Positive offsets still delay the whole mix via the delay line (lip-sync fix);
negative offsets now ARM once at StartLive and drop |N| ms off the pipe's write
head so audio events land earlier when audio runs BEHIND video. Slider relabeled
AUDIO SYNC, Min −500, locked while live/recording (IsEditMode). LayoutStore and
VM clamp to −500..500.
OBS reference for eat-the-head negative sync: https://obsproject.com/kb/obs-studio/buffering-time (negative sync values pull audio earlier by discarding buffered player audio).
Test: StartLive_NegativeOffset_AdvancesAudio_ByDroppingTheStreamHead (6x0.9 head
must be eaten before 0.2 bed reaches the wire).
AudioMixer.Start() begins mic/loopback capture at app startup to feed the
level meters, so both 2s ring buffers fill with pre-live audio. StartLive()
drained from the oldest tail sample, putting every recorded event ~2.2s late
in the audio track (confirmed by clap analysis + cross-correlation on two
takes: +2.11 to +2.22s).
Fix: AudioMixer.StartLive() now runs _micBuffer.Clear() + _loopbackBuffer.Clear()
immediately after the pipe starts, before the drain task runs. Recording now
begins at go-live; the <=10ms in-flight chunk evicted by Clear() is imperceptible.
Regression test: StartLive_DiscardsPreLiveBacklog_SoFirstAudioIsCurrent
saturates the loopback ring with stale 0.8 pre-live audio, then asserts the
wire carries fresh post-live 0.2 (max < 0.3).
Reference (external, per derivative-work rule): OBS 'Audio mixer' keeps its
buffers fed continuously and syncs the stream start timestamp at record time
rather than replaying pre-live capture; a go-live flush of the capture buffer
is the accepted pattern for live tools restarting a stream.
Docs: HANDOFF.md (fix shipped), MyMistakes.md (A/V sync measurement recipe).
gramps