The keyboard preview crashed on first record with a
`__abort_with_payload` deep inside Speech's
`DictationTranscriber`. The disassembly surfaced three
preconditions checked before a `brk #0x1`:
+620 "Audio sample data must be 16-bit signed integers"
+848 "Multi-channel audio is not supported"
+1072 "Client info not fully initialized"
We hit the first one. `DictationTranscriber` (iOS 26's new
`SpeechAnalyzer`-backed engine) is strict about its input
format: only Int16 PCM, not the Float32 PCM that the iOS 18
`SFSpeechRecognizer` path accepted. Our audio-tap and
`AudioBufferSnapshot.samples: [Float]` are Float32 all the
way down — that was the SFSpeech shape, and the previous
`AppleSpeechASR` adapted internally. With iOS 26 as the
deployment target, the only ASR backend is
`SpeechAnalyzerASR`, and the conversion needed to happen at
the `AnalyzerInput` boundary.
Fix:
- `transcribe` builds the `AVAudioFormat` as
`.pcmFormatInt16, 16 kHz, 1 ch, interleaved: true` (the
canonical layout for Int16 Speech input).
- `makeInputStream` runs the per-sample conversion
`Int16(round(clamp(s * 32767, -32768, 32767)))` into the
`AVAudioPCMBuffer`'s `int16ChannelData[0]`. The explicit
clip is required (a `s == 1.5` from a gain-overflow at the
audio-engine boundary would otherwise wrap to a negative
Int16 after the implicit truncation). `round()` (not
truncate) preserves DC balance — `0.5` quantises to
`+16384`, not `+16383`, matching what audio DAWs expect.
- The conversion helper is exposed as
`ASRServiceFactory.convertFloat32ToInt16` so unit tests
can lock the math without instantiating the full pipeline.
Why not change `AudioBufferSnapshot` to `[Int16]` instead
(see earlier first-principles discussion): the snapshot is a
transport format that both `AudioCaptureService` (in the
ext) and `PreviewASRController` (in the main app) produce.
Float32 is the natural shape coming out of `AVAudioEngine`,
and pushing the conversion to the ASR service keeps the
transport contract platform-agnostic — a future second
backend with different format needs can have its own
adaptation without dragging everyone else.
Tests:
- `testFloat32ToInt16EdgeCases` — 0, ±1, ±0.5, ±1.5
(gain-overflow case).
- `testFloat32ToInt16RoundTrip` — quantisation step is
1/32767 (so the asymmetric Int16 range is honoured: -32768
has no exact Float source).
- `testFloat32ToInt16Empty` — `sourceCount == 0` with nil
pointers is a no-op (function guards on count before
dereferencing).
- All 25 tests pass (22 existing + 3 new).
- BUILD SUCCEEDED.
🤖 Generated with Claude Code
Rocky
a227309059