 None
+    """
+    if fade_out <= 0:
+        return None
+    if target_duration <= 0:
+        return None
+    if fade_out >= target_duration:
+        return None
+    return target_duration - fade_out
+
+
+def estimate_bgm_processing_duration(
+    bgm_duration: float,
+    target_duration: float,
+    loop_enabled: bool = True,
+) -> float:
+    """估算 BGM 预处理后的实际输出时长.
+
+    正常情况下应该等于 target_duration，
+    但在某些边界情况下可能不同。
+
+    Args:
+        bgm_duration: BGM 原始时长
+        target_duration: 目标时长
+        loop_enabled: 是否允许循环
+
+    Returns:
+        预估输出时长（秒）
+    """
+    if target_duration <= 0:
+        return 5.0  # 兜底时长
+
+    # 不需要循环的情况：如果 BGM 够长，截断到 target_duration
+    if not loop_enabled and bgm_duration >= target_duration:
+        return target_duration
+
+    # 需要循环或 BGM 太短：截断到 target_duration
+    return target_duration
diff --git a/tests/unit/test_bgm_mixer_pure.py b/tests/unit/test_bgm_mixer_pure.py
new file mode 100755
index 000000000..1b4d43ed1
--- /dev/null
+++ b/tests/unit/test_bgm_mixer_pure.py
@@ -0,0 +1,658 @@
+"""BGM 混音纯逻辑单元测试."""
+
+from __future__ import annotations
+
+import pytest
+from video_processing.bgm_mixer_pure import (
+    BGMPureConfig,
+    build_bgm_filter_chain,
+    build_sidechain_mix_filter,
+    build_simple_mix_filter,
+    calculate_fade_out_start,
+    calculate_loop_count,
+    calculate_sidechain_ratio,
+    estimate_bgm_processing_duration,
+    normalize_bgm_config,
+    should_loop_bgm,
+    validate_bgm_config,
+)
+
+# ─────────────────────────────────────────────────────────────────────────────
+# should_loop_bgm 测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestShouldLoopBGM:
+    """BGM 循环判断测试."""
+
+    def test_need_loop_when_much_shorter(self):
+        """BGM 远短于目标时长，需要循环."""
+        assert should_loop_bgm(10, 100, True) is True
+
+    def test_no_loop_when_long_enough(self):
+        """BGM 够长，不需要循环."""
+        assert should_loop_bgm(100, 100, True) is False
+
+    def test_no_loop_when_just_slightly_shorter(self):
+        """BGM 只差一点点（>90%），不循环."""
+        assert should_loop_bgm(95, 100, True) is False
+
+    def test_threshold_90_percent(self):
+        """刚好 90% 阈值，不循环（<90% 才循环）."""
+        assert should_loop_bgm(90, 100, True) is False
+
+    def test_just_below_threshold(self):
+        """略低于 90%，需要循环."""
+        assert should_loop_bgm(89, 100, True) is True
+
+    def test_loop_disabled(self):
+        """禁用循环，即使 BGM 很短也不循环."""
+        assert should_loop_bgm(10, 100, False) is False
+
+    def test_zero_bgm_duration(self):
+        """BGM 时长为 0，不循环."""
+        assert should_loop_bgm(0, 100, True) is False
+
+    def test_negative_bgm_duration(self):
+        """BGM 时长为负，不循环."""
+        assert should_loop_bgm(-5, 100, True) is False
+
+    def test_zero_target_duration(self):
+        """目标时长为 0，不循环."""
+        assert should_loop_bgm(10, 0, True) is False
+
+    def test_negative_target_duration(self):
+        """目标时长为负，不循环."""
+        assert should_loop_bgm(10, -10, True) is False
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# calculate_loop_count 测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestCalculateLoopCount:
+    """循环次数计算测试."""
+
+    def test_exact_multiple(self):
+        """刚好整数倍."""
+        # 100/10 = 10, +2 = 12
+        assert calculate_loop_count(10, 100) == 12
+
+    def test_not_exact_multiple(self):
+        """不是整数倍."""
+        # 100/30 = 3, +2 = 5
+        assert calculate_loop_count(30, 100) == 5
+
+    def test_bgm_longer_than_target(self):
+        """BGM 比目标长，至少 1 次."""
+        assert calculate_loop_count(200, 100) == 1
+
+    def test_zero_bgm_duration(self):
+        """BGM 时长为 0，返回 1."""
+        assert calculate_loop_count(0, 100) == 1
+
+    def test_negative_bgm_duration(self):
+        """BGM 时长为负，返回 1."""
