Groundwork for bitstreaming TrueHD the way other players do (#1804).
Android will not bitstream raw TrueHD on the TV routes measured so far. Both
connected Android TV boxes report ENCODING_DOLBY_TRUEHD as offload-only while
reporting ENCODING_IEC61937 at 192kHz/7.1 as bitstream-capable, and the
reporter's box takes a raw TrueHD AudioTrack and then never advances its
playback head. Kodi models this split explicitly: it offers an "AudioTrack
(IEC)" sink where it packs the carrier itself and treats handing raw TrueHD to
Android as the fallback, and even that fallback runs at 192kHz. Media3 only
ever does the raw form, at the stream rate.
This adds the packer half: split a sample into TrueHD access units, assemble
MAT frames with timing-derived padding, and emit IEC 61937 bursts. It is a port
of FFmpeg's spdif_header_truehd rather than Kodi's CAEBitstreamPacker, because
Kodi's is a thin wrapper over an already-assembled buffer while the MAT code
placement and padding live in FFmpeg's stateful packer.
Details the port has to get right. Media3's Matroska path concatenates 16
syncframes into one sample, so access units are split here; reading a single
input_timing for sixteen frames would desynchronise the carrier. Burst buffers
alternate and are reused rather than allocated, because a fresh 61,440 byte
array every 20ms is roughly 3MB/s of garbage on the low-power hardware this
runs on. A 44.1kHz-family stream rides a 176.4kHz carrier instead of 192kHz,
which changes the whole AudioTrack tuple, so it is reported as unsupported for
the caller to decode instead.
A wrong byte here is not subtle — the receiver drops sync or renders full-scale
noise — so the test compares against FFmpeg's own output byte for byte, using
its input and output as fixtures.
No caller yet; the sink that routes TrueHD through this follows.
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