Phases 0-3 of design/audio-quality-and-latency.md, host side.
**WP2.1 — the 2026-08-03 root cause.** The client-only loopback preference took Steam's
Streaming *Microphone* render endpoint over real hardware unconditionally, because it is
silent on the host. But that endpoint exists to carry remote VOICE, and nothing checked
whether it could carry music: on the reporter's box it won all 31 loopback opens across 25
sessions while a clean AMD HD Audio endpoint sat idle, and the whole desktop mix went
through it before reaching Opus. A silent sink now has to EARN its preference — if its mix
format narrows the mix it drops below real hardware. It is still taken when nothing better
exists (narrow audio beats no audio), but flagged so the capture side says why.
`plan_with_formats` takes a probe rather than reading WASAPI, so all 26 wiring-plan tests
still run on every platform. An unknown format counts as fine, which is asserted:
`unknown_formats_reproduce_the_formatless_plan` proves a probe failure can never make the
plan worse than it was before formats existed.
**WP0.1 — log the endpoint's ACTUAL mix format.** Everything the old log printed ("48 kHz
f32 channels=2") was our REQUEST; with `autoconvert` WASAPI converts silently from whatever
the endpoint really runs. That is why a 3,600-line log filed over an audio-quality
complaint contained nothing that could diagnose it.
**WP0.2 — count what we drop.** The capture->encode handoff was a silent lossy `try_send`:
a stalled encode thread lost chunks, the encoder concatenated across the hole, and nothing
recorded it — a click plus a permanent shift of everything after. Now counted and warned,
alongside per-window peak/RMS/delivered% so a quiet host, a broken endpoint and a stream we
are damaging ourselves stop looking identical.
**WP2.4 — stop the default-device tug-of-war.** In Assert mode the capture is bound to the
planned endpoint EXPLICITLY, so a hijacked default changes only where apps render — the old
full reopen tore the capture down for nothing. The field log shows the cost: something
re-set the default every ~4 s and each round was a teardown, a wiring pass with
IPolicyConfig writes, and an audible dropout — seven in sixteen seconds, one ending in a
2 s error backoff. Now: put the default back, keep the stream, and after four rounds in
twenty seconds concede for a minute and say so once.
**WP1.1/1.2 — encode quality.** Constrained VBR (the hard-CBR comment justifies itself with
GameStream's audio FEC, which this plane does not have) and `AudioTier::High` by default:
stereo 128 -> 256 kbps, ~1 % of a 20 Mbps session. GameStream's encoder is deliberately
untouched — its FEC really does need fixed-size packets.
**WP3.1 — redundant `0xD2` plane**, sent when the client asked for it.
**WP2.2 — `audio.output_mode`** as a first-class setting (`client_only` / `host_and_client`
/ `follow_default`), superseding the two undocumented env vars, which stay honoured. The
enum lives in pf-host-config, which is deliberately dependency-free, so the tier table stays
in core where the codec knowledge is.
`capture_policy.rs` is split out for the same reason `wiring_plan.rs` is: both encode field
behaviour, so their tests must run on Linux CI, not only on a Windows box. That split
immediately earned itself — `capture_stats_separate_silence_from_signal` caught RMS being
divided by the FRAME count while summed over interleaved SAMPLES, which inflated it by
sqrt(channels) and made a sine report an RMS equal to its own peak.
WP4.5 (open the loopback at the minimum device period) is deliberately NOT done: in shared
mode `IAudioClient::Initialize` cannot change the engine period at all, so it would be a
no-op at best and a new failure path at worst. Recorded in the code. WP2.3 (force the parked
endpoint's volume) is deferred — `wasapi` keeps IMMDevice private, so it needs new raw COM
on a path this tree cannot compile, let alone test; its diagnostic half ships as the RMS
line above.
Verified: punktfunk-host + pf-host-config clippy --all-targets -D warnings and the audio
test suite under Linux/docker (gate proven non-vacuous with a planted type error); 26
wiring-plan tests standalone; fmt. The Windows-only halves of wasapi_cap.rs and
audio_control.rs are NOT compile-verified anywhere yet.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
enricobuehlerandClaude Opus 5
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