+        assert calculate_loop_count(-5, 100) == 1
+
+    def test_zero_target_duration(self):
+        """目标时长为 0，返回 1."""
+        assert calculate_loop_count(10, 0) == 1
+
+    def test_negative_target_duration(self):
+        """目标时长为负，返回 1."""
+        assert calculate_loop_count(10, -10) == 1
+
+    def test_very_short_bgm(self):
+        """非常短的 BGM，循环次数多."""
+        # 100/1 = 100, +2 = 102
+        assert calculate_loop_count(1, 100) == 102
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# build_bgm_filter_chain 测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestBuildBGMFilterChain:
+    """BGM 预处理滤镜链构建测试."""
+
+    def test_basic_volume_only(self):
+        """只有音量调节."""
+        result = build_bgm_filter_chain(
+            bgm_duration=200,
+            target_duration=100,
+            volume=0.5,
+        )
+        assert "volume=0.500" in result
+        assert "aloop" not in result
+        assert "afade=t=in" not in result
+        assert "afade=t=out" not in result
+        assert "atrim=0:100.000" in result
+        assert "asetpts=N/SR/TB" in result
+
+    def test_with_loop(self):
+        """需要循环的情况."""
+        result = build_bgm_filter_chain(
+            bgm_duration=10,
+            target_duration=100,
+            volume=0.3,
+            loop_enabled=True,
+        )
+        assert "aloop=loop=" in result
+        assert "volume=0.300" in result
+
+    def test_no_loop_when_disabled(self):
+        """禁用循环，即使 BGM 短也不循环."""
+        result = build_bgm_filter_chain(
+            bgm_duration=10,
+            target_duration=100,
+            volume=0.3,
+            loop_enabled=False,
+        )
+        assert "aloop" not in result
+
+    def test_fade_in_only(self):
+        """只有淡入."""
+        result = build_bgm_filter_chain(
+            bgm_duration=200,
+            target_duration=100,
+            volume=1.0,
+            fade_in=2.5,
+        )
+        assert "afade=t=in:st=0:d=2.500" in result
+        assert "afade=t=out" not in result
+        assert "volume=" not in result  # volume=1.0 不加
+
+    def test_fade_out_only(self):
+        """只有淡出."""
+        result = build_bgm_filter_chain(
+            bgm_duration=200,
+            target_duration=100,
+            volume=1.0,
+            fade_out=3.0,
+        )
+        assert "afade=t=out:st=97.000:d=3.000" in result
+        assert "afade=t=in" not in result
+
+    def test_fade_in_and_out(self):
+        """淡入+淡出."""
+        result = build_bgm_filter_chain(
+            bgm_duration=200,
+            target_duration=100,
+            volume=1.0,
+            fade_in=1.5,
+            fade_out=2.0,
+        )
+        assert "afade=t=in:st=0:d=1.500" in result
+        assert "afade=t=out:st=98.000:d=2.000" in result
+
+    def test_volume_1_0_skipped(self):
+        """音量为 1.0 时不添加 volume 滤镜."""
+        result = build_bgm_filter_chain(
+            bgm_duration=200,
+            target_duration=100,
+            volume=1.0,
+        )
+        assert "volume=" not in result
+
+    def test_volume_0(self):
+        """音量为 0."""
+        result = build_bgm_filter_chain(
+            bgm_duration=200,
+            target_duration=100,
+            volume=0.0,
+        )
+        assert "volume=0.000" in result
+
+    def test_volume_clamped_high(self):
+        """音量超过 1.0 被钳制."""
+        result = build_bgm_filter_chain(
+            bgm_duration=200,
+            target_duration=100,
+            volume=1.5,
+        )
+        assert "volume=1.000" not in result  # 1.0不加
+        # 钳制到1.0后和1.0一样，不加volume滤镜
+        # 但因为abs(1.0 - 1.0) < 0.001，所以不添加
+        assert "volume=" not in result
+
+    def test_volume_clamped_low(self):
+        """音量为负被钳制到 0."""
+        result = build_bgm_filter_chain(
+            bgm_duration=200,
+            target_duration=100,
+            volume=-0.5,
+        )
+        assert "volume=0.000" in result
+
+    def test_fade_out_longer_than_duration(self):
+        """淡出时长超过总时长，不加淡出."""
+        result = build_bgm_filter_chain(
+            bgm_duration=200,
+            target_duration=10,
+            volume=1.0,
+            fade_out=20.0,
+        )
+        assert "afade=t=out" not in result
+
+    def test_fade_out_equal_to_duration(self):
+        """淡出时长等于总时长，不加淡出."""
