The DVD AC-3 sub-stream probe recorded the FIRST decodable frame of each
physical 0x8x sub-stream as its channel count. A DVD feature opens with
logos/warnings whose audio is often a thin 2.0 bed on 0x80 before the
real 5.1 main mix begins a fraction of a second later. The probe locked
onto that opening 2.0 frame and reported 0x80=2, missing the 5.1
entirely (confirmed on Greenland: 0x80's head frames are acmod=2, then
acmod=7+lfe). With no 6-channel sub-stream found, channel-match routing
fell back to the ordinal map — harmless on Greenland, but on a disc where
the 5.1 lives on a non-ordinal sub-stream the wrong-substream bug stays
unfixed.
Fix: scan EVERY 0x0B77 frame of each sub-stream in the probe window and
keep the MAXIMUM channel count (the sub-stream's real main-mix
capability), advancing frame-by-frame via ac3_frame_size so a frame body
can't be mistaken for a new sync. Also bump PROBE_SECTORS 512->1024: the
1 MiB head window saw ONLY 0x80; 2 MiB reliably contains a frame of every
physical sub-stream.
Greenland tag=dvd.substream: before 0x80=2 (only); after 0x80=6, 0x81=2,
0x82=2 — matching the IFO and the decoded output.
Adds probe_reads_max_channels_no_cross_contamination regression test.
Fixes the "Silence of the Lambs" R2 PAL wrong-substream rip: the feature's
IFO declares one 5.1 AC-3 stream, but the scan assigned it the on-wire
sub-stream id 0x80 purely by per-codec ordinal (ifo::assign_audio_sub_stream_ids).
On this disc the physical 0x80 carries the 2.0 down-mix and the 5.1 main mix
lives at a different 0x8x sub-stream, so the rip muxed 2.0 while labelling it
"Dolby Digital 5.1" (the acmod fixup in mkv.rs then corrected only the Channels
element, surfacing the mismatch as the "IFO claimed 6 but acmod says 2" warning
— too late to re-route).
New src/disc/dvd_audio_probe.rs probes each physical AC-3 sub-stream's real
channel count from the head of the feature (the acmod/lfeon of its first frame
after the 0x0B77 sync) and re-routes each IFO-declared AC-3 stream onto the
physical sub-stream whose actual channel count matches the declared count,
instead of trusting the ordinal. Wired into both mux demux paths
(DiscStream::new and resolve::build_iso_pipeline) over the decrypting reader,
so it works on CSS discs and the autorip ISO-remux path alike. Bounded
512-sector best-effort read; an empty/unreadable probe degrades to the original
ordinal mapping (no regression on normal discs).
The cell selection is left unchanged: the feature's cell 0 (cat=0x02, 302.4s)
is chapter 1 of the movie (matches MakeMKV's chapter map and 1h53 duration
exactly), so it must NOT be dropped — the perceived "wrong video at the start"
was the wrong 2.0 audio over the opening, the same root cause.
Diagnostics (--log-level 3): new tag=dvd.substream rows dump the ACTUAL acmod
channel count of each physical 0x8x sub-stream read from the VOB, and the
per-cell tag=dvd.cell verdict now spells out the keep/skip reason. With the
existing tag=dvd.aattr (IFO declared sub_id + channels) a bug log alone now
shows whether the ordinal 0x80 really carries the declared layout — no disc
needed to diagnose this class.
expose ac3::find_ac3_sync as pub(crate) for the probe.
Matthew Jackson