diff --git a/docs/filters.md b/docs/filters.md
index 48ae3b3..f357b71 100644
--- a/docs/filters.md
+++ b/docs/filters.md
@@ -21,6 +21,9 @@
 
 - **每个级由自己的值开关**：`flow_ema_alpha >= 1`、`force_median_window <= 1`、
   `force_lowpass_cutoff_hz <= 0` 都是"关"。
+- **配置没写的键 = 关**：缺省不是"我们觉得合理的那档"，而是"不滤波"。因此**没有 `filters`
+  段的配置文件什么都不滤**，出厂 bundle 也把三个值显式写成全关 —— 想让哪一级跑起来，
+  必须在文件里写出来（数值或 `preset`）。这样"文件说的"和"设备跑的"才总是一致。
 - **非法值 → 关**，绝不回落到"默认开"。
 - `filters.enabled` 是**废弃别名**，显式 `false` 表示"三级全关"；没有任何开关能覆盖已写的值。
 - `filters.preset` 可引用下面的预设；**显式键优先于预设值**（可从预设出发只改一个旋钮）。
@@ -32,7 +35,7 @@
 |---|---|---|---|---|---|
 | `off` | 1.0 | 1 | 0.0 | 0 帧 | 原始读数；文件说什么就跑什么 |
 | `low_latency` | 1.0 | 3 | 0.0 | ≈1 帧（33 ms） | 延迟敏感，只去尖峰 |
-| `balanced` | 0.5 | 3 | 0.0 | ≈2 帧（67 ms） | 默认建议：EMA 干掉力级滤波够不到的光流抖动 |
+| `balanced` | 0.5 | 3 | 0.0 | ≈2 帧（67 ms） | 建议起步档：EMA 干掉力级滤波够不到的光流抖动 |
 | `low_noise` | 0.3 | 3 | 10.0 | ≈3 帧（100 ms） | 底噪优先，读数再低通一道 |
 
 延迟换算 @60fps 减半；数字来自 `_perf/`（对照表见 `orisys-sdk-perf-plan.md`）。
diff --git a/examples/filters/README.md b/examples/filters/README.md
index 64f2c96..6340b81 100644
--- a/examples/filters/README.md
+++ b/examples/filters/README.md
@@ -4,6 +4,8 @@
 到 `sensors/<name>/sensor.json` 的 `"filters"` 段即可。原有的 `filters` 段请整段替换，
 不要只改其中一行（避免出现 `preset` 与数值混写的歧义——显式键优先于预设值，混写容易看错）。
 
+配置里**没写**的滤波键等于"关"：整段删掉就是什么都不滤，和 `off` 那段等价。
+
 背景、代价与可追溯性约定见 `docs/filters.md`；延迟/噪声实测见 `_perf/` 与
 `orisys-sdk-perf-plan.md`。
 
diff --git a/src/orisys/sensor.py b/src/orisys/sensor.py
index 452756e..7ec6b54 100644
--- a/src/orisys/sensor.py
+++ b/src/orisys/sensor.py
@@ -32,16 +32,24 @@ from .signal_processing import (
 )
 
 # Default signal-conditioning settings (config: top-level "filters" block).
-# Measured on dd02-0703 at 280x300 (see the noise analysis in the perf notes):
-# flow EMA 0.5 + median 3 costs ~2 frames at 30 fps, cuts steady-press force
-# jitter by roughly half and removes ~85% of the heavy-tailed spikes, while also
-# stabilising vnormal/contact (which a force-level filter cannot do).
-# The force low-pass is OFF by default: on top of the EMA it added no measurable
-# noise benefit at the same latency, so it is kept as an opt-in stage for sites
-# with different noise characteristics.
+#
+# A key a config does not state is **off**, not "whatever we thought was
+# sensible": the file has to name what runs, or a device can be filtering a
+# measurement nobody asked it to filter and no reader of the file could tell.
+# The shipped bundles state all three values explicitly, so they are unaffected
+# by these defaults; a config with no "filters" block at all filters nothing.
+#
+# Which settings are worth asking for is measured, not guessed. On dd02-0703 at
+# 280x300 (see the noise analysis in the perf notes) flow EMA 0.5 + median 3
+# costs ~2 frames at 30 fps and cuts steady-press force jitter by roughly half
+# while also stabilising vnormal/contact (which a force-level filter cannot do);
+# that combination is the "balanced" preset. The force low-pass stays off there
+# too: on top of the EMA it added no measurable noise benefit at the same
+# latency, so it is an opt-in stage for sites with different noise
+# characteristics (the "low_noise" preset).
 DEFAULT_FORCE_LOWPASS_HZ = 0.0
-DEFAULT_FORCE_MEDIAN_WINDOW = 3
-DEFAULT_FLOW_EMA_ALPHA = 0.5
+DEFAULT_FORCE_MEDIAN_WINDOW = 1
+DEFAULT_FLOW_EMA_ALPHA = 1.0
 
 #: Named filter presets, so a recommendation is *executable* instead of a
 #: comment in a JSON file (``json.load`` cannot carry comments). A config may
diff --git a/tests/test_filter_presets.py b/tests/test_filter_presets.py
index 3120c44..dd27fef 100644
--- a/tests/test_filter_presets.py
+++ b/tests/test_filter_presets.py
@@ -10,6 +10,7 @@ when a copy stops matching them.
 from __future__ import annotations
 
 import json
+import shutil
 from pathlib import Path
 
 import pytest
@@ -88,3 +89,29 @@ def test_shipped_bundles_report_all_off(bundle: str):
     assert fp["measurement"] == {"flow_ema_alpha": 1.0}
     assert fp["readout"]["force_median_window"] == 1
     assert fp["readout"]["force_lowpass_cutoff_hz"] == 0.0
+
+
+def test_a_config_with_no_filters_block_filters_nothing(tmp_path: Path):
+    """Absence is off, not "the values we thought were sensible".
+
+    A key that is missing has to mean the stage is not running: otherwise a
+    device filters a measurement nobody asked for, and nothing in the file says
+    so. Callers with a hand-written config (an old customer file, a bench
+    script) rely on this.
+    """
+    copy = tmp_path / "box"
+    shutil.copytree(REPO_ROOT / "sensors" / "box", copy)
+    path = copy / "sensor.json"
+    cfg = json.loads(path.read_text(encoding="utf-8"))
+    cfg.pop("filters", None)
+    path.write_text(json.dumps(cfg, indent=2), encoding="utf-8")
+
+    sensor = Sensor("external", config_name=str(path), verbose=False, backend="cpu")
+    try:
+        state = sensor.filters_state()
+    finally:
+        sensor.disconnect()
+    assert state["measurement"]["flow_ema_alpha"] == 1.0
+    assert state["readout"]["force_median_window"] == 1
+    assert state["readout"]["force_lowpass_cutoff_hz"] == 0.0
+    assert state["any_stage_applied"] is False