+        result = build_bgm_filter_chain(
+            bgm_duration=200,
+            target_duration=10,
+            volume=1.0,
+            fade_out=10.0,
+        )
+        assert "afade=t=out" not in result
+
+    def test_zero_target_duration_fallback(self):
+        """目标时长为 0，兜底 5 秒."""
+        result = build_bgm_filter_chain(
+            bgm_duration=3,
+            target_duration=0,
+            volume=0.5,
+        )
+        assert "atrim=0:5.000" in result
+
+    def test_negative_target_duration_fallback(self):
+        """目标时长为负，兜底 5 秒."""
+        result = build_bgm_filter_chain(
+            bgm_duration=3,
+            target_duration=-5,
+            volume=0.5,
+        )
+        assert "atrim=0:5.000" in result
+
+    def test_full_chain_with_all_effects(self):
+        """完整滤镜链：循环+音量+淡入淡出+截断+重置."""
+        result = build_bgm_filter_chain(
+            bgm_duration=10,
+            target_duration=100,
+            volume=0.4,
+            fade_in=1.0,
+            fade_out=2.0,
+            loop_enabled=True,
+        )
+        parts = result.split(",")
+        # 顺序：aloop -> volume -> afade in -> afade out -> atrim -> asetpts
+        assert len(parts) >= 6
+        assert "aloop" in parts[0]
+        assert "volume" in parts[1]
+        assert "afade=t=in" in parts[2]
+        assert "afade=t=out" in parts[3]
+        assert "atrim" in parts[4]
+        assert "asetpts" in parts[5]
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# calculate_sidechain_ratio 测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestCalculateSidechainRatio:
+    """Sidechain 压缩比计算测试."""
+
+    def test_default_ratio_0_3(self):
+        """默认 0.3."""
+        # 1 / (1 - 0.3) = 1.428... 但下限是 2.0
+        assert calculate_sidechain_ratio(0.3) == pytest.approx(2.0, rel=0.01)
+
+    def test_ratio_0_5(self):
+        """比例 0.5."""
+        # 1 / (1 - 0.5) = 2.0
+        assert calculate_sidechain_ratio(0.5) == pytest.approx(2.0, rel=0.01)
+
+    def test_ratio_0_8(self):
+        """比例 0.8."""
+        # 1 / (1 - 0.8) = 5.0
+        assert calculate_sidechain_ratio(0.8) == pytest.approx(5.0, rel=0.01)
+
+    def test_ratio_0_9(self):
+        """比例 0.9."""
+        # 1 / (1 - 0.9) = 10.0
+        assert calculate_sidechain_ratio(0.9) == pytest.approx(10.0, rel=0.01)
+
+    def test_ratio_0(self):
+        """比例 0，返回下限 2.0."""
+        assert calculate_sidechain_ratio(0.0) == 2.0
+
+    def test_ratio_negative(self):
+        """比例为负，返回下限 2.0."""
+        assert calculate_sidechain_ratio(-0.5) == 2.0
+
+    def test_ratio_1_0(self):
+        """比例 1.0，返回上限 10.0."""
+        assert calculate_sidechain_ratio(1.0) == 10.0
+
+    def test_ratio_greater_than_1(self):
+        """比例超过 1.0，返回上限 10.0."""
+        assert calculate_sidechain_ratio(2.0) == 10.0
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# build_simple_mix_filter 测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestBuildSimpleMixFilter:
+    """普通混音滤镜构建测试."""
+
+    def test_contains_amix(self):
+        """包含 amix."""
+        result = build_simple_mix_filter()
+        assert "amix=inputs=2" in result
+
+    def test_contains_volume_compensation(self):
+        """包含 volume=2 补偿."""
+        result = build_simple_mix_filter()
+        assert "volume=2" in result
+
+    def test_output_label(self):
+        """输出标签为 [final]."""
+        result = build_simple_mix_filter()
+        assert "[final]" in result
+
+    def test_duration_first(self):
+        """duration=first，以主音频时长为准."""
+        result = build_simple_mix_filter()
+        assert "duration=first" in result
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# build_sidechain_mix_filter 测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestBuildSidechainMixFilter:
+    """Sidechain 混音滤镜构建测试."""
+
+    def test_contains_sidechaincompress(self):
+        """包含 sidechaincompress."""
+        result = build_sidechain_mix_filter()
+        assert "sidechaincompress=" in result
+
+    def test_threshold_param(self):
+        """threshold 参数正确."""
+        result = build_sidechain_mix_filter(threshold=-30.0)
+        assert "threshold=-30.0dB" in result
+
+    def test_attack_param(self):
+        """attack 参数正确."""
+        result = build_sidechain_mix_filter(attack=0.05)
+        assert "attack=0.050" in result
+
+    def test_release_param(self):
+        """release 参数正确."""
+        result = build_sidechain_mix_filter(release=0.8)
+        assert "release=0.800" in result
+
+    def test_knee_param(self):
+        """knee=6 参数."""
+        result = build_sidechain_mix_filter()
+        assert "knee=6" in result
+
+    def test_contains_amix(self):
+        """包含 amix 混音."""
+        result = build_sidechain_mix_filter()
+        assert "amix=inputs=2" in result
+
+    def test_volume_compensation(self):
+        """volume=1.5 轻微补偿."""
+        result = build_sidechain_mix_filter()
+        assert "volume=1.5" in result
+
+    def test_bgmc_comp_label(self):
+        """包含 [bgm_comp] 中间标签."""
+        result = build_sidechain_mix_filter()
+        assert "[bgm_comp]" in result
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# normalize_bgm_config 测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestNormalizeBGMConfig:
+    """配置规范化测试."""
+
+    def test_empty_dict_defaults(self):
+        """空字典返回默认值."""
+        result = normalize_bgm_config({})
+        assert result["volume"] == 0.3
+        assert result["fade_in"] == 0.0
+        assert result["fade_out"] == 0.0
+        assert result["loop_enabled"] is True
+        assert result["sidechain_enabled"] is False
+        assert result["sidechain_ratio"] == 0.3
+
+    def test_volume_clamped(self):
+        """音量钳制."""
+        result = normalize_bgm_config({"volume": 1.5})
+        assert result["volume"] == 1.0
+        result2 = normalize_bgm_config({"volume": -0.5})
+        assert result2["volume"] == 0.0
+
+    def test_fade_in_negative(self):
+        """淡入为负钳制到 0."""
+        result = normalize_bgm_config({"fade_in": -1})
+        assert result["fade_in"] == 0.0
+
+    def test_fade_out_negative(self):
+        """淡出为负钳制到 0."""
+        result = normalize_bgm_config({"fade_out": -1})
+        assert result["fade_out"] == 0.0
+
+    def test_sidechain_ratio_clamped(self):
+        """sidechain_ratio 钳制."""
+        result = normalize_bgm_config({"sidechain_ratio": 1.5})
+        assert result["sidechain_ratio"] == 1.0
+        result2 = normalize_bgm_config({"sidechain_ratio": -0.1})
+        assert result2["sidechain_ratio"] == 0.0
+
+    def test_sidechain_attack_min(self):
+        """attack 最小值 0.001."""
+        result = normalize_bgm_config({"sidechain_attack": 0})
+        assert result["sidechain_attack"] == 0.001
+
+    def test_sidechain_release_min(self):
+        """release 最小值 0.01."""
+        result = normalize_bgm_config({"sidechain_release": 0})
+        assert result["sidechain_release"] == 0.01
+
+    def test_string_values_converted(self):
+        """字符串数值被转换."""
+        result = normalize_bgm_config(
+            {
+                "volume": "0.5",
+                "fade_in": "2.0",
+            }
+        )
+        assert result["volume"] == 0.5
+        assert result["fade_in"] == 2.0
+
+    def test_loop_enabled_truthy(self):
+        """loop_enabled 真值转换."""
+        result = normalize_bgm_config({"loop_enabled": 1})
+        assert result["loop_enabled"] is True
+        result2 = normalize_bgm_config({"loop_enabled": 0})
+        assert result2["loop_enabled"] is False
+
+    def test_preserves_unknown_keys(self):
+        """未知 key 不保留."""
+        result = normalize_bgm_config({"unknown_key": "value", "volume": 0.5})
+        assert "unknown_key" not in result
+        assert result["volume"] == 0.5
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# validate_bgm_config 测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestValidateBGMConfig:
+    """配置验证测试."""
+
+    def test_valid_config(self):
+        """合法配置."""
+        ok, errors = validate_bgm_config(
+            {
+                "volume": 0.5,
+                "fade_in": 1.0,
+                "fade_out": 2.0,
+                "sidechain_ratio": 0.3,
+            }
+        )
+        assert ok is True
+        assert len(errors) == 0
+
+    def test_volume_not_number(self):
+        """volume 不是数字."""
+        ok, errors = validate_bgm_config({"volume": "high"})
+        assert ok is False
+        assert any("volume" in e for e in errors)
+
+    def test_volume_out_of_range(self):
+        """volume 超出范围."""
+        ok, errors = validate_bgm_config({"volume": 1.5})
+        assert ok is False
+        assert any("volume" in e for e in errors)
+
+    def test_fade_in_negative(self):
+        """fade_in 为负."""
+        ok, errors = validate_bgm_config({"fade_in": -1})
+        assert ok is False
+        assert any("fade_in" in e for e in errors)
+
+    def test_fade_out_negative(self):
+        """fade_out 为负."""
+        ok, errors = validate_bgm_config({"fade_out": -1})
+        assert ok is False
+        assert any("fade_out" in e for e in errors)
+
+    def test_sidechain_ratio_out_of_range(self):
+        """sidechain_ratio 超出范围."""
+        ok, errors = validate_bgm_config({"sidechain_ratio": 2.0})
+        assert ok is False
+        assert any("sidechain_ratio" in e for e in errors)
+
+    def test_multiple_errors(self):
+        """多个错误同时报告."""
+        ok, errors = validate_bgm_config(
+            {
+                "volume": 2.0,
+                "fade_in": -1,
+                "sidechain_ratio": -0.5,
+            }
+        )
+        assert ok is False
+        assert len(errors) >= 3
+
+    def test_empty_config_valid(self):
+        """空配置（全用默认值）视为合法."""
+        ok, errors = validate_bgm_config({})
+        assert ok is True
+        assert len(errors) == 0
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# calculate_fade_out_start 测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestCalculateFadeOutStart:
+    """淡出开始时间计算测试."""
+
+    def test_normal_case(self):
+        """正常情况."""
+        assert calculate_fade_out_start(100, 3) == pytest.approx(97.0)
+
+    def test_zero_fade_out(self):
+        """淡出时长为 0，返回 None."""
+        assert calculate_fade_out_start(100, 0) is None
+
+    def test_negative_fade_out(self):
+        """淡出时长为负，返回 None."""
+        assert calculate_fade_out_start(100, -1) is None
+
+    def test_zero_duration(self):
+        """总时长为 0，返回 None."""
+        assert calculate_fade_out_start(0, 3) is None
+
+    def test_fade_out_longer_than_duration(self):
+        """淡出超过总时长，返回 None."""
+        assert calculate_fade_out_start(10, 20) is None
+
+    def test_fade_out_equal_to_duration(self):
+        """淡出等于总时长，返回 None."""
+        assert calculate_fade_out_start(10, 10) is None
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# estimate_bgm_processing_duration 测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestEstimateBGMProcessingDuration:
+    """BGM 处理时长估算测试."""
+
+    def test_normal_case_with_loop(self):
+        """正常循环情况，输出目标时长."""
+        assert estimate_bgm_processing_duration(10, 100, True) == 100
+
+    def test_bgm_longer_no_loop(self):
+        """BGM 够长，不循环，截断到目标时长."""
+        assert estimate_bgm_processing_duration(200, 100, False) == 100
+
+    def test_bgm_shorter_no_loop(self):
+        """BGM 短但不循环，仍然截断到目标时长（实际会更短，但 atrim 会截断）."""
+        assert estimate_bgm_processing_duration(10, 100, False) == 100
+
+    def test_zero_target(self):
+        """目标时长为 0，兜底 5 秒."""
+        assert estimate_bgm_processing_duration(10, 0, True) == 5.0
+
+    def test_negative_target(self):
+        """目标时长为负，兜底 5 秒."""
+        assert estimate_bgm_processing_duration(10, -5, True) == 5.0
+
+
+# ─────────────────────────────────────────────────────────────────────────────
+# BGMPureConfig 测试
+# ─────────────────────────────────────────────────────────────────────────────
+
+
+class TestBGMPureConfig:
+    """BGMPureConfig 数据类测试."""
+
+    def test_default_values(self):
+        """默认值正确."""
+        config = BGMPureConfig()
+        assert config.volume == 0.3
+        assert config.fade_in == 0.0
+        assert config.fade_out == 0.0
+        assert config.loop_enabled is True
+        assert config.sidechain_enabled is False
+        assert config.sidechain_ratio == 0.3
+        assert config.sidechain_attack == 0.02
+        assert config.sidechain_release == 0.5
+        assert config.sidechain_threshold == -25.0
+
+    def test_custom_values(self):
+        """自定义值."""
+        config = BGMPureConfig(
+            volume=0.7,
+            fade_in=1.0,
+            fade_out=2.0,
+            loop_enabled=False,
+            sidechain_enabled=True,
+            sidechain_ratio=0.5,
+            sidechain_attack=0.05,
+            sidechain_release=0.8,
+            sidechain_threshold=-30.0,
+        )
+        assert config.volume == 0.7
+        assert config.loop_enabled is False
+        assert config.sidechain_enabled is True
+        assert config.sidechain_threshold == -30.0