tapir/docs/research/audio-length-ceiling-2026-10-01.md

Audio length ceiling — measured before designing chunking (tapir#33, 2026-10-01)

Question (#33, "the main technical risk"): at what recording length does Swedish transcription through the LiteLLM gateway first fail, and with what failure mode? Measured before any chunking design, with iguana not holding a large ollama model, so the result is the ceiling and not the Metal OOM of infra#273.

Answer

No transcription ceiling was found at or below 90 minutes. A single request carrying 90 minutes of 16 kHz mono PCM (174 MB) transcribed in one pass in 440 s. The first thing that actually fails is not whisper: it is the public gateway ingress, which rejects the upload with 413 at roughly 5.5 minutes of WAV.

Route	First failure	Failure mode
https://llm-api.d-ma.be (openresty ingress)	between 8.0 MB and 10.5 MB body (~4–5.5 min of WAV)	413 Request Entity Too Large, HTML body from openresty, returned instantly
http://koala:30401 (LiteLLM NodePort) → iguana/kb-whisper	none up to 90 min / 174 MB	—
http://koala:30401 → berget/kb-whisper	none up to 22 min / 42 MB (largest tried)	—

The ingress limit is the reason the 8:20 / 15 MB recording in #33 "failed repeatedly" and a 30-second slice succeeded. That failure was attributed to a Metal OOM; at least in the size-dependent form described, a body-size cap explains it without one. (The OOM mechanism in infra#273 is separately real; this run did not provoke it.)

The exact ingress cap was bracketed (8.0 MB passed, 10.5 MB failed), not read from the ingress configuration, which lives in infra. "About 10 MB" is an inference.

Method

• Audio. No known-good Swedish recording is available that is safe to use: the only real one (IMG_1233.wav) is the private /bygge recording (#31 custody still open) and was deliberately not touched. Instead: macOS say -v Alva (Swedish) on iguana, a 1,171-char five-paragraph meeting script → 82.5 s of 16 kHz mono PCM WAV (ffmpeg -ar 16000 -ac 1). Concatenated ×4, ×8, ×16, ×32, ×44, ×66 with Python wave (the issue's own recipe). Nothing here is a real transcript; none of it is committed.
• Request. POST /v1/audio/transcriptions, multipart file=, model=iguana/kb-whisper, response_format=verbose_json, -H 'Expect:' (per #33's traps), master key supplied only through op run and never on argv.
• Confound control. ollama /api/ps on iguana was read before every step. Resident: nothing, or nomic-embed-text (0.37 GB). Never a large model.

Results (iguana/kb-whisper, NodePort)

Audio	WAV size	HTTP	Wall	Realtime factor	Segments	Chars
0:30	0.96 MB	200	2.8 s	11×	—	—
2:30 / 3:20 / 4:10	4.8 / 6.4 / 8.0 MB	200 (ingress)	12.9 / 16.0 / 19.9 s	12×	—	—
5:30 (×4)	10.5 MB	413 ingress, 200 NodePort in 27 s	27 s	12×	121	4,557
11:00 (×8)	21.1 MB	200	53 s	12×	241	9,094
22:00 (×16)	42.3 MB	200	105 s	13×	482	18,201
44:01 (×32)	84.5 MB	200	209 s	13×	966	36,438
60:32 (×44)	116.2 MB	200	291 s	12×	1,332	50,145
90:48 (×66)	174.3 MB	200	440 s	12×	2,005	75,165

• Throughput is flat at ~12× realtime. 90 minutes costs ~7.5 minutes of wall time. This is a minutes-long job, not a request/response one (see ADR-033 on ADR-018).
• Segments per source repetition stay at 30.1–30.4 from ×4 to ×66, and segment times are monotonic with last_end within 0.2 s of duration. No drift and no repetition-loop hallucination appeared across 90 minutes. This is a weak check — the input is a repeated loop and synthetic voice — not evidence of accuracy.
• Only iguana/kb-whisper's x4 run reported last_end 360.0 s against duration 330.2 s. Whisper's final segment can end past the audio; clamp to duration.

Findings that change the design

1. Use the NodePort route, not the public hostname. Tapir runs in-cluster beside the gateway; llm-api.d-ma.be is the external door and has a body cap. #33 said "use the gateway, never the host" and named the public URL; the gateway is right, that URL is not. TAPIR_GATEWAY_URL already points at koala:30401/v1.
2. response_format=srt survives the LiteLLM hop, but wrapped. The response is JSON {"text": "<the SRT>", "usage": …}, not a bare SRT body — a client that parses the body as SRT gets a parse error on {. #33 question 2 is answered: it passes through, inside an envelope.
3. The two backends return different timestamp granularity. iguana/kb-whisper segments are whole seconds (0→1, 1→4, 4→7); berget/kb-whisper returns real sub-second times (0.051→1.192) and a different language token (swedish vs sv). verbose_json and the default JSON were byte-identical on iguana. An adapter that falls back between the two must normalise both, and "which endpoint served this" is a quality fact, not just a log line.
4. The existing chat path cannot answer about most of a long recording. At 828 chars/min (this fixture; real speech is plausibly a little denser), TAPIR_MAX_TRANSCRIPT_CHARS's default of 18,000 is ~21.7 minutes. A 60-minute transcript is 50 k chars, a 90-minute one 75 k. chat.Service truncates by keeping the head (internal/adapters/chat/chat.go), so questions about the last two-thirds of a one-hour meeting get an answer from a transcript that does not contain it. #33 says chat "retrieves rather than generates" and therefore has no ceiling; it stuffs and truncates, and its ceiling is the model context. See ADR-033.
5. Chunking is not needed for v1 on length. Nothing failed up to 90 minutes. The chunking-and-stitching recipe in #33 (offset timestamps, renumber cues, strip \r) is not built. It stays the answer if a longer cap is wanted or the OOM mechanism forces smaller requests — see ADR-033's resolution triggers.

What this did not measure

• Metal OOM under contention. Deliberately avoided. A big resident ollama model during a long transcription is still the known way to wedge iguana permanently.
• Real speech. TTS is clean, single-voice, no crosstalk or room noise. WER and speaker overlap in a Teams export are untested. Speed is probably representative; accuracy is not claimed.
• Compressed input. Only WAV was sent. Whether m4a/mp3/opus can reach whisper without a server-side transcode is not established here (ADR-033 treats it as needing one).
• Concurrency. One request at a time. Two simultaneous 90-minute jobs on iguana were not tried.
• berget/kb-whisper beyond 22 min / 42 MB. Enough to know the fallback is not size-capped at meeting length for 22 minutes; not proven at 90.
• Time-of-day / other tenants. One afternoon, one run per length.

Scripts were throwaway shell/Python in a session scratchpad and are not committed; the method above reproduces them.