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method: ask whether an instrument returns zero for no difference

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Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Wuu56cE8vJGTBtn1ppsk8v
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@@ -2670,3 +2670,33 @@ untouched copy and **passed a wrong contract**.
a checked value** — and this one was created by the fix for a different navigation a checked value** — and this one was created by the fix for a different navigation
problem. Forward markers here now name their target by **date and subject** rather problem. Forward markers here now name their target by **date and subject** rather
than reproducing its heading. than reproducing its heading.
## Before asking whether an instrument can measure a difference, ask whether it returns zero for no difference
`sylpheed-port`'s rule, after three cheap tests disqualified a difference-signal
path of theirs: source against a **second decode of itself** reached −inf (the
pipeline was fine), but a **lossless** encode of the identical fold reached only
14.2 dB where it must reach ~90. **An instrument that cannot verify an encode known
to preserve every sample says nothing about a lossy one** — so every difference
number in that thread was an artefact of the lag search, not a measurement.
✅ **Applied to my coherence estimator, which had never had the test.** Its
"positive control" was a *filtered* copy reading **0.94**, and I had taken that as
the ceiling. It is not:
| | all bands |
|---|---|
| wave 0 **against itself** | **1.0000** |
| a linear filter, **no delay** | **1.0000** |
| the same filter **+ 12 ms delay** | 0.9288–0.9380 |
The estimator is exact; the 0.94 was the **delay's windowing cost**. The
instrument passed, and the finding it supports got *stronger* — the real ceiling is
1.0, so the measured 0.027 sits further below it than I had claimed.
📌 **A positive control that is merely "high" hides the difference between an exact
instrument and a lossy one.** Mine read 0.94 for two different reasons — a correct
estimator plus a windowed delay — and I could not have separated them without the
identity case. ⚠️ Note what this cost to find: **one line, no new data**, and it
was available from the day the tool was written. Their equivalent test cost one
decode and no encoder, and they say the same thing about it.