DTS audio pass-through doesn't work

[Christian Weisgerber]

Configuration:
FreeBSD 7.2-STABLE amd64
snd_hda, S/PDIF, AC3 and DTS-capable amplifier
dev.pcm.2.play.vchans=0, mixer vol 100 pcm 100

What works:
mplayer AC3 and DTS pass-through (-ac hwac3,hwdts) for DVDs etc
works fine.

What doesn't:
Playing DTS audio that masquerades as 44.1kHz PCM data fails, e.g.
DTS audio CDs or corresponding WAV files.  XMMS and mplayer should
just output this unchanged over S/PDIF and the amplifier should
recognize it as DTS and decode it.  Except it doesn't.  It reports
PCM 44.1 and plays noise.

When I play the same audio data on OpenBSD, the amplifier recognizes
it as DTS-encoded and everything works fine.

There must be an element in the FreeBSD audio systems that performs
some kind of transform on the audio data, killing the DTS signature.
Any ideas?

-- 
Christian "naddy" Weisgerber                          naddy at mips.inka.de