A DTS:X stream in the standard presentation is DTS-HD Master Audio with a 7.1
bed and an extension at the end of every XLL frame, behind the syncword the
decoder already recognises (DCA_SYNCWORD_XLL_X). The extension is a 22-byte
header followed by a regular XLL channel set: four channels, every one coded
in full (no residual against the core), no speaker mask, its own NAVI table
with the frame's segment geometry, then a two-byte trailer. Its channels are
the front and rear height pairs, and the encoder folds each of them into the
bed speaker below it (L, R, Lsr, Rsr) at -3 dB, gain code 23170/32768, so the
7.1 bed plays them on players that ignore the extension.
The decoder now parses that channel set with the existing channel set code,
outputs 7.1.4 (TFL, TFR, TBL, TBR), and removes the fold from the bed at
output resolution. A "downmix" request for 7.1 keeps the bed as coded, heights
folded in, which is what the decoder output before; the requests for 5.1 and
stereo skip the extension as well, so their output is unchanged. Streams
without the extension are not affected. A frame whose extension does not
decode keeps the 7.1.4 layout with silent heights and the bed as coded, and
the frame still ends where it did before. The alternate presentations, which
carry objects behind DCA_SYNCWORD_XLL_X_IMAX, are not touched.
The format facts (header size, channel order, the fold) come from the public
analysis in harletty-bridge's dca crate and kristofferR's SpatialAudioBridge;
this is an implementation on FFmpeg's own XLL code, not a translation.
Signed-off-by: Vincent Herbst <superuser404@tuta.com>
Vincent Herbst